Energy expenditure and subsequent nutrient in overfed young men SUSAN BYRON ELLEN
intakes
B. ROBERTS, VERNON R. YOUNG, PAUL FUSS, MARIA A. FIATARONE, RICHARD, HELEN RASMUSSEN, DAVID WAGNER, LYNDON JOSEPH, HOLEHOUSE, AND WILLIAM J. EVANS
Laboratory of Human Physiology, United States Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston 02111; the Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge 02139; and the Shriners Burns Institute, Boston, MA 02114
ROBERTS, SUSAN B., VERNON R. YOUNG, PAUL Fuss, MARIA A. FIATARONE, BYRON RICHARD, HELEN RASMUSSEN, DAVID WAGNER, LYNDON JOSEPH, ELLEN HOLEHOUSE, AND WILLIAM J. EVANS. Energy expenditure and subsequent nutrient intakes in overfed young men. Am. J. Physiol. 259 (Reg-
ulatory Integrative Comp. Physiol. 28): R461-R469,1990.-We investigated the mechanisms of body weight regulation in young men of normal body weight leading unrestricted lives. Changes in total and resting energy expenditure, body composition, and subsequent voluntary nutrient intakes in response to overeating by 4,230 t 115 (SE) kJ/day (1,011 * 27 kcal/day) for 21 days were measured in seven subjects consuming a typical diet. On average, 85-90% of the excess energy intake was deposited (with 87% of this amount in fat and 13% in protein on average). There was no detectable difference between individuals in susceptibility to energy deposition. The resting metabolic rate, averaged for fasting and fed states, increased during overfeeding (mean k SE, 628 t 197 kJ/day, P C O.Ol), but at least some of this amount was obligatory expenditure associated with nutrient assimilation. No significant increase in energy expenditure for physical activity or thermoregulation resulted from overfeeding. Thus energy expenditure did not substantially adapt to increased energy intake. However, significant decreases in voluntary energy intake (1,991 t 824 kJ/day, P < 0.05) and fat intake (48 * 11 g/day, P < 0.01) followed overeating, indicating that adaptive changes in nutrient intakes can contribute significantly to body weight regulation after overeating. doubly labeled water; fat; obesity
METHODS
from a remarkable balance between energy intake and energy expenditure. It can be calculated that a discrepancy of only 5% between energy intake and expenditure will result in a body fat gain of lo-15 lb during the course of a year, leading rapidly to obesity if continued. This regulation of body weight and energy balance must involve adaptive fluctuations in energy expenditure or energy intake over periods of several days or longer, because diet digestibility is relatively unaffected by energy intake (6, 14, 28) and day-to-day variations in energy intake and expenditure are substantial but independent (7). However, the question of whether energy intake or energy expenditure is the primary factor. maintaining energy balance remains a subject of major controversy (10). SUCCESSFUL
BODY
WEIGHT
Agreement on this fundamental issue would be of importance in helping to define the causes of obesity and in designing improved methods for prevention and treatment of excess body fat gain. We report here an overfeeding study in healthy young adult men, which was designed to investigate the normal mechanisms of body weight and energy regulation. The study differed from previous overfeeding studies (l-3, 6, 11,14-16,26,28) by including three important elements. First, we utilized recent methodological advances (17,19, 21) to measure the changes in all the principal components of energy balance resulting from overeating, including total energy expenditure (TEE), body energy content, and the subsequent voluntary energy intake. Second, we conducted the investigation in subjects pursuing normal daily activities. This aspect of the study was important because we anticipated that confining the subjects to a metabolic ward during the investigation might well suppress natural mechanisms of energy regulation, e.g., those associated with thermoregulation and physical activity. Third, we focused on assessing the causes of apparent individual variability in the susceptibility to weight gain during overeating, specifically to see whether the reported differences (11, 14, 15) are due to genuine physiological differences between individuals or to methodological error.
regulation
0363-6119/90
results
$1.50
Copyright
Subjects. The subjects were seven male students or laboratory staff of normal body weight and fat content (Table 1). None smoked or had any recent illness or history of an endocrinopathy. At the time of the study all were healthy as judged by a normal physical examination, routine blood and urine examinations, and electrocardiogram. The study was conducted in the Metabolic Research Unit (MRU) at the United States Department of Agriculture Human Nutrition Research Center with approval from the New England Medical Center Human Investigation Review Committee. Written informed consent was obtained from the subjects. Protocol. The subjects were able and encouraged to pursue a normal lifestyle during the study, and all continued their usual occupations. They were required to sleep at the MRU for the nights before measurements of
0 1990 the American
Physiological
Society
R461
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R462
MECHANISMS
BODY
WEIGHT
REGULATION
IN
YOUNG
MEN
body weight (kl g) and standard anthropometric measures (waist, hip, midupper arm, thigh and calf circumferParameters Values ences, and triceps, biceps, subscapular, and suprailiac skinfold thicknesses) were recorded in the morning be7 23.7kO.4 be, Yr fore the subjects had consumed breakfast and after they Starting weight, kg 76.3t4.7 had voided, on study days 0, 10, 20, 29, 41, and then at Height, cm 178.322.4 approximately weekly intervals until day 77. Body mass index, kg/m2 24.0t1.3 Diets. All food and caloric beverages consumed by the Body fat before overfeeding, %wt 13.7t1.2 subjects during phases 1 and 2 were provided by the Values are means t SE; n, no. of men. MRU. The MRU also provided food during the first 10 days of phase 3, but subjects were also permitted to [j PHASE 2 PHASE 3 consume additional caloric beverages provided that 0 10 20 30 77 40 I I I I // Study day 1 I amounts and types were reported. Meals consisted of normal food items divided between three meals plus an weight weight maintenance ad Energy intake t?aintenan%? plus 1000 kcal/day )+libitum ) evening snack each day. There were three different daily (provided) Activity monitors menus provided on a rotating basis to the subjects. The 4 b menus were tailored to individual preferences for foods. Total energy expenditure d-b+ b Coffee and tea were included in the menus in fixed daily amounts if they formed part of the subjects’ normal diet. Resting metabolic rate/ ** * * thermic effect of meal At least 1 meal/day was consumed in the MRU, and Urine collection 4 b other meals were consumed at the MRU or at the subjects’ residence. Body density * * The nutrient content of the diets provided during Anthropometry * * * * * * * * * phase 1 were designed to mimic a typical high-protein, FIG. 1. Study design. high-fat American diet (5). Approximately 1.5 g/kg body wt of mixed protein was provided, and an average of 55% resting energy expenditure (described below) but otherof the nonprotein energy was derived from carbohydrate wise could reside at home. Physical activity was moniand 45% from fat. Sodium intakes were dictated by the tored but was not restricted, with the exception that preference of the subjects. During the first 7 days of subjects were requested to not engage in an unusual phase 1, the energy intake required for weight mainteamount of activity for the specific purpose of preventing nance was determined. The initial energy intake provided weight gain during overfeeding. was 1.5 times estimated basal metabolic rate (30). MeasThe study was divided into three phases as shown in urements of body weight were taken, and appropriate Fig. 1. During phase 1, which lasted for 10 days, the adjustments to energy intake were made to maintain subjects’ energy intakes for body weight maintenance weight within 500 g of the value on day 0. Subjects were were determined. Phase 2 lasted for 21 days and started allowed to leave partial portions of food, which were within 1 mo of the end of phase 1. Subjects were requested saved for composition analysis (see below). During the to maintain body weight between these phases. During last 3 days of phase 1, subjects were given the calculated phase 2, subjects consumed -4,200 kJ/day (1,000 kcal/ day) more than during phase 1. Phase 3 lasted for 46 average of energy consumed during the first 7 days, with days and was consecutive with phase 2. Voluntary food the dietary composition specified above, and were reintake was measured during the first 10 days, and After quired to completely consume all portions of food and rinse and scrape food containers. Exactly 25% of daily this time subjects were monitored to assess long-term fat, protein, and carbohydrate were provided in the trends in body weight and anthropometric variables. TEE was measured throughout phases 1 and 2 using the breakfast meal, and the remaining nutrients were partidoubly labeled water technique (17, 21). Complete 24-h tioned between the other two meals and snack. Duringphase 2, subjects were provided with the menus urine collections were made during this time. All urine was collected in dark containers containing 15 ml 4.2 N fixed on the last 3 days of phase 1 and in addition were HCl with the exception of daily specimens required for given a daily low-protein, high-energy supplement conthe doubly labeled water analyses, which were stored in taining an average of 2 g protein, 55 g fat, and 127 g [to provide ~4,200 kJ/day (1,000 kcal/ nonacidified dark bottles. Body density was measured at carbohydrate day)]. In the first three subjects, the additional nutrients the beginning and end of phase 2 using the underwater weighing technique with published correction factors for were provided in the form of fruit juices, margarine, corn oil, and cookies. Subsequent subjects were given a pallung residual volume (27). Physical activity was moniatable beverage consisting of sherbet, corn oil, nondairy tored qualitatively in phases 1 and 2 and during the first 10 days of phase 3 using motion recorders worn around creamer, sucrose, and flavoring (Kool-aid). The supplethe waist (Caltrac, Hemokinetics, Madison, WI) and the ment was divided between meals, with exactly 25% given with breakfast. Subjects were required to completely wrist of the nondominant hand (Timex model 101motion consume all portions. The p-amino benzoic acid (PABA) recorder, Kaulins and Willis, Middlebury, CT). During this time, subjects kept a record of the duration and types test (19) was used to verify food consumption in five of strenuous physical activities performed. Body weight subjects during phase 2 between study days 13 and 30 (tlO0 g) was measured daily during phase 1. In addition, (one of the other subjects was allergic to PABA, and TABLE
1. Details
OF
of subjects
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MECHANISMS
OF
BODY
WEIGHT
technical problems prevented correct use of the method in another subject). This method involves incorporation of 240 mg/day of the potassium salt of PABA into the diet and quantitative measurement of its recovery in complete 24-h urine collections. A mean recovery rate of 92.6% in each subject (98.6 t 2.2%), indicating both complete consumption of provided foods and complete urine collections. During the first 10 days of phase 3, subjects were requested to consume as much or as little food and drink as they required to feel normally satiated and were given specific instructions to not deliberately change their diet to lose weight gained during phase 2. They were also asked not to weigh themselves. As in phases 1 and 2, there was a 3-day menu plan, but the number of items of food included in the menu was increased threefold, and subjects were allowed to specify as many or as few items and number of portions as they wished. They were not required to completely consume the portions, and leftovers were saved for composition analysis. After these 10 days, subjects provided their own food and could lose any weight gained during overfeeding if they so wished. Freeze-dried, homogenized portions of identical replicas of menus given to the subjects in phases 1 and 2 and the leftover foods were analyzed for gross energy by adiabatic bomb calorimetry. The measured gross energy contents of the 3-day menu cycles were 2.9 t 0.6% (P < 0.01) lower than values calculated using information on the fat, protein, and carbohydrate contents from food tables (GRAND database, Grand Forks Human Nutrition Research Center, Grand Forks, ND), assuming energy contents of 39.6, 23.6, and 16.6 kJ/g, respectively (23). Metabolizable energy (ME) intakes during phases 1 and 2 were calculated as (measured gross energy x 0.96) - (urinary nitrogen X 7.9) (23). Gross energy intakes during phase 3 were calculated from the weights of foods provided and gross energy contents estimated from GRAND food composition data (corrected for the difference between gross energy calculated from food tables and measured by bomb calorimetry in individual subjects for the phase 1 menus) and from the gross energy contents of the leftover foods, assuming as in phase 1 that the efficiency of absorption was 96% and that body nitrogen balance was at equilibrium. Measurements of resting energy expenditure. The volume of O2 and COZ (VO, and VCO~, respectively) were measured under thermoneutral temperature conditions in the fasting and fed states on four occasions during the study, twice during the last 3 days of phase 1 and once each during the first and second halves of phase 2. The subjects were instructed to relax and avoid hyperventilation, fidgeting, and sleeping during measurements, which were made while the subjects breathed through a low-resistance mouthpiece. Expired air was collected in a calibrated Tissot spirometer and analyzed for O2 and
REGULATION
IN
YOUNG
MEN
R463
COe contents at regular intervals (02 analyzer model N22M, Applied Electrochemistry, Sunnyvale, CA, and CO2 analyzer model LB-2, Beckman Instruments, Anaheim, CA). The measurements during the fasted state were made for 20 min before breakfast, lo-13 h after the last meal. The purpose of having this relatively long measurement duration was to reduce the possibility that persistent hyperventilation could influence the results obtained. After the fasting measurements, subjects consumed their usual study breakfast containing exactly 25% of the daily intakes of protein, fat, and carbohydrate. The measurements of Vos and VCO~ were resumed 30 min after the start of breakfast and continued during 20 min of every 30 min until 4 h after the start of breakfast. Subjects remained resting during the lo-min breaks. During one of the calorimetry sessions in each phase, small serial blood samples were drawn through an indwelling butterfly catheter inserted into an antecubital vein at least 15 min before the start of calorimetry measurements. Values for resting metabolic rate (RMR) were calculated from each determination of Vos and VCO~ using Weir’s equation (29). Two estimates of the 24-h RMR were made for each study period. Estimate 1 was calculated from the mean of all 20-min determinations for the study day. Estimate 2 was determined assuming that the 24-h day consisted of four 4-h energy expenditure cycles as measured between breakfast and the end of the measurement plus an 8-h period during which energy expenditure decreased linearly from the final determination of energy expenditure in the session to the initial fasting value. Measurements of TEE. Three sequential lo-day doubly labeled water studies (17, 21) were conducted in each subject to measure TEE throughout phases 1 and 2. The protocol used was designed to minimize possible errors arising from small changes in isotopic backgrounds and de novo lipogenesis. Concerning minimizing the effects of any isotopic background changes, a relatively short study period (10 days) was used and, in addition, the 2H20 intake was reduced in the second and third isotope doses (see below) to maintain the ratio of 2H-to-180 abundances in body water approximately constant. Two control subjects (data not shown), who completed phases 1 and 2 but received no 2H2180, were subsequently found to have only very small changes in isotopic backgrounds. The potential for de novo lipogenesis was minimized by ensuring that dietary fat intake was greater than the expected rate of fat deposition during overfeeding by a factor of two on average. The absence of net lipogenesis was subsequently confirmed with the observation of a relatively small increase in average resting respiratory quotient (RQ) during overfeeding (described in RESULTS).
Mixed ‘H2180 doses containing 0.15 g/kg H2180 and 1) or 0.055 (measurements 2 and 3) g/kg 2H20 were given orally midmorning on study days 0, 10, and 20 and were followed with two 50-ml rinses of tap water. Subjects consumed breakfast before the doses were given and were fasted for 3 h after the administration of isotope. Urine specimens were collected before 0.06 (measurement
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R464
MECHANISMS
OF
BODY
WEIGHT
administration of each isotope dose, at 3,4, and 5 h after each isotope administration, and thereafter at approximate daily intervals for 10 days after each dose until study day 30. Abundances of 2H and 180 in dilutions of the isotope doses and in at least 19 urine specimens/ subject (the baseline, 5-h, l-day, 2day, 8-day, g-day, and lo-day samples for each isotope study) were analyzed using isotope-ratio mass spectrometry (SIRA-10, VG Isogas, Middlewich, Cheshire, England). The samples collected at 3 and 4 h after dosing were not analyzed routinely, because preliminary analyses indicated that the isotopes were not fully equilibrated in urine at these times. Samples at the beginning and end of each isotope curve, rather than samples spaced evenly throughout each study period, were used because this approach should increase the precision of intercepts and rate constants for isotope disappearance. Samples were prepared for 2H/1H analysis using the zinc reduction technique and for 1802/1602 analysis in CO2 using the H20-CO2 equilibrator system (31). On average, quadruplicate analyses of each sample were performed. The coefficients of variation for day-to-day repeated measures of 2H and 180 abundances were 0.2%0 and 0.8%0, respectively. The doubly labeled water data were processed using DLW software (G. E. Dallal and S. B. Roberts, unpublished observations). Residual plots were obtained for two single exponential models of isotope disappearance over time (a nonlinear least-squares model and a linear least-squares model using the natural logs of the isotope data). Residuals generated in the nonlinear least-squares model showed no systematic variability over time, and thus the intercepts and rate constants from this model were used in subsequent calculations. Isotope-dilution spaces were then calculated from the intercept data (18). The ratio of the measured 2H20-to-H2180 dilution spaces should have been constant over time within subjects (because body composition changes were small), but none of the variance was accounted for by subject or study period (values ranged between 1.020 and 1.011). The intraclass correlation coefficient for the measurements was -0.1191. The variability in the ratio of the dilution spaces occurred despite the fact that potential sources of error had been minimized by mixing the isotopes before dosing and analyzing the isotope doses for isotopic abundances after appropriate dilution to give isotopic abundances similar to those in the urine specimens collected at 5 h after dosing. Because of the variation, we calculated carbon dioxide production rates (Rco,) using the equation of Schoeller et al. (22), which involves assumption of a fixed ratio between the dilution spaces.The value for total body water (N) used in the equation was determined as described by Schoeller et al. (22) (i.e., H2180 space/l.Ol), with a small correction for changes in body water during the measurement period made by assuming proportional changes in body weight and water (18). The value for N could also have been calculated from the average of both the dilution spaces, but this different approach would have had only a very small effect on the calculated Rco,. Values for TEE were calculated from R co, using Weir’s equation (29), using estimates for a modified food quotient (MFQ) determined
REGULATION
IN
YOUNG
MEN
from the known nutrient intakes (G. E. Dallal and S. B. Roberts, unpublished observations). In this calculation, it is necessary to correct for body fat mobilization or deposition during the measurement. However, provided that accurate data on dietary nutrient intakes and nitrogen balance are available, as in this study, it is not necessary to estimate body fat balance because this can be calculated by solving for energy expenditure, RQ, and ME intake simultaneously using the equations (G. E. Dallal and S. B. Roberts, unpublished observations) TEE = Rco, (88.278/RQ 1355.65F 973.OAL MFQ = 1918.05F 1461.OAL
+ + -
+ 24.774)
718.52P + 731.08C + (1427.OFB + 4881.25NB) 888.7231 + 731.08C + (2019.OFB + 6037.5NB)
(0
(2)
where F, P, C, AL are dietary intakes of fat, protein, carbohydrate, and alcohol in g/day, and FB and NB are body fat and nitrogen balances in g/day ME intake = 8.987F + 5.198P + 3.8906C + 7.1AL - 7.9U
(3)
where U is urine nitrogen excretion (g/day). Equation 2 assumes no significant change in body carbohydrate stores and no de novo lipogenesis during the study. It can be calculated that failure to correct for a net gain or loss of as much as 500 g of glycogen/lO-day study would result in an error in the calculated energy expenditure of only -1%. Body composition. Body fat contents at the beginning and end of phase 2 were calculated from the measurements of body density using the equations of Siri (27). Nitrogen balance was determined during phases 1 and 2 to provide information on the change in body protein content resulting from overfeeding. For this measurement, the nitrogen contents of pooled 24-h urine collections obtained during phases 1 and 2 and of the food aliquots were determined using a modification of the Kjeldahl technique involving microwave digestion of the samples (Kjelfast, CEM, Matthews, NC). Digestible nitrogen intakes were calculated from gross intakes assuming an average digestibility of 92% (23). Nitrogen balance was assumed to be zero during phase 1, and miscellaneous nitrogen losses (in sweat, hair, nails, etc.) were calculated for each subject as the difference between digestible nitrogen intake and nitrogen output in urine. These factors were then applied to phase 2 data to calculate nitrogen balance as the difference between digestible nitrogen intake and the sum of nitrogen losses (urine plus miscellaneous). Protein balance was calculated as nitrogen balance X 6.25. Energy balance during phase 2 was calculated as the sum of fat and protein depositions, assuming energy contents of 39.6 and 23.6 kcal/g, respectively (23). Energy balances during both phases 1 and 2 were also determined as the difference between the measured rates of ME intake and TEE. Statistical analyses. Values are expressed as means t SE. Differences between measurements and phases were analyzed using analysis of variance and Student’s paired t test. Regression analysis was used to assess associations
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MECHANISMS
OF
BODY
WEIGHT
among initial body fat content, excess energy intake, and the energy deposition during phase 2 and the voluntary energy intake duringphase 3. Differences between groups were considered significant at P < 0.05. The calculations were performed using CLINFO software (BBN Software Products, Cambridge, MA). RESULTS
Table 2 shows the values for measured ME intakes, and gross nutrient intakes calculated from food weights and published compositions, during weight maintenance in phase 1, overfeeding in phase 2, and during the first 10 days of phase 3 when food intake was ad libitum. ME intake was significantly higher in phase 2 than in phase 1 and significantly lower in phase 3 than in phase 1. The reduced ME intake in phase 3 was due to a significantly lower intake of fat. There was no significant trend in energy intake over time during phase 3. The body weights of the subjects are shown in Fig. 2. There was no significant change in body weight during phase 1 and no significant change between phases 1 and 2 (the change was 242 t 115 g). However, there was a significant increase of 118 g/day throughout phase 2. There was no significant difference between weight gain during the first and second halves of phase 2, and means were 142 t 36 and 92 t 26 g/day, respectively (P = 0.3347). There was substantial variation between sub2. Energy and nutrient intakes during weight maintenance (phase l), overfeeding (phase 2), and 10 days after overfeeding (phase 3)
TABLE
Phase 1
Metabolizable
energy,
H/day Protein,
Fat,
g/day
g/day
Carbohydrate, Ethanol, g/day
g/day
13,949+841
Phase 2
Phase 3
18,113+874$
11,958*523*
114t6 153t10
116k7 207+10$
12Ok9 105+6j-
409225 Ok0
542&27$
376t25
Values are means t SE. Significantly 1: * P c 0.05, t P < 0.01, $ P c 0.001.
l&l different
1426 from
values
for phase
2.5 9 O
2.0
5 8
1.5
E
1.0
8 5 3 !F (3 iii 3
050.0 -0.5
1
Phase1
STUDY DAY 1 Phase2 1
Lowest weight
Phase3
FIG. 2. Body weight change of subjects from study day 0. Values are means t SE of data corrected for weight change between phases 1 and Z(242 t 115g). ** P < 0.01 compared with values at start of overfeeding on study day 10.
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jects in weight gain over the entire overfeeding period, with values ranging from 13 to 142 g/day. The differences in weight gain between individuals were linearly related to body fat changes (P < 0.01, r = 0.9190), which averaged +l.53 t 0.45% of body weight between the 1st and 20th days ofphase 2 (P = 0.015). There was no significant relationship between weight change during overfeeding and changes in the skinfold thicknesses (triceps, biceps, subscapular, suprailiac) or waist, hip, or midupper arm circumferences. There was also no significant change in these variables between the beginning and end of overfeeding, although the change for the sum of all four skinfold thicknesses approached significance (4.8 t 2.2 mm, P = 0.0789). Differences between subjects in weight and fat gain paralleled subjective responses to overfeeding, with subjects exhibiting relatively high weight and fat gains finding it difficult to consume the excess energy, and subjects exhibiting low gains finding it relatively easy. Body weights fell on average during the first 10 days of phase 3 but were still significantly elevated compared with before overfeeding. The mean weight at the end of the study was similar to that observed on the 11th morning of phase 3 (study day 41) but was not significantly different from the weight before overfeeding because of increased variability in weight between subjects. The lowest weights measured during phase 3 for each subject differed from weights at the start of overfeeding by only -0.005 t 0.458 kg. Only one subject reported conscious dieting after completion of the food intake component of the study, to lose weight gained during phase 2.
Table 3 summarizes all the data on energy expenditure and physical activity throughout phase 1 and during phase 2 days l-10 and 1 l-20. There was no difference between the first and second halves of phase 2 for any parameter. There was no significant difference between phases 1 and 2 in the fasting RMR expressed either per subject or per kilogram of fat-free mass. However, the estimated 24-h RMR, calculated as described in METHODS from the mean (estimate 1) or weighted mean (estimate 2) of data from the fasted and fed states was significantly higher in phase 2 than in phase 1, by 594628 kJ/day on average. RQ averaged for fasting and fed states was slightly higher in phase 2 than in phase 1, but the difference was not quite significant (P = 0.0845). The values for RQ are slightly lower than the RQ values estimated from dietary intake and body fat balance, but the difference between phases 1 and 2 were very similar in the two methods, indicating that lipogenesis was minimal. The average Hz180 dilution spaces determined during the doubly labeled water measurements were significantly greater in phase 2 than in phase 1. However, there was no significant difference in TEE between phases, and values in phase 2 differed from those in phase 1 by only +423 t 795 kJ/day (+lOl t 190 kcal/day). There was considerable individual variability in TEE differences between phases 1 and 2. Thus the 95% confidence limits for the difference were relatively large, at -1,528, 2,371 kJ/day. The lack of a significant increase in energy
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R466 TABLE
MECHANISMS
3. Energy expenditure
OF BODY
WEIGHT
REGULATION
IN YOUNG
MEN
data Phase 1 (Study days O-9)
Calorimetry
Fasting RMR, kJ/day Fasting RMR, kJ . kg fat-free mass-’ day-’
(Study
RQ
24-h RMR 1, kJ/day* 24-h RMR 2, kJ/day* Doubly
Mean Hz”0 space, moles Rate of 2H disappearance, day-l Rate of la0 disappearance, day-l Modified food quotient TEE, kJ/day TEE/RMR
labeled
7,774+381 118t3 0.839IkO.017 9,439+377 9,276+372
water
Phase 2 days 20-29)
7,778+339 11824 0.854~0.011 9,569+389 9,393+356
data
2,479+113$ 0.09451+0.00860~ 0.12021kO.OO877"f 0.854+0.014t 13,883+774 1.851t0.010 Activity
(Study
data
7,544*381 115t2 0.821t0.013 8,878+477$ 8,740+456$
l
Phase 2 days 10-19)
2,565f133 0.10262kO.00931 0.12973t0.00961 0.880t0.007 14,665+678 1.890t0.035
2,580+127 0.10157*0.010 0.12750t0.011 0.891t0.007 13,945+745 1.793t0.059
data
Caltrac trunk monitor, kcal/day 2,547+223 2,618+276 2,717+283 Wrist motion sensor, units/day 1,947+221 2,009+220 2,417+388 Reported strenuous activity, min/day 31~10 24dO 26212 Values are means t SE. * See text for calculations. RMR, resting metabolic rate; RQ, respiration quotient; TEE, total energy expenditure. Significantly different from values for mean phase 2 values (days I-20 combined): t P < 0.05, $ P < 0.01. There was no significant difference between values for phase 2 (days l-l 0) and phase 2 (days 1 l-20).
expenditure for physical activity or thermoregulation during overfeeding is indicated by the similarity between the changes in the 24-h RMR and TEE. Qualitative estimates of physical activity provided by the motion sensors worn around the waist and wrist and the reported amount of time devoted to strenuous activity did not differ significantly between phases 1 and 2. Table 4 summarizes the energy balance of the subjects during overfeeding in phase 2. The mean increment in ME intake above that required for weight maintenance in phase 1 was 4,046 kJ/day. This value was very similar to the excess energy intake calculated as the difference between ME intake in phase 2 and TEE in phase 1 (4,230 kJ/day), indicating that on average the subjects were in energy balance during weight maintenance in phase 1. A mean increase in the maintenance energy expenditure of 14-E% resulted from overfeeding. The mean energy 4. Summary of changes in energy balance and body composition during overfeeding
TABLE
Parameters
Values
Energy,
kJ/day
Metabolizable energy intake Calculated energy excess (energy intake during phase 2 - total energy expenditure in phase 1) 24-h resting metabolic rate 1* 24-h resting metabolic rate 2” Total energy expenditure Energy deposition (fat + protein deposition) Energy deposition (energy intake expenditure) Body
composition,
+4,046+113 +4,230-r-1,150 +628*197 +594,t176 +423*795 +3,594*828 +3,807+448 g/day
Weight +118t21 Fat (by underwater weighing) +79t20 +20*3 Protein (by nitrogen balance) Values are means -+ SE. * See text for calculations.
deposition resulting from overfeeding, calculated from either measured fat and protein depositions or the difference between ME intake and TEE during overfeeding, accounted for 8590% of the excess energy intake. There was no significant difference between the two values for energy deposition, and means differed by only 213 t 498 kJ/day. Fat and protein depositions constituted 67 and 17% respectively, of body weight gain during overfeedix, and the average energy content of weight gain was 30 kJ/g (7.3 kcal/g). Body water gain during the first half of overfeeding, indicated by the HZ”0-dilution space change, averaged 27 g/day. This value is very slightly higher than the difference between average weight gain and fat plus protein depositions during phase 2. This slight discrepancy is not unexpected, because all the methods used were independent. Moreover, weight gain in the first half of phase 2 was greater than during the second half, although not significantly so. Thus, it seems likely that the higher weight gain during the first half of phase 2 was due to slightly greater water gain. To assess the cause of variability in body weight and energy gain, individual values for excess energy intake during overfeeding (calculated as the diffe rence between ME intake during phase 2 and TEE in phase 1) were regressed against body energy gain calculated from measured fat and protein depositions as shown in Fig. 3. There was a significant linear relationship between these parameters, and 90% of the variance in energy deposition between subjects was accounted for by differences in excess energy intake. The slope of the line was 0.665 t 0.094 kJ/kJ. The intercept of the line, 778 t 477 kJ/day, was not significantly different from zero. Significant relationships were also observed between the calculated excess energy intake and energy deposition determined as the difference between ME intake in phase 2 and TEE in phase 2 (P < 0.05) and body weight gain (P < 0.05). There was no indication of any effect of weight change 9
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MECHANISMS
0 0
OF
BODY
I
I
I
I
1
2
4
6
8
10
ENERGY
INTAKE
EXCESS
WEIGHT
(MJ/day)
FIG. 3. Relationship between excess metabolizable energy intake during overfeeding (calculated as difference between ME intake during overfeeding in phase 2 and total energy expenditure during prior weight maintenance during phase 1) and energy deposition in fat and protein in 7 subjects.
during phase 1, body fat content before overfeeding, or the reported amount of strenuous activity on the relationship between excess energy intake and energy deposition during phase 2. There was no relationship between weight change during phase 1 and energy balance during phase 1 (calculated as the difference between ME intake and energy expenditure). Individual variability in voluntary energy intake after overfeeding was also examined to explore the cause of the significant decreases in ME and fat intakes. No significant relationship was observed between energy deposition during overfeeding and the subsequent decrease in energy intake below normal energy needs (calculated as the difference between ME intake in phase 3 and TEE in phase 1 ), initial body fat content, or reported strenuous activity. DISCUSSION
Overfeeding has been a classical technique for investigating the mechanisms of body weight regulation in humans. Two distinctly different results have emerged from previous studies. In investigations focusing on measurements of energy expenditure, relatively small increases in expenditure were generally observed in response to increased energy intake (3,6,14, 16), implying that most of the excess energy must have been deposited, and suggesting a limited role for energy-wasting mechanisms in the regulation of body weight. However, in the studies focusing on measurements of body composition, only small increases in body weight or energy content in relation to excess energy intake were frequently observed (2, 11, 15, 2% implying that unmeasured adaptive responses to overfeeding involving an increase in energy expenditure must have occurred to prevent the expected body weight and fat gain in some or all of the individuals studied. Attempts to reconcile these diametrically opposed conclusions have usually concentrated on the different study durations and diet compositions (9, 25). However, the studies focusing on measurements of energy expenditure were usually performed under restric-
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R467
tive experimental conditions, whereas the studies emphasizing measurements of body composition were performed under conditions more closely resembling normal life. Thus noncompliance with the free-living protocols, or a suppression of energy-wasting mechanisms in the more restrictive protocols (for example, associated with thermoregulation or components of physical activity such as duration of activities, the energy costs of activities, or the extent of nonessential movements such as fidgeting), may well have contributed to the different results obtained. Because of these concerns and because recent advances in stable isotope methodology for measuring energy expenditure (17, 21) have permitted much more accurate and precise measurements of TEE in free-living subjects than were possible previously, we initiated a new overfeeding study in healthy young men leading normal lives and consuming a typical American diet. The measured increment in energy intake, averaging 4,046 kJ/day (967 kcal/day), was well within the reported normal day-today variations in energy intake (13). Thus the study mimicked and extended a change in energy intake within the normal pattern. Confidence in the food intake data was obtained by use of our new test of dietary compliance involving incorporation of PABA into the provided foods (19) and by the observation that the mean measured increase in energy intake was very similar to the mean sum of energy deposition plus energy expenditure changes during overfeeding. In the group of subjects taken as a whole, we observed no evidence of a significant energy-wasting mechanism. Two independent measures of body energy gain indicated that, on average, 85-90s of the excess energy intake provided during overfeeding was deposited. There was no significant increase in the energy expenditure for physical activity or thermoregulation, despite the fact that the subjects were able to lead essentially normal lives. In addition, as reported by most other investigators (12, 16, 25), the fasting RMR corrected for changes in lean tissue did not increase with increased energy intake. However, there was a significant increase in the 24-h RMR (which included the increase in energy expenditure after eating) that accounted for an average of l4-15% of the excess ME, an amount comparable to that reported in previous whole body calorimeter studies (16, 24). The issue of whether this energy expenditure is entirely obligatory expenditure associated with digestion and absorption of the extra food and deposition of fat and protein or whether a small component results from a nonessential energy-wasting process cannot be determined with certainty. However, it is noteworthy that the reported energy costs of fat and protein deposition (20) can completely account for the observed increase in metabolic rate. In addition, if any energy-wasting mechanism was operational in these subjects, it would have to be viewed as essentially unsuccessful in regulating energy balance because most of the excess dietary energy was deposited rather than expended. There was a wide variation between subjects in body weight and fat gain during the overfeeding phase of the study, with some subjects gaining little fat and others
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R468
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gaining more energy than the allocated increase in energy intake. Similar findings have been observed previously and suggested as evidence of a genetic component to the susceptibility of individuals to body weight gain when energy intake is increased (11, 14, 15). However, the validity of that interpretation rests on the accuracy with which excess energy intake during overfeeding can be quantified. By necessity, excess energy intake in previous studies has been calculated as the difference between dietary energy intake during overfeeding and the estimated or measured dietary energy requirement for approximate body weight maintenance (11, 15). However, we observed that body weight maintenance over as long a period of time as 10 days does not necessarily indicate body energy maintenance, and significant changes in energy balance can occur in the absence of body weight change. As in previous studies, we based energy intakes during overfeeding on estimations of energy intake for weight maintenance, but in addition it was possible for us to retrospectively calculate more exactly the excess energy intake of each subject during overfeeding, as the difference between ME intake during overfeeding and TEE during weight maintenance. Within our population of subjects, we observed a wide variation in true excess energy intake during overfeeding, which accounted for 90% of the between-subject variance in body energy gain. This finding does not refute the existence of individual differences in the susceptibility to body weight gain during overeating but indicates that the differences may be much smaller than previous studies would indicate (11, 14, 15).
In contrast to the finding of no significant involvement of energy expenditure in the regulation of energy balance, we found evidence consistent with an important role for long-term adaptive variations in voluntary nutrient intakes in these normal young subjects. After overfeeding, there was a significant decrease in voluntary energy intake below that consumed during weight maintenance, resulting in a rapid loss of weight gained during overeating. This decrease in energy intake occurred despite the subjects being specifically requested to refrain from dieting. While we cannot rule out the possibility that they were consciously limiting their food intake, we believe this unlikely since they all reported thinking that they were eating a normal or even increased amount of food relative to phase 1 and, in addition, only one subject felt the extra body fat to be a sufficient problem that he subsequently dieted. It is of interest that the decrease in energy intake after overfeeding was due to reduced fat intake and that no significant decrease in carbohydrate or protein intake was observed. Recent findings in the mouse (8) that body carbohydrate stores are closely regulated by day-to-day adjustments in food intake are consistent with earlier speculations (4) that successful body weight regulation might occur through the separate regulation of body carbohydrate and fat stores, rather than the regulation of a single compartment such as total body energy, body weight, or body fat as traditionally proposed in the “setpoint” hypothesis (9). A logical extension of this proposal is that there are separate hunger and satiety signals
REGULATION
IN
YOUNG
MEN
associated with body carbohydrate and fat balances. In this case, significant depletion of either reserve will result in hunger, whereas supplementation will result in satiety. It is likely that when one reserve is deficient and the other is in excess, the metabolic need for repletion will predominate and result in hunger. Further investigations are required to explore the extent to which the decreased fat intake and unaltered carbohydrate intake of our subjects was influenced by nutrient-specific satiety. However, it is possible to speculate that this alteration in dietary nutrient balance may have enabled the subjects to experience normal satiety while in negative energy balance. This is because the decrease in fat intake can be expected to have resulted in negative body fat balance, while at the same time the high carbohydrate intake may have prevented hunger induced by depletion of body carbohydrate stores. In summary, our results suggest that regulation of energy intake rather than energy expenditure is the primary determinant of energy balance in young adult men of normal body weight overeating a typical diet. Our findings also indicate that voluntary consumption of a low-fat, high-carbohydrate diet after overeating may have played an important role in the ability of the subjects to lose weight while apparently having normal patterns of hunger after overeating. Further studies are needed to examine the effect of dietary composition on energy deposition and energy utilization during overeating and to explore the mechanisms of the control of energy intake, which are as yet inadequately understood. In addition, studies are now needed to investigate further the extent to which manipulation of fat and carbohydrate proportions in the diet can aid natural mechanisms of energy regulation and thereby provide improved methods for the prevention and treatment of excess body fat gain and obesity. This study was made possible by the dedication of the subjects and the staff of the Metabolic Research Unit and Nutrition Evaluation Laboratory at the United States Department of Agriculture (USDA) Human Nutrition Research Center. We also thank I. Rosenberg for support of this investigation and A. Greenberg and J. Flanagan for valuable advice during preparation of the manuscript. The study was funded in part with federal funds from the USDA Agriculture Research Service under contract 53-3KO6-5-10 and in part with National Institutes of Health Grant AC-07388 The contents of this publication do not necessarily reflect the views or policies of the USDA, nor does mention of trade names, commercial products, or organizations imply endorsement by the US Government. Address for reprint requests: S. B. Roberts, USDA Human Nutrition Research Center on Aging at Tufts University, 711 Washington St., Boston, MA 02111. Received
13 February
1990; accepted
in final
form
26 April
1990.
REFERENCES APFELBAUM, M., J. BOSTSARRON, AND D. LACATIS. Effect of caloric restriction and excessive caloric intake on energy expenditure. Am. J. Clin. Nutr. 24: 1405-1409, 1971. ASHWORTH, N., S. CREEDY, N. J. HUNT, S. MAHON, AND P. NEWLAND. Effect of nightly food supplements on food intake in man. Lancet 2: 685-687,1962. BANDINI, L. G., D. A. SCHOELLER, J. EDWARDS, V. R. YOUNG, S. H. OH, AND W. H. DIETZ. Energy expenditure during carbohydrate overfeeding in obese and nonobese adolescents. Am. J. Physiol. 256 (Endocrinol. Metczb. 19): E357-E367, 1989.
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OF BODY
WEIGHT
4. BRAY, G. A. Obesity-a disease of nutrient or energy balance? Nutr. Rev. 45: 33-43, 1987. 5. COMMITTEE ON DIET AND HEALTH, FOOD AND NUTRITION BOARD, COMMISSION ON LIFE, AND NATIONAL RESEARCH COUNCIL. Diet and Health. Implications for Reducing Chronic Disease. Washing-
ton, DC: Natl. Acad., 1989. H. M., AND W. P. T. JAMES. Whole-body calorimeter studies in adult men 1. The effect of fat over-feeding on 24-h energy expenditure. Br. J. Nutr. 52: 49-64, 1984. 7. EDHOLM, 0. G., J. M. ADAM, M. J. R. HEALY, H. S. WOLFF, R. GOLDSMITH, AND T. W. BEST. Food intake and energy expenditure of army recruits. Br. J. Nutr. 24: 1091-1107, 1970. 8. FLATT, J. P. Effect of spontaneous running activity on the carbohydrate, fat and energy balances in ad libitum-fed mice. FASEB
6. DALLOSSO,
J. 2: A1084, 9. GARROW,
1988. S. Energy
J. Balance and Obesity in Man. Amsterdam: Elsevier/North-Holland Amsterdam, 1982. 10. HIRSCH, J., AND R. L. LEIBEL. New light on obesity. N. Engl. J. Med. 318: 509-510,1988. 11. MILLER, D. S., AND
REGULATION
intake and metabolizable
Thermogenesis
P. MUMFORD, AND M. J. STOCK. Gluttony 2. in overeating man. Am. J. Clin. Nutr. 20: 1223-
1229,1967. 13. NELSON,
M., A. E. BLACK, J. A. MORRIS, AND T. J. COLE. Between- and within-subject variation in nutrient intake from infancy to old age: estimating the number of days required to rank dietary intakes with desired precision. Am. J. Clin. Nutr. 50: 155-
167,1989. 14. NORGAN,
15.
16.
17.
18.
N. G., AND J. V. G. A. DURNIN. The effect of 6 weeks of overfeeding on the body weight, body composition, and energy metabolism of young men. Am. J. Clin. Nutr. 33: 978-988, 1980. POEHLMAN, E. T., A. TREMBLAY, J.-P. DESPRES, E. FONTAINE, L. PERUSSE, G. THERIAULT, AND C. BOUCHARD. Genotype-controlled changes in body composition and fat morphology following overfeeding in twins. Am. J. Clin. Nutr. 43: 723-731, 1986. RAVUSSIN, E., Y. SCHUTZ, K. J. ACHESON, M. DUSMET, L. BOURQUIN, AND E. JEQUIER. Short-term, mixed-diet overfeeding in man: no evidence for “luxuskonsumption”. Am. J. Physiol. 249 (Endocrinol. Metab.) 12: E470-E477, 1985. ROBERTS, S. B. A review of the doubly labeled water method for measurement of total energy expenditure, total body water, water intake and metabolizable energy intake in humans and small animals. Can. J. Physiol. Pharmacol. In press. ROBERTS, S. B., W. A. COWARD, K.-H. SCHLINGENSEIPEN, V. NOHRIA, AND A. LUCAS. Comparison of the doubly labeled water (“H2”O) method with indirect calorimetry and a nutrient balance study for simultaneous determination of energy expenditure, water
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energy intake in preterm infants. Am. J.
Clin. Nutr. 44: 315-322, 1986. 19. ROBERTS, S. B., F. J. MORROW, E. DALLAL, C. N. MEREDITH,
W. J. EVANS, D. C. SHEPARD, G. AND V. R. YOUNG. Use of paminobenzoic acid to monitor compliance with prescribed dietary regimens during metabolic balance studies in man. Am. J. Clin.
Nutr. 51: 485-488,199O. 20. ROBERTS, S. B., AND
V. R. YOUNG. Energy costs of fat and protein deposition in the human infant. Am. J. Clin. Nutr. 48: 951-955,
1988. 21. SCHOELLER,
D. A. Measurement of energy expenditure in freeliving humans by using doubly labeled water. J. Nutr. 118: 1278-
1289,1988. 22. SCHOELLER, BAERTSCHI,
D. A., E. RAVUSSIN, Y. SCHUTZ, K. J. ACHESON, P. E. JEQUIER. Energy expenditure by doubly labeled water: validation in humans and proposed calculation. Am.
J. Physiol. R830,1986. 23. SCHUTZ,
AND
250 (Regulatory
Integrative
Comp.
Physiol.
19): R823-
Y. Glossary of energy terms and factors used for calculations of energy metabolism in human studies (provisional version). In: Human Energy Metabolism, Physical Activity and Energy Ex-
P. MUMFORD. Gluttony 1. An experimental study of overeating low- or high-protein diets. Am. J. Clin. Nutr.
20: 1212-1222,1967. 12. MILLER, D. S.,
IN YOUNG
penditure Measurements in Epidemiological Research Based Upon Direct and Indirect Calorimetry, edited by A. J. H. v. Es. Wagineningen, The Netherlands: Euro-Nutr, 1984, p. 169-181. 24. SCHUTZ, Y., K. J. ACHESON, AND E. JEQUIER. Twenty-four-hour
energy expenditure and thermogenesis: response to progressive carbohydrate overfeeding in man. Int. J. Obesity. 9, Suppl. 2: lll114,1985. 25. SIMS, E.
A. H., AND E. DANFORTH. Expenditure and storage of energy in man. J. CZin. Invest. 79: 1019-1025, 1987. 26. SIMS, E. A. H., E. DANFORTH, E. S. HORTON, G. A. BRAY, J. A. GLENNON, AND L. B. SALANS. Endocrine and metabolic effects of experimental obesity in man. Recent Prog. Horm. Res. 29: 457-496, 1973. 27. SIRI,
W. E. Body composition from fluid spaces and density: analysis of methods. In: Techniques for Measuring Body Composition, edited by J. Brozek and A. Henschel. Washington, DC: Natl. Acad. Sci., 1961, p. 223-244. 28. WEBB, P., AND J. F. ANNIS. Adaptation to overeating in lean and overweight men and women. Hum. Nutr. Clin. Nutr. 37C: 117-131, 1983. 29. WEIR,
J. B. DE V. New method for calculating metabolic rate with special reference to protein metabolism. J. Physiol. Land. 109: l-
9, 1949.
WHO/FAO/UNO. Energy and Protein Requirements. Geneva: World Health Org., 1985. (Tech. Rep. Ser. 724) 31. WONG, W. W., L. S. LEE, AND P. D. KLEIN. Deuterium and oxygen18 measurements on microliter samples of urine, plasma, saliva, and human milk. Am. J. Clin. Nutr. 45: 905-913, 1987. 30.
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intakes
B. ROBERTS, VERNON R. YOUNG, PAUL FUSS, MARIA A. FIATARONE, RICHARD, HELEN RASMUSSEN, DAVID WAGNER, LYNDON JOSEPH, HOLEHOUSE, AND WILLIAM J. EVANS
Laboratory of Human Physiology, United States Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston 02111; the Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge 02139; and the Shriners Burns Institute, Boston, MA 02114
ROBERTS, SUSAN B., VERNON R. YOUNG, PAUL Fuss, MARIA A. FIATARONE, BYRON RICHARD, HELEN RASMUSSEN, DAVID WAGNER, LYNDON JOSEPH, ELLEN HOLEHOUSE, AND WILLIAM J. EVANS. Energy expenditure and subsequent nutrient intakes in overfed young men. Am. J. Physiol. 259 (Reg-
ulatory Integrative Comp. Physiol. 28): R461-R469,1990.-We investigated the mechanisms of body weight regulation in young men of normal body weight leading unrestricted lives. Changes in total and resting energy expenditure, body composition, and subsequent voluntary nutrient intakes in response to overeating by 4,230 t 115 (SE) kJ/day (1,011 * 27 kcal/day) for 21 days were measured in seven subjects consuming a typical diet. On average, 85-90% of the excess energy intake was deposited (with 87% of this amount in fat and 13% in protein on average). There was no detectable difference between individuals in susceptibility to energy deposition. The resting metabolic rate, averaged for fasting and fed states, increased during overfeeding (mean k SE, 628 t 197 kJ/day, P C O.Ol), but at least some of this amount was obligatory expenditure associated with nutrient assimilation. No significant increase in energy expenditure for physical activity or thermoregulation resulted from overfeeding. Thus energy expenditure did not substantially adapt to increased energy intake. However, significant decreases in voluntary energy intake (1,991 t 824 kJ/day, P < 0.05) and fat intake (48 * 11 g/day, P < 0.01) followed overeating, indicating that adaptive changes in nutrient intakes can contribute significantly to body weight regulation after overeating. doubly labeled water; fat; obesity
METHODS
from a remarkable balance between energy intake and energy expenditure. It can be calculated that a discrepancy of only 5% between energy intake and expenditure will result in a body fat gain of lo-15 lb during the course of a year, leading rapidly to obesity if continued. This regulation of body weight and energy balance must involve adaptive fluctuations in energy expenditure or energy intake over periods of several days or longer, because diet digestibility is relatively unaffected by energy intake (6, 14, 28) and day-to-day variations in energy intake and expenditure are substantial but independent (7). However, the question of whether energy intake or energy expenditure is the primary factor. maintaining energy balance remains a subject of major controversy (10). SUCCESSFUL
BODY
WEIGHT
Agreement on this fundamental issue would be of importance in helping to define the causes of obesity and in designing improved methods for prevention and treatment of excess body fat gain. We report here an overfeeding study in healthy young adult men, which was designed to investigate the normal mechanisms of body weight and energy regulation. The study differed from previous overfeeding studies (l-3, 6, 11,14-16,26,28) by including three important elements. First, we utilized recent methodological advances (17,19, 21) to measure the changes in all the principal components of energy balance resulting from overeating, including total energy expenditure (TEE), body energy content, and the subsequent voluntary energy intake. Second, we conducted the investigation in subjects pursuing normal daily activities. This aspect of the study was important because we anticipated that confining the subjects to a metabolic ward during the investigation might well suppress natural mechanisms of energy regulation, e.g., those associated with thermoregulation and physical activity. Third, we focused on assessing the causes of apparent individual variability in the susceptibility to weight gain during overeating, specifically to see whether the reported differences (11, 14, 15) are due to genuine physiological differences between individuals or to methodological error.
regulation
0363-6119/90
results
$1.50
Copyright
Subjects. The subjects were seven male students or laboratory staff of normal body weight and fat content (Table 1). None smoked or had any recent illness or history of an endocrinopathy. At the time of the study all were healthy as judged by a normal physical examination, routine blood and urine examinations, and electrocardiogram. The study was conducted in the Metabolic Research Unit (MRU) at the United States Department of Agriculture Human Nutrition Research Center with approval from the New England Medical Center Human Investigation Review Committee. Written informed consent was obtained from the subjects. Protocol. The subjects were able and encouraged to pursue a normal lifestyle during the study, and all continued their usual occupations. They were required to sleep at the MRU for the nights before measurements of
0 1990 the American
Physiological
Society
R461
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R462
MECHANISMS
BODY
WEIGHT
REGULATION
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YOUNG
MEN
body weight (kl g) and standard anthropometric measures (waist, hip, midupper arm, thigh and calf circumferParameters Values ences, and triceps, biceps, subscapular, and suprailiac skinfold thicknesses) were recorded in the morning be7 23.7kO.4 be, Yr fore the subjects had consumed breakfast and after they Starting weight, kg 76.3t4.7 had voided, on study days 0, 10, 20, 29, 41, and then at Height, cm 178.322.4 approximately weekly intervals until day 77. Body mass index, kg/m2 24.0t1.3 Diets. All food and caloric beverages consumed by the Body fat before overfeeding, %wt 13.7t1.2 subjects during phases 1 and 2 were provided by the Values are means t SE; n, no. of men. MRU. The MRU also provided food during the first 10 days of phase 3, but subjects were also permitted to [j PHASE 2 PHASE 3 consume additional caloric beverages provided that 0 10 20 30 77 40 I I I I // Study day 1 I amounts and types were reported. Meals consisted of normal food items divided between three meals plus an weight weight maintenance ad Energy intake t?aintenan%? plus 1000 kcal/day )+libitum ) evening snack each day. There were three different daily (provided) Activity monitors menus provided on a rotating basis to the subjects. The 4 b menus were tailored to individual preferences for foods. Total energy expenditure d-b+ b Coffee and tea were included in the menus in fixed daily amounts if they formed part of the subjects’ normal diet. Resting metabolic rate/ ** * * thermic effect of meal At least 1 meal/day was consumed in the MRU, and Urine collection 4 b other meals were consumed at the MRU or at the subjects’ residence. Body density * * The nutrient content of the diets provided during Anthropometry * * * * * * * * * phase 1 were designed to mimic a typical high-protein, FIG. 1. Study design. high-fat American diet (5). Approximately 1.5 g/kg body wt of mixed protein was provided, and an average of 55% resting energy expenditure (described below) but otherof the nonprotein energy was derived from carbohydrate wise could reside at home. Physical activity was moniand 45% from fat. Sodium intakes were dictated by the tored but was not restricted, with the exception that preference of the subjects. During the first 7 days of subjects were requested to not engage in an unusual phase 1, the energy intake required for weight mainteamount of activity for the specific purpose of preventing nance was determined. The initial energy intake provided weight gain during overfeeding. was 1.5 times estimated basal metabolic rate (30). MeasThe study was divided into three phases as shown in urements of body weight were taken, and appropriate Fig. 1. During phase 1, which lasted for 10 days, the adjustments to energy intake were made to maintain subjects’ energy intakes for body weight maintenance weight within 500 g of the value on day 0. Subjects were were determined. Phase 2 lasted for 21 days and started allowed to leave partial portions of food, which were within 1 mo of the end of phase 1. Subjects were requested saved for composition analysis (see below). During the to maintain body weight between these phases. During last 3 days of phase 1, subjects were given the calculated phase 2, subjects consumed -4,200 kJ/day (1,000 kcal/ day) more than during phase 1. Phase 3 lasted for 46 average of energy consumed during the first 7 days, with days and was consecutive with phase 2. Voluntary food the dietary composition specified above, and were reintake was measured during the first 10 days, and After quired to completely consume all portions of food and rinse and scrape food containers. Exactly 25% of daily this time subjects were monitored to assess long-term fat, protein, and carbohydrate were provided in the trends in body weight and anthropometric variables. TEE was measured throughout phases 1 and 2 using the breakfast meal, and the remaining nutrients were partidoubly labeled water technique (17, 21). Complete 24-h tioned between the other two meals and snack. Duringphase 2, subjects were provided with the menus urine collections were made during this time. All urine was collected in dark containers containing 15 ml 4.2 N fixed on the last 3 days of phase 1 and in addition were HCl with the exception of daily specimens required for given a daily low-protein, high-energy supplement conthe doubly labeled water analyses, which were stored in taining an average of 2 g protein, 55 g fat, and 127 g [to provide ~4,200 kJ/day (1,000 kcal/ nonacidified dark bottles. Body density was measured at carbohydrate day)]. In the first three subjects, the additional nutrients the beginning and end of phase 2 using the underwater weighing technique with published correction factors for were provided in the form of fruit juices, margarine, corn oil, and cookies. Subsequent subjects were given a pallung residual volume (27). Physical activity was moniatable beverage consisting of sherbet, corn oil, nondairy tored qualitatively in phases 1 and 2 and during the first 10 days of phase 3 using motion recorders worn around creamer, sucrose, and flavoring (Kool-aid). The supplethe waist (Caltrac, Hemokinetics, Madison, WI) and the ment was divided between meals, with exactly 25% given with breakfast. Subjects were required to completely wrist of the nondominant hand (Timex model 101motion consume all portions. The p-amino benzoic acid (PABA) recorder, Kaulins and Willis, Middlebury, CT). During this time, subjects kept a record of the duration and types test (19) was used to verify food consumption in five of strenuous physical activities performed. Body weight subjects during phase 2 between study days 13 and 30 (tlO0 g) was measured daily during phase 1. In addition, (one of the other subjects was allergic to PABA, and TABLE
1. Details
OF
of subjects
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MECHANISMS
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WEIGHT
technical problems prevented correct use of the method in another subject). This method involves incorporation of 240 mg/day of the potassium salt of PABA into the diet and quantitative measurement of its recovery in complete 24-h urine collections. A mean recovery rate of 92.6% in each subject (98.6 t 2.2%), indicating both complete consumption of provided foods and complete urine collections. During the first 10 days of phase 3, subjects were requested to consume as much or as little food and drink as they required to feel normally satiated and were given specific instructions to not deliberately change their diet to lose weight gained during phase 2. They were also asked not to weigh themselves. As in phases 1 and 2, there was a 3-day menu plan, but the number of items of food included in the menu was increased threefold, and subjects were allowed to specify as many or as few items and number of portions as they wished. They were not required to completely consume the portions, and leftovers were saved for composition analysis. After these 10 days, subjects provided their own food and could lose any weight gained during overfeeding if they so wished. Freeze-dried, homogenized portions of identical replicas of menus given to the subjects in phases 1 and 2 and the leftover foods were analyzed for gross energy by adiabatic bomb calorimetry. The measured gross energy contents of the 3-day menu cycles were 2.9 t 0.6% (P < 0.01) lower than values calculated using information on the fat, protein, and carbohydrate contents from food tables (GRAND database, Grand Forks Human Nutrition Research Center, Grand Forks, ND), assuming energy contents of 39.6, 23.6, and 16.6 kJ/g, respectively (23). Metabolizable energy (ME) intakes during phases 1 and 2 were calculated as (measured gross energy x 0.96) - (urinary nitrogen X 7.9) (23). Gross energy intakes during phase 3 were calculated from the weights of foods provided and gross energy contents estimated from GRAND food composition data (corrected for the difference between gross energy calculated from food tables and measured by bomb calorimetry in individual subjects for the phase 1 menus) and from the gross energy contents of the leftover foods, assuming as in phase 1 that the efficiency of absorption was 96% and that body nitrogen balance was at equilibrium. Measurements of resting energy expenditure. The volume of O2 and COZ (VO, and VCO~, respectively) were measured under thermoneutral temperature conditions in the fasting and fed states on four occasions during the study, twice during the last 3 days of phase 1 and once each during the first and second halves of phase 2. The subjects were instructed to relax and avoid hyperventilation, fidgeting, and sleeping during measurements, which were made while the subjects breathed through a low-resistance mouthpiece. Expired air was collected in a calibrated Tissot spirometer and analyzed for O2 and
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COe contents at regular intervals (02 analyzer model N22M, Applied Electrochemistry, Sunnyvale, CA, and CO2 analyzer model LB-2, Beckman Instruments, Anaheim, CA). The measurements during the fasted state were made for 20 min before breakfast, lo-13 h after the last meal. The purpose of having this relatively long measurement duration was to reduce the possibility that persistent hyperventilation could influence the results obtained. After the fasting measurements, subjects consumed their usual study breakfast containing exactly 25% of the daily intakes of protein, fat, and carbohydrate. The measurements of Vos and VCO~ were resumed 30 min after the start of breakfast and continued during 20 min of every 30 min until 4 h after the start of breakfast. Subjects remained resting during the lo-min breaks. During one of the calorimetry sessions in each phase, small serial blood samples were drawn through an indwelling butterfly catheter inserted into an antecubital vein at least 15 min before the start of calorimetry measurements. Values for resting metabolic rate (RMR) were calculated from each determination of Vos and VCO~ using Weir’s equation (29). Two estimates of the 24-h RMR were made for each study period. Estimate 1 was calculated from the mean of all 20-min determinations for the study day. Estimate 2 was determined assuming that the 24-h day consisted of four 4-h energy expenditure cycles as measured between breakfast and the end of the measurement plus an 8-h period during which energy expenditure decreased linearly from the final determination of energy expenditure in the session to the initial fasting value. Measurements of TEE. Three sequential lo-day doubly labeled water studies (17, 21) were conducted in each subject to measure TEE throughout phases 1 and 2. The protocol used was designed to minimize possible errors arising from small changes in isotopic backgrounds and de novo lipogenesis. Concerning minimizing the effects of any isotopic background changes, a relatively short study period (10 days) was used and, in addition, the 2H20 intake was reduced in the second and third isotope doses (see below) to maintain the ratio of 2H-to-180 abundances in body water approximately constant. Two control subjects (data not shown), who completed phases 1 and 2 but received no 2H2180, were subsequently found to have only very small changes in isotopic backgrounds. The potential for de novo lipogenesis was minimized by ensuring that dietary fat intake was greater than the expected rate of fat deposition during overfeeding by a factor of two on average. The absence of net lipogenesis was subsequently confirmed with the observation of a relatively small increase in average resting respiratory quotient (RQ) during overfeeding (described in RESULTS).
Mixed ‘H2180 doses containing 0.15 g/kg H2180 and 1) or 0.055 (measurements 2 and 3) g/kg 2H20 were given orally midmorning on study days 0, 10, and 20 and were followed with two 50-ml rinses of tap water. Subjects consumed breakfast before the doses were given and were fasted for 3 h after the administration of isotope. Urine specimens were collected before 0.06 (measurement
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R464
MECHANISMS
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administration of each isotope dose, at 3,4, and 5 h after each isotope administration, and thereafter at approximate daily intervals for 10 days after each dose until study day 30. Abundances of 2H and 180 in dilutions of the isotope doses and in at least 19 urine specimens/ subject (the baseline, 5-h, l-day, 2day, 8-day, g-day, and lo-day samples for each isotope study) were analyzed using isotope-ratio mass spectrometry (SIRA-10, VG Isogas, Middlewich, Cheshire, England). The samples collected at 3 and 4 h after dosing were not analyzed routinely, because preliminary analyses indicated that the isotopes were not fully equilibrated in urine at these times. Samples at the beginning and end of each isotope curve, rather than samples spaced evenly throughout each study period, were used because this approach should increase the precision of intercepts and rate constants for isotope disappearance. Samples were prepared for 2H/1H analysis using the zinc reduction technique and for 1802/1602 analysis in CO2 using the H20-CO2 equilibrator system (31). On average, quadruplicate analyses of each sample were performed. The coefficients of variation for day-to-day repeated measures of 2H and 180 abundances were 0.2%0 and 0.8%0, respectively. The doubly labeled water data were processed using DLW software (G. E. Dallal and S. B. Roberts, unpublished observations). Residual plots were obtained for two single exponential models of isotope disappearance over time (a nonlinear least-squares model and a linear least-squares model using the natural logs of the isotope data). Residuals generated in the nonlinear least-squares model showed no systematic variability over time, and thus the intercepts and rate constants from this model were used in subsequent calculations. Isotope-dilution spaces were then calculated from the intercept data (18). The ratio of the measured 2H20-to-H2180 dilution spaces should have been constant over time within subjects (because body composition changes were small), but none of the variance was accounted for by subject or study period (values ranged between 1.020 and 1.011). The intraclass correlation coefficient for the measurements was -0.1191. The variability in the ratio of the dilution spaces occurred despite the fact that potential sources of error had been minimized by mixing the isotopes before dosing and analyzing the isotope doses for isotopic abundances after appropriate dilution to give isotopic abundances similar to those in the urine specimens collected at 5 h after dosing. Because of the variation, we calculated carbon dioxide production rates (Rco,) using the equation of Schoeller et al. (22), which involves assumption of a fixed ratio between the dilution spaces.The value for total body water (N) used in the equation was determined as described by Schoeller et al. (22) (i.e., H2180 space/l.Ol), with a small correction for changes in body water during the measurement period made by assuming proportional changes in body weight and water (18). The value for N could also have been calculated from the average of both the dilution spaces, but this different approach would have had only a very small effect on the calculated Rco,. Values for TEE were calculated from R co, using Weir’s equation (29), using estimates for a modified food quotient (MFQ) determined
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from the known nutrient intakes (G. E. Dallal and S. B. Roberts, unpublished observations). In this calculation, it is necessary to correct for body fat mobilization or deposition during the measurement. However, provided that accurate data on dietary nutrient intakes and nitrogen balance are available, as in this study, it is not necessary to estimate body fat balance because this can be calculated by solving for energy expenditure, RQ, and ME intake simultaneously using the equations (G. E. Dallal and S. B. Roberts, unpublished observations) TEE = Rco, (88.278/RQ 1355.65F 973.OAL MFQ = 1918.05F 1461.OAL
+ + -
+ 24.774)
718.52P + 731.08C + (1427.OFB + 4881.25NB) 888.7231 + 731.08C + (2019.OFB + 6037.5NB)
(0
(2)
where F, P, C, AL are dietary intakes of fat, protein, carbohydrate, and alcohol in g/day, and FB and NB are body fat and nitrogen balances in g/day ME intake = 8.987F + 5.198P + 3.8906C + 7.1AL - 7.9U
(3)
where U is urine nitrogen excretion (g/day). Equation 2 assumes no significant change in body carbohydrate stores and no de novo lipogenesis during the study. It can be calculated that failure to correct for a net gain or loss of as much as 500 g of glycogen/lO-day study would result in an error in the calculated energy expenditure of only -1%. Body composition. Body fat contents at the beginning and end of phase 2 were calculated from the measurements of body density using the equations of Siri (27). Nitrogen balance was determined during phases 1 and 2 to provide information on the change in body protein content resulting from overfeeding. For this measurement, the nitrogen contents of pooled 24-h urine collections obtained during phases 1 and 2 and of the food aliquots were determined using a modification of the Kjeldahl technique involving microwave digestion of the samples (Kjelfast, CEM, Matthews, NC). Digestible nitrogen intakes were calculated from gross intakes assuming an average digestibility of 92% (23). Nitrogen balance was assumed to be zero during phase 1, and miscellaneous nitrogen losses (in sweat, hair, nails, etc.) were calculated for each subject as the difference between digestible nitrogen intake and nitrogen output in urine. These factors were then applied to phase 2 data to calculate nitrogen balance as the difference between digestible nitrogen intake and the sum of nitrogen losses (urine plus miscellaneous). Protein balance was calculated as nitrogen balance X 6.25. Energy balance during phase 2 was calculated as the sum of fat and protein depositions, assuming energy contents of 39.6 and 23.6 kcal/g, respectively (23). Energy balances during both phases 1 and 2 were also determined as the difference between the measured rates of ME intake and TEE. Statistical analyses. Values are expressed as means t SE. Differences between measurements and phases were analyzed using analysis of variance and Student’s paired t test. Regression analysis was used to assess associations
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MECHANISMS
OF
BODY
WEIGHT
among initial body fat content, excess energy intake, and the energy deposition during phase 2 and the voluntary energy intake duringphase 3. Differences between groups were considered significant at P < 0.05. The calculations were performed using CLINFO software (BBN Software Products, Cambridge, MA). RESULTS
Table 2 shows the values for measured ME intakes, and gross nutrient intakes calculated from food weights and published compositions, during weight maintenance in phase 1, overfeeding in phase 2, and during the first 10 days of phase 3 when food intake was ad libitum. ME intake was significantly higher in phase 2 than in phase 1 and significantly lower in phase 3 than in phase 1. The reduced ME intake in phase 3 was due to a significantly lower intake of fat. There was no significant trend in energy intake over time during phase 3. The body weights of the subjects are shown in Fig. 2. There was no significant change in body weight during phase 1 and no significant change between phases 1 and 2 (the change was 242 t 115 g). However, there was a significant increase of 118 g/day throughout phase 2. There was no significant difference between weight gain during the first and second halves of phase 2, and means were 142 t 36 and 92 t 26 g/day, respectively (P = 0.3347). There was substantial variation between sub2. Energy and nutrient intakes during weight maintenance (phase l), overfeeding (phase 2), and 10 days after overfeeding (phase 3)
TABLE
Phase 1
Metabolizable
energy,
H/day Protein,
Fat,
g/day
g/day
Carbohydrate, Ethanol, g/day
g/day
13,949+841
Phase 2
Phase 3
18,113+874$
11,958*523*
114t6 153t10
116k7 207+10$
12Ok9 105+6j-
409225 Ok0
542&27$
376t25
Values are means t SE. Significantly 1: * P c 0.05, t P < 0.01, $ P c 0.001.
l&l different
1426 from
values
for phase
2.5 9 O
2.0
5 8
1.5
E
1.0
8 5 3 !F (3 iii 3
050.0 -0.5
1
Phase1
STUDY DAY 1 Phase2 1
Lowest weight
Phase3
FIG. 2. Body weight change of subjects from study day 0. Values are means t SE of data corrected for weight change between phases 1 and Z(242 t 115g). ** P < 0.01 compared with values at start of overfeeding on study day 10.
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jects in weight gain over the entire overfeeding period, with values ranging from 13 to 142 g/day. The differences in weight gain between individuals were linearly related to body fat changes (P < 0.01, r = 0.9190), which averaged +l.53 t 0.45% of body weight between the 1st and 20th days ofphase 2 (P = 0.015). There was no significant relationship between weight change during overfeeding and changes in the skinfold thicknesses (triceps, biceps, subscapular, suprailiac) or waist, hip, or midupper arm circumferences. There was also no significant change in these variables between the beginning and end of overfeeding, although the change for the sum of all four skinfold thicknesses approached significance (4.8 t 2.2 mm, P = 0.0789). Differences between subjects in weight and fat gain paralleled subjective responses to overfeeding, with subjects exhibiting relatively high weight and fat gains finding it difficult to consume the excess energy, and subjects exhibiting low gains finding it relatively easy. Body weights fell on average during the first 10 days of phase 3 but were still significantly elevated compared with before overfeeding. The mean weight at the end of the study was similar to that observed on the 11th morning of phase 3 (study day 41) but was not significantly different from the weight before overfeeding because of increased variability in weight between subjects. The lowest weights measured during phase 3 for each subject differed from weights at the start of overfeeding by only -0.005 t 0.458 kg. Only one subject reported conscious dieting after completion of the food intake component of the study, to lose weight gained during phase 2.
Table 3 summarizes all the data on energy expenditure and physical activity throughout phase 1 and during phase 2 days l-10 and 1 l-20. There was no difference between the first and second halves of phase 2 for any parameter. There was no significant difference between phases 1 and 2 in the fasting RMR expressed either per subject or per kilogram of fat-free mass. However, the estimated 24-h RMR, calculated as described in METHODS from the mean (estimate 1) or weighted mean (estimate 2) of data from the fasted and fed states was significantly higher in phase 2 than in phase 1, by 594628 kJ/day on average. RQ averaged for fasting and fed states was slightly higher in phase 2 than in phase 1, but the difference was not quite significant (P = 0.0845). The values for RQ are slightly lower than the RQ values estimated from dietary intake and body fat balance, but the difference between phases 1 and 2 were very similar in the two methods, indicating that lipogenesis was minimal. The average Hz180 dilution spaces determined during the doubly labeled water measurements were significantly greater in phase 2 than in phase 1. However, there was no significant difference in TEE between phases, and values in phase 2 differed from those in phase 1 by only +423 t 795 kJ/day (+lOl t 190 kcal/day). There was considerable individual variability in TEE differences between phases 1 and 2. Thus the 95% confidence limits for the difference were relatively large, at -1,528, 2,371 kJ/day. The lack of a significant increase in energy
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R466 TABLE
MECHANISMS
3. Energy expenditure
OF BODY
WEIGHT
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data Phase 1 (Study days O-9)
Calorimetry
Fasting RMR, kJ/day Fasting RMR, kJ . kg fat-free mass-’ day-’
(Study
RQ
24-h RMR 1, kJ/day* 24-h RMR 2, kJ/day* Doubly
Mean Hz”0 space, moles Rate of 2H disappearance, day-l Rate of la0 disappearance, day-l Modified food quotient TEE, kJ/day TEE/RMR
labeled
7,774+381 118t3 0.839IkO.017 9,439+377 9,276+372
water
Phase 2 days 20-29)
7,778+339 11824 0.854~0.011 9,569+389 9,393+356
data
2,479+113$ 0.09451+0.00860~ 0.12021kO.OO877"f 0.854+0.014t 13,883+774 1.851t0.010 Activity
(Study
data
7,544*381 115t2 0.821t0.013 8,878+477$ 8,740+456$
l
Phase 2 days 10-19)
2,565f133 0.10262kO.00931 0.12973t0.00961 0.880t0.007 14,665+678 1.890t0.035
2,580+127 0.10157*0.010 0.12750t0.011 0.891t0.007 13,945+745 1.793t0.059
data
Caltrac trunk monitor, kcal/day 2,547+223 2,618+276 2,717+283 Wrist motion sensor, units/day 1,947+221 2,009+220 2,417+388 Reported strenuous activity, min/day 31~10 24dO 26212 Values are means t SE. * See text for calculations. RMR, resting metabolic rate; RQ, respiration quotient; TEE, total energy expenditure. Significantly different from values for mean phase 2 values (days I-20 combined): t P < 0.05, $ P < 0.01. There was no significant difference between values for phase 2 (days l-l 0) and phase 2 (days 1 l-20).
expenditure for physical activity or thermoregulation during overfeeding is indicated by the similarity between the changes in the 24-h RMR and TEE. Qualitative estimates of physical activity provided by the motion sensors worn around the waist and wrist and the reported amount of time devoted to strenuous activity did not differ significantly between phases 1 and 2. Table 4 summarizes the energy balance of the subjects during overfeeding in phase 2. The mean increment in ME intake above that required for weight maintenance in phase 1 was 4,046 kJ/day. This value was very similar to the excess energy intake calculated as the difference between ME intake in phase 2 and TEE in phase 1 (4,230 kJ/day), indicating that on average the subjects were in energy balance during weight maintenance in phase 1. A mean increase in the maintenance energy expenditure of 14-E% resulted from overfeeding. The mean energy 4. Summary of changes in energy balance and body composition during overfeeding
TABLE
Parameters
Values
Energy,
kJ/day
Metabolizable energy intake Calculated energy excess (energy intake during phase 2 - total energy expenditure in phase 1) 24-h resting metabolic rate 1* 24-h resting metabolic rate 2” Total energy expenditure Energy deposition (fat + protein deposition) Energy deposition (energy intake expenditure) Body
composition,
+4,046+113 +4,230-r-1,150 +628*197 +594,t176 +423*795 +3,594*828 +3,807+448 g/day
Weight +118t21 Fat (by underwater weighing) +79t20 +20*3 Protein (by nitrogen balance) Values are means -+ SE. * See text for calculations.
deposition resulting from overfeeding, calculated from either measured fat and protein depositions or the difference between ME intake and TEE during overfeeding, accounted for 8590% of the excess energy intake. There was no significant difference between the two values for energy deposition, and means differed by only 213 t 498 kJ/day. Fat and protein depositions constituted 67 and 17% respectively, of body weight gain during overfeedix, and the average energy content of weight gain was 30 kJ/g (7.3 kcal/g). Body water gain during the first half of overfeeding, indicated by the HZ”0-dilution space change, averaged 27 g/day. This value is very slightly higher than the difference between average weight gain and fat plus protein depositions during phase 2. This slight discrepancy is not unexpected, because all the methods used were independent. Moreover, weight gain in the first half of phase 2 was greater than during the second half, although not significantly so. Thus, it seems likely that the higher weight gain during the first half of phase 2 was due to slightly greater water gain. To assess the cause of variability in body weight and energy gain, individual values for excess energy intake during overfeeding (calculated as the diffe rence between ME intake during phase 2 and TEE in phase 1) were regressed against body energy gain calculated from measured fat and protein depositions as shown in Fig. 3. There was a significant linear relationship between these parameters, and 90% of the variance in energy deposition between subjects was accounted for by differences in excess energy intake. The slope of the line was 0.665 t 0.094 kJ/kJ. The intercept of the line, 778 t 477 kJ/day, was not significantly different from zero. Significant relationships were also observed between the calculated excess energy intake and energy deposition determined as the difference between ME intake in phase 2 and TEE in phase 2 (P < 0.05) and body weight gain (P < 0.05). There was no indication of any effect of weight change 9
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MECHANISMS
0 0
OF
BODY
I
I
I
I
1
2
4
6
8
10
ENERGY
INTAKE
EXCESS
WEIGHT
(MJ/day)
FIG. 3. Relationship between excess metabolizable energy intake during overfeeding (calculated as difference between ME intake during overfeeding in phase 2 and total energy expenditure during prior weight maintenance during phase 1) and energy deposition in fat and protein in 7 subjects.
during phase 1, body fat content before overfeeding, or the reported amount of strenuous activity on the relationship between excess energy intake and energy deposition during phase 2. There was no relationship between weight change during phase 1 and energy balance during phase 1 (calculated as the difference between ME intake and energy expenditure). Individual variability in voluntary energy intake after overfeeding was also examined to explore the cause of the significant decreases in ME and fat intakes. No significant relationship was observed between energy deposition during overfeeding and the subsequent decrease in energy intake below normal energy needs (calculated as the difference between ME intake in phase 3 and TEE in phase 1 ), initial body fat content, or reported strenuous activity. DISCUSSION
Overfeeding has been a classical technique for investigating the mechanisms of body weight regulation in humans. Two distinctly different results have emerged from previous studies. In investigations focusing on measurements of energy expenditure, relatively small increases in expenditure were generally observed in response to increased energy intake (3,6,14, 16), implying that most of the excess energy must have been deposited, and suggesting a limited role for energy-wasting mechanisms in the regulation of body weight. However, in the studies focusing on measurements of body composition, only small increases in body weight or energy content in relation to excess energy intake were frequently observed (2, 11, 15, 2% implying that unmeasured adaptive responses to overfeeding involving an increase in energy expenditure must have occurred to prevent the expected body weight and fat gain in some or all of the individuals studied. Attempts to reconcile these diametrically opposed conclusions have usually concentrated on the different study durations and diet compositions (9, 25). However, the studies focusing on measurements of energy expenditure were usually performed under restric-
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tive experimental conditions, whereas the studies emphasizing measurements of body composition were performed under conditions more closely resembling normal life. Thus noncompliance with the free-living protocols, or a suppression of energy-wasting mechanisms in the more restrictive protocols (for example, associated with thermoregulation or components of physical activity such as duration of activities, the energy costs of activities, or the extent of nonessential movements such as fidgeting), may well have contributed to the different results obtained. Because of these concerns and because recent advances in stable isotope methodology for measuring energy expenditure (17, 21) have permitted much more accurate and precise measurements of TEE in free-living subjects than were possible previously, we initiated a new overfeeding study in healthy young men leading normal lives and consuming a typical American diet. The measured increment in energy intake, averaging 4,046 kJ/day (967 kcal/day), was well within the reported normal day-today variations in energy intake (13). Thus the study mimicked and extended a change in energy intake within the normal pattern. Confidence in the food intake data was obtained by use of our new test of dietary compliance involving incorporation of PABA into the provided foods (19) and by the observation that the mean measured increase in energy intake was very similar to the mean sum of energy deposition plus energy expenditure changes during overfeeding. In the group of subjects taken as a whole, we observed no evidence of a significant energy-wasting mechanism. Two independent measures of body energy gain indicated that, on average, 85-90s of the excess energy intake provided during overfeeding was deposited. There was no significant increase in the energy expenditure for physical activity or thermoregulation, despite the fact that the subjects were able to lead essentially normal lives. In addition, as reported by most other investigators (12, 16, 25), the fasting RMR corrected for changes in lean tissue did not increase with increased energy intake. However, there was a significant increase in the 24-h RMR (which included the increase in energy expenditure after eating) that accounted for an average of l4-15% of the excess ME, an amount comparable to that reported in previous whole body calorimeter studies (16, 24). The issue of whether this energy expenditure is entirely obligatory expenditure associated with digestion and absorption of the extra food and deposition of fat and protein or whether a small component results from a nonessential energy-wasting process cannot be determined with certainty. However, it is noteworthy that the reported energy costs of fat and protein deposition (20) can completely account for the observed increase in metabolic rate. In addition, if any energy-wasting mechanism was operational in these subjects, it would have to be viewed as essentially unsuccessful in regulating energy balance because most of the excess dietary energy was deposited rather than expended. There was a wide variation between subjects in body weight and fat gain during the overfeeding phase of the study, with some subjects gaining little fat and others
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R468
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gaining more energy than the allocated increase in energy intake. Similar findings have been observed previously and suggested as evidence of a genetic component to the susceptibility of individuals to body weight gain when energy intake is increased (11, 14, 15). However, the validity of that interpretation rests on the accuracy with which excess energy intake during overfeeding can be quantified. By necessity, excess energy intake in previous studies has been calculated as the difference between dietary energy intake during overfeeding and the estimated or measured dietary energy requirement for approximate body weight maintenance (11, 15). However, we observed that body weight maintenance over as long a period of time as 10 days does not necessarily indicate body energy maintenance, and significant changes in energy balance can occur in the absence of body weight change. As in previous studies, we based energy intakes during overfeeding on estimations of energy intake for weight maintenance, but in addition it was possible for us to retrospectively calculate more exactly the excess energy intake of each subject during overfeeding, as the difference between ME intake during overfeeding and TEE during weight maintenance. Within our population of subjects, we observed a wide variation in true excess energy intake during overfeeding, which accounted for 90% of the between-subject variance in body energy gain. This finding does not refute the existence of individual differences in the susceptibility to body weight gain during overeating but indicates that the differences may be much smaller than previous studies would indicate (11, 14, 15).
In contrast to the finding of no significant involvement of energy expenditure in the regulation of energy balance, we found evidence consistent with an important role for long-term adaptive variations in voluntary nutrient intakes in these normal young subjects. After overfeeding, there was a significant decrease in voluntary energy intake below that consumed during weight maintenance, resulting in a rapid loss of weight gained during overeating. This decrease in energy intake occurred despite the subjects being specifically requested to refrain from dieting. While we cannot rule out the possibility that they were consciously limiting their food intake, we believe this unlikely since they all reported thinking that they were eating a normal or even increased amount of food relative to phase 1 and, in addition, only one subject felt the extra body fat to be a sufficient problem that he subsequently dieted. It is of interest that the decrease in energy intake after overfeeding was due to reduced fat intake and that no significant decrease in carbohydrate or protein intake was observed. Recent findings in the mouse (8) that body carbohydrate stores are closely regulated by day-to-day adjustments in food intake are consistent with earlier speculations (4) that successful body weight regulation might occur through the separate regulation of body carbohydrate and fat stores, rather than the regulation of a single compartment such as total body energy, body weight, or body fat as traditionally proposed in the “setpoint” hypothesis (9). A logical extension of this proposal is that there are separate hunger and satiety signals
REGULATION
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associated with body carbohydrate and fat balances. In this case, significant depletion of either reserve will result in hunger, whereas supplementation will result in satiety. It is likely that when one reserve is deficient and the other is in excess, the metabolic need for repletion will predominate and result in hunger. Further investigations are required to explore the extent to which the decreased fat intake and unaltered carbohydrate intake of our subjects was influenced by nutrient-specific satiety. However, it is possible to speculate that this alteration in dietary nutrient balance may have enabled the subjects to experience normal satiety while in negative energy balance. This is because the decrease in fat intake can be expected to have resulted in negative body fat balance, while at the same time the high carbohydrate intake may have prevented hunger induced by depletion of body carbohydrate stores. In summary, our results suggest that regulation of energy intake rather than energy expenditure is the primary determinant of energy balance in young adult men of normal body weight overeating a typical diet. Our findings also indicate that voluntary consumption of a low-fat, high-carbohydrate diet after overeating may have played an important role in the ability of the subjects to lose weight while apparently having normal patterns of hunger after overeating. Further studies are needed to examine the effect of dietary composition on energy deposition and energy utilization during overeating and to explore the mechanisms of the control of energy intake, which are as yet inadequately understood. In addition, studies are now needed to investigate further the extent to which manipulation of fat and carbohydrate proportions in the diet can aid natural mechanisms of energy regulation and thereby provide improved methods for the prevention and treatment of excess body fat gain and obesity. This study was made possible by the dedication of the subjects and the staff of the Metabolic Research Unit and Nutrition Evaluation Laboratory at the United States Department of Agriculture (USDA) Human Nutrition Research Center. We also thank I. Rosenberg for support of this investigation and A. Greenberg and J. Flanagan for valuable advice during preparation of the manuscript. The study was funded in part with federal funds from the USDA Agriculture Research Service under contract 53-3KO6-5-10 and in part with National Institutes of Health Grant AC-07388 The contents of this publication do not necessarily reflect the views or policies of the USDA, nor does mention of trade names, commercial products, or organizations imply endorsement by the US Government. Address for reprint requests: S. B. Roberts, USDA Human Nutrition Research Center on Aging at Tufts University, 711 Washington St., Boston, MA 02111. Received
13 February
1990; accepted
in final
form
26 April
1990.
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