Endurance training decreases plasma glucose turnover and oxidation during mod rate-intensity exercise in men ANDREW R. COGGAN, WENDY M. KOHRT, ROBERT J. SPINA, DENNIS M. BIER, AND JOHN 0. HOLLOSZY Section of Applied Physiology and Metabolism Division, Department of Medicine, and Irene Walter Johnson Institute of Rehabilitation, Washington University School of Medicine, St. Louis, Missouri 63110 COGGAN, ANDREW R., WENDY M. KOHRT, ROBERT J. SPINA, DENNIS M. BIER, AND JOHN 0. HOLLOSZY. Endurance
65-70% maximal
training decreasesplasma glucose turnover and oxidation during moderate-intensity exercise in men. J. Appl. Physiol. 68(3): 990996, 1990.-To assess the effects of endurance training on plasma glucose kinetics during moderate-intensity exercise in men, seven men were studied before and after 12 wk of strenuous exercise training (3 days/wk running, 3 days/wk cycling).
plasma glucose disappearance between trained and untrained subjects during high-intensity exercise. Recently,
After
priming
of the glucose
and bicarbonate
pools,
[U-‘“Cl
02 uptake (v02 max) (14, 34). Kjaer et
al. (25) also found
no difference
in tracer-determined
however, Jansson and Kaijser (20) have reported that leg
glucose uptake, as determined from the arterial-femoral venous difference for glucose and estimated leg blood flow, is lower in well-trained
cyclists compared
with
glucose was infused continuously during 2 h of cycle ergometer exercise at 60% of pretraining peak O2 uptake (vo2) to deter-
untrained men during exercise at 65% VOWmax. Somewhat paradoxically, based on the known adap-
mine glucose turnover
tations
and oxidation.
Training
increased
cycle
ergometer peak Vo2 by 23% and decreased the respiratory exchange ratio during the final 30 min of exercise from 0.89 + 0.01 to 0.85 -t 0.01 (SE) (P c 0.001). Plasma glucose turnover during exercise decreased from 44.6 t 3.5 pmol. kg fat-free mass (FFM)-l min-l before training to 31.5 $- 4.3 after training (P < O.OOl), whereas plasma glucose clearance (i.e., rate of disappearance/plasma glucose concentration) fell from 9.5 t 0.6 to 6.4 t 0.8 ml kg FFM-l. min-’ (P < 0.001). Oxidation of plasma-derived glucose, which accounted for -90% of plasma l
l
glucose disappearance
in both the untrained
and trained
states,
to endurance
exercise training,
it might be rea-
sonable to expect that utilization of plasma glucose during exercise would be increased after training. During exercise,
intramuscular
glucose
6-phosphate
(G-6-P)
concentrations are lower in the trained than in the untrained state (8, 20). This decrease, along with an increase in hexokinase
(HK) activity
(30), might protect
against inhibition of glucose phosphorylation within the muscle cell and thereby increase the uptake and oxidation of plasma glucose. Exercise also increases insulin
decreased from 41.1 & 3.4 pmol kg FFM-l . min-’ before training to 27.7 * 4.8 after training (P < 0.001). This decrease could
sensitivity (24), and insulin levels tend to decrease less during exercise in the trained state (13,43). Insulin might
account
therefore play a greater role in promoting
l
for roughly
one-half
of the total
reduction
in the
amount of carbohydrate utilized during the final 30 min of exercise in the trained compared with the untrained state.
effects of training
carbohydrate metabolism; glucose kinetics; stable isotopes
ENDURANCE EXERCISE TRAINING results in a decreased
reliance
on carbohydrate
submaximal
as an energy source during
exercise, as indicated
by a decrease in the
respiratory exchange ratio (RER) (6) and a slower rate of depletion of muscle glycogen in humans (22) or rats (I, 10) during exercise performed
at the same absolute
intensity before and after training. In rats, training also decreases blood glucose turnover and oxidation during moderate intensity exercise (3), and therefore results in a slower depletion of liver glycogen during exercise in the trained state (1, 8). Consequently, trained rats are better able to maintain plasma glucose concentration during prolonged exercise (1, 3). In humans, however, the effect of training on the utilization of plasma glucose is not clear. Studies in which only one leg has been trained have not shown any significant differences in glucose uptake between the trained and untrained legs during two-legged exercise at 990
0161-7567/90
$1.50
Copyright
glucose uptake
during exercise after training. The purpose of the present study was to examine the on plasma glucose metabolism
during
exercise in man. Plasma glucose turnover and oxidation were determined by using [ UJ3C]glucose during exercise in seven men before and after adaptation to a strenuous exercise training program. The data indicate that endurance training results in a decreased reliance on plasma glucose as an energy source during exercise performed at the same absolute intensity before and after training. METHODS
Subjects. Seven young [age 26 t 1 (SE) yr] men volunteered for this study, which was approved by the Human Studies Committee at Washington University School of Medicine. All subjects were healthy, as indicated by medical history, physical examination, and standard blood and urine chemistries, and all had normal plasma glucose and insulin responses to a 75-g oral glucose challenge. None of the men had performed regular endurance exercise for at least 6 mo before the study. Preliminary testing. VO zmax was determined during a continuous treadmill test, at a constant speed with the
0 1990 the American
Physiological
Society
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TRAINING
AND
GLUCOSE
KINETICS
DURING
EXERCISE
991
grade increasing 2% every 2 min. Peak O2 consumption After completion of the pretraining experiments, sub(VO,) during cycle ergometry was also determined during jects trained for 12 wk by running and cycling, using a a continuous exercise test in which power output was training program described in detail previously (15). increased by 25 W every 2. min. In all tests, subjects Briefly, on 3 days each week, the men exercised by exercised until fatigue, with Vo2 maxand peak Vo2 defined running as fast as possible for 40 min. On 3 other days by a plateau in Vo2 with increasing work rate. To2 was each week, subjects performed six work intervals of 5calculated every 30 s during exercise using a computermin duration on the cycle ergometer, at a power output based system that incorporated a Parkinson-Cowan CD- designed to elicit 90-100% peak VOW, separated by 2- to 4 dry gas meter, electronic O2 (Applied Electrochemistry 3-min recovery periods of pedalling at 40-50% peak Vo2. S3-A) and COz (Beckman LB-2) analyzers, and a &liter vozrnax and peak vos were reassessed every 2-3 wk during training, and running pace and power output on mixing chamber. Body composition was determined by hydrostatic the cycle ergometer increased to maintain a constant weighing (4), with residual lung volume determined by training stimulus. 02 dilution (41). Sample collection and analysis. Plasma was obtained Study design and exercise training program. Subjects by collecting 3 ml of blood into heparinized tubes and at 4°C. Glucose concentration was deterwere studied both before and after training during 2 h of centrifuging exercise on an electrically braked Lode cycle ergometer. mined using the glucose oxidase method (Glucose AnaExercise was performed at 60% of pretraining peak vo2, lyzer 2, Beckman Instruments, Fullerton, CA). FFA conwith the same absolute power output (124 t 9 W) em- centration was assayed using an enzymatic calorimetric technique (Wako NEFA C kit, Wako Chemicals, Dallas, ployed after training. Testing was conducted in the early afternoon -6 h after a light breakfast and, in the postTX). An additional 3 ml of blood were placed in chilled training experiments, between 48 and 72 h after the final tubes containing 7.5 mg EDTA, 2,000 kallikrein inhibitraining session. Food intake was constant in terms of tory units of aprotinin, and the separated plasma used of insulin (27) and glucagon (9). both quantity and composition for 72 h before the pre- for the determination and posttraining studies. Then 1 ml of blood was deproteinized in 2 ml of 1 M An indwelling catheter. was placed in retrograde fash- perchloric acid, centrifuged, and the supernatant fluid ion in a hand vein and the hand placed in a box heated used for the determination of lactate (12). to 65°C for the sampling of arterialized venous blood For the determination of [ U-‘“Cl glucose enrichment, plasma samples were deproteinized with (29). A second catheter was placed in an antecubital vein duplicate in the contralateral arm for the infusion of sterile, py- Ba(OH)2 and ZnS04 and passed rapidly over cation- and rogen-free [U-13C]glucose (91.6 mol%, Cambridge Iso- anion-exchange resins (Dowex AG50W-X8 and AGl-X8, Bio-Rad Laboratories) to remove charged metabolites, tope Laboratories, Woburn, MA) dissolved in saline. Preexercise blood and expired gas samples (see below) were and lyophilized (2). The butylboronate monoacetate deobtained after the subjects had been sitting quietly on rivative of glucose was prepared (2, 40) and samples the cycle ergometer for 5 min. Thereafter, subjects began analyzed using electron-impact (70 keV) gas chromatography-mass spectrometry (Finnigan 3300 GCMS system, pedalling at the required power output, with priming doses of NaH13C03 [lo0 pmol/kg fat-free mass (FFM); Sunnyvale, CA). After isothermal (200°C) separation on 90 atom%; MSD Isotopes, Rahway, NJ] and [U-‘“Cla 3% W-17 column (1 m X 0.3 mm), selected ion glucose (9 pmol/kg FFM) administered through the an- monitoring was performed at m/z 297 and 303, representing unlabeled and uniformly labeled species, respectecubital catheter at the onset of exercise. [U-13C]glucose was then infused throughout exercise at 0.3 prnol. kg tively. Standards of known enrichment ranging from 0.25 FFM-l. min-’ using a calibrated Harvard syringe pump. to 10 mol% excess were analyzed before and after each vo2 and RER were measured at rest and every 10 min set of unknowns. The relative coefficients of variation at during exercise using the previously described system, 0.5 and 1.0 mol% excess were 4.2 and 2.0%, respectively. and expired breath samples were collected for determiGlucose production determined in this manner reprenation of 13C02 enrichment (see below). Heart rate (HR) sents total glucose production because the probability of was also determined by electrocardiogram (ECG) every [ U-‘“Cl glucose recycling as a uniformly labeled species 10 min during exercise. Blood samples were obtained at is minute (38). Enrichments at m/z 298 to m/z 302 (as were also rest and every 10 min during exercise and used for the the result of recycling of the [U-‘“Clglucose) measurement of plasma glucose concentration and [Umonitored but were generally close to or below the limit 13C] glucose enrichment (see below). Additional blood of detection (-0.1 mol% excess). samples were drawn at rest and every 30 min during Expired gas samples for the determination of 13C02 exercise and used for the determination of lactate, free enrichment were collected in triplicate in 20-ml evacuated tubes (Tracer Technologies, Somerville, MA) difatty acids (FFA), insulin, and glucagon concentrations rectly from the mixing chamber by means of a needle (see below). Four of the subjects (2 when untrained, 2 when port. Samples were analyzed via isotope-ratio mass spectrained) were also studied to determine the effects of trometry, using an automated instrument similar to that exercise alone on background 13C02 production (44). described by Schoeller and Klein (35). 13C02 enrichment These experiments followed the same general procedures was determined against a known standard, after correction for mass spectrometer background, abundance senas described above, except that no isotope was infused and no blood samples were obtained. sitivity, and 170 abundance. Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (128.059.222.107) on January 17, 2019.
992
TRAINING
AND GLUCOSE
KINETICS
Calculation of glucose turnover and oxidation. Tracer was infused only during exercise and not at rest to minimize the possibility of incorporation of the label into liver and muscle glycogen stores (36). However, a dynamic steady state was achieved with respect to 13COZ production and [U-13C]glucose enrichment during the final 30 min of exercise in both the untrained and trained states (see RESULTS). The rate of appearance (R,) of plasma glucose (in pmol . kg FFM-l. min-‘) was therefore calculated during this time period using the steady-state tracer-dilution equation modified for use with stable isotopes (2, 7)
R a = Fm[(IEinf*IE,,-l)
0 l]
(1) where F is the infusion rate of [U-13C]glucose (expressed as prnol. kg FFM-l. min-‘) and IEinf and IE,, represent the isotopic enrichment of the infusate and of plasma glucose at plateau (both in mol% excess). The rate of disappearance of plasma glucose (RJ and plasma glucose turnover (R) were assumed to be equal to Rae The rate of [ U-‘“Cl glucose infusion was calculated by multiplying the infusion pump rate by the infusate glucose concentration determined using the glucose analyzer. The clearance rate of plasma glucose (R,, in ml. kg FFM-1 min-‘) was calculated as l
R
C
= Rd'[pg]-'
(2)
where [pg] is the plasma glucose concentration (expressed as pmol/ml). Non-steady-state Rap Rd, and R, were also calculated throughout exercise (37). A three-point moving average technique was used to minimize large deviations in glucose kinetics as the result of experimental error in the determination of IE,, and [pg] (5). The rate of oxidation (R,,) of plasma-derived glucose (in pmol . kg FFM-l. min-‘) during the final 30 min of exercise was calculated as R ox = IEco, IE,g’ l
l
Vco2 6-l l
(3)
where IEco, is the isotopic enrichment in expired breath COa (in mol% excess), ho2 is the rate of CO2 production (in pmol . kg FFM-l. min-‘), and division by six accounts for the fact that six 13C02 molecules are produced for every [U-13C]glucose molecule completely oxidized. Based on the results of Wolfe et al. (45), it was assumed that all the 13C02 produced by oxidation of [U-‘“Clglucose during exercise would appear in expired breath, and therefore no correction was applied for bicarbonate retention. Similarly, results of the preliminary experiments in which no [U-13C]glucose was infused indicated that a change in background 13C02 production as the result of exercise alone (44) could have contributed Cl% to the plateau in 13C02 production observed during [U13C]glucose infusion, and no correction was therefore necessary. Rates of total carbohydrate and fat oxidation were estimated from Tjo2 and RER using the tables of Lusk (28), assuming a nonprotein RER. StatisticaL analyses. Data were analyzed by repeated measures analysis of variance (ANOVA). When statistical significance (P 5 0.05) was indicated by ANOVA,
DURING
EXERCISE
differences were isolated using Newman-Keul post hoc tests. Where appropriate, before- and after-training means were compared using a paired t test. RESULTS
Training resulted in a slight (2%) decrease in body weight but no significant changes in percent body fat or FFM (Table 1). VO zrnaxand peak VOWboth increased (P c 0.001) by 9-10 ml. kg-l. min-’ in response to training, representing increases of 19 and 23%, respectively (Table 1). Exercise at 124 t 9 W therefore demanded 61% of peak i702 before training but only 49% of peak vo2 after training (P c 0.001; Table 2). Further evidence of a training effect is provided by the significant reductions in HR and RER during submaximal exercise (Table 2) ‘Plasma glucose concentration, [U- 13C]glucoseenrichment, and l3CO2 production. At rest, plasma glucose
concentration averaged 5.08 t 0.18 and 5.14 t 0.20 mM in the untrained and trained states, respectively, and did not change significantly during exercise either before or after training (Fig. 1). [U-13C]glucose enrichment in plasma decreased steadily during exercise in the untrained state, before plateauing during the final 30 min of exercise (Fig. 1). After training, [U-13C]glucose enrichment was constant throughout exercise and significantly higher than before training from 50 min until the end of exercise (Fig. 1). In both the untrained and trained states, 13C02 production initially fell during the first 20-30 min of exercise, representing decay of the NaH13C03 prime (Fig. 1). In the untrained state, 13C02 production then rose steadily before plateauing during the final 30 min of exercise. This increase, however, appeared to be delayed by lo-20 min after training, possibly as a result of the slower turnover of the glucose pool (see below). 13C02 production was therefore significantly lower after training from 40 min until 80 min of exercise (Fig. 1). However, 13C02 production did not differ significantly with time or beTABLE
1. Physical characteristics of subjects Before Training
After Training
Weight, kg 74.9t2.4 73.3k2.4” %Fat 14.3t1.6 13.3t1.1 FFM, kg 63.9k1.5 63.5t1.6 VO 2max,ml kg-’ . min-’ 47.3k2.3 56.2+3.0? TO 2peak, IIll 0kg-’ min-’ 43.2k2.1 53.1*2.2-t Values are means t SE for 7 subjects. FFM, fat-free mass; VO, max, maximal 02 uptake measured during treadmill running; i702peak, peak O2 uptake measured during cycle ergometry. * P < 0.05; t P < 0.001. l
l
TABLE 2. vo2, HR, and RER during 90. to 120-min period of exercise Before Training
After Training
V02, l/min 1.98,tO.ll 1.90t0.11 %Peak i702 61.01kO.7 48.9*1.3* HR, beats/min 158k3 13odz3* RER 0.89kO.01 0.85kO.01' Values are means t SE for 7 subjects. \io2, O2 consumption; HR, heart rate; RER, respiratory exchange ratio. * P < 0.001.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (128.059.222.107) on January 17, 2019.
TRAINING
AND GLUCOSE
KINETICS
DURING
-2
o-
993
EXERCISE
......... ......... ......... ......... ......... ......... ........ ......... ........ ......... ..................... ......... ..................... ......... ......... ......... ......... ........ ................. ---------1 Glucose
turnover
......... ......... ......... ......... ......... ......... ......... ......... ......... ......... ..... ..... ......... ......... ......... ......... ......... Glucose
oxidation
FIG. 2. Plasma glucose turnover and oxidation during final 30 min of exercise before and after training. FFM, fat-free mass. Bars, group means; circles and lines, individual subjects. i P c 0.001.
08. 069 .
also decreased after training, averaging 9.5 t 0.6 ml. kg FFM-1 min-’ before training and 6.4 t 0.8 ml* kg FFM-l. min-’ after training (-33%; P < 0.001; Table 3). Non-steady-state analyses indicated that Ra, Rd, and Rc were significantly lower after training from 50 min through the end of exercise (data not shown). Most of the glucose cleared from the plasma during exercise was directed to oxidation both before and after training, with -90% of the infused 13C label recovered as 13COz at isotopic plateau (Table 3). Oxidation of glucose derived from plasma (R,,) was therefore 33% lower during the final 30 min of exercise after training (Table 3). The rate of plasma glucose turnover (R) was significantly correlated with the rate of total carbohydrate oxidation as estimated from VOWand RER (Fig. 3). Similar relationships were observed when plasma glucose clearance (R,) or oxidation of plasma-derived glucose (R,,) were related to total carbohydrate oxidation (Fig. 3) . l
. 0402 . O120 100 80 60 40 20 0 11
0
11
1
20
40
II
11
60 80 TIME (min)
’
11
100
1
120
FIG. 1. Plasma glucose concentration, [U-13C]glucose enrichment, and 13C02 production before (0) and after (0) training. Values are means t SE. * P < 0.05.
Contribution of fat, carbohydrate, and glucose oxidation to total energy expenditure. VOW did not differ signifi-
cantly between the untrained and trained states at any time during exercise. Total energy expenditure estimated from vo2 and RER was also not different during the first 90 min of exercise. However, total energy expenditure TABLE 3. Plasma glucose kinetics during 90- to 120-min was slightly (4%) but significantly lower during the final period of exercise 30 min of exercise after training (Table 4) because of the Before Training After Training consistent decrease in RER. Oxidation of carbohydrate accounted for almost twoPlasma glucose, mmol/l 4.69k0.13 4.93t0.14 31.5*4.3* thirds of total energy expenditure during the final 30 min Rt, pmol . kg FFM-1 min-’ 44.6t3.5 R,, ml. kg FFM-’ .min-’ 9.5k0.6 6.4kO.8* of exercise in the untrained state (Table 4). After train92.2k2.6 85.424.5 % of R, oxidized ing, the estimated rate of total carbohydrate oxidation 41.1t3.4 27.7t4.8* R 0x,pmol . kg FFM-l. min-’ was significantly reduced, whereas that of fat was signifValues are means * SE for 7 subjects. FFM, fat-free mass; R, icantly increased (Table 4). plasma glucose turnover; R. plasma glucose clearance; R,,, oxidation Before training, oxidation of plasma-derived glucose rate. *P < 0.001. represented one-third of total carbohydrate oxidation cause of training during the final 30 min of exercise (Fig. and one-fifth of total energy expenditure during the final 30 min of exercise (Table 4). Training resulted in a 1) . smaller proportion of both total carbohyTurnover, clearance, and oxidation of plasma glucose. significantly drate oxidation and total energy expenditure being deDuring the final 30 min of exercise, steady-state plasma rived from plasma glucose (Table 4). The decrease in the glucose turnover was lower by an average of 29% after glucose could account for training (Table 3). All subjects demonstrated a slower oxidation of plasma-derived roughly one-half of the decrease in total carbohydrate turnover of the plasma glucose pool after training, with individual decreases ranging from 17 to 54% (Fig. 2). oxidation during the last 30 min of exercise (Table 4). Lactate, FFA, glucagon, and insulin responsesto exerBecause plasma glucose concentration was similar before and after training (Table 3), the rate of clearance of cise. Blood lactate concentrations were lower during the plasma glucose during the final 30 min of exercise was final 30 min of exercise after training, whereas plasma l
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994
TRAINING
AND GLUCOSE
KINETICS
DURING
EXERCISE
5. Blood lactate and plasma FFA, insulin, and glucagon concentrations during 90to 1ZO-min period of exercise
TABLE
Before Training
Blood lactate, mM 1.98t0.15 Plasma FFA, mM 1.26kO.21 Plasma insulin, pU/ml 7.2t1.3 Plasma glucagon, pg/ml 185*31 Values are means ,t SE for 7 subjects. FFA, free 0.05; t P c 0.001.
p
65-70% maximal
training decreasesplasma glucose turnover and oxidation during moderate-intensity exercise in men. J. Appl. Physiol. 68(3): 990996, 1990.-To assess the effects of endurance training on plasma glucose kinetics during moderate-intensity exercise in men, seven men were studied before and after 12 wk of strenuous exercise training (3 days/wk running, 3 days/wk cycling).
plasma glucose disappearance between trained and untrained subjects during high-intensity exercise. Recently,
After
priming
of the glucose
and bicarbonate
pools,
[U-‘“Cl
02 uptake (v02 max) (14, 34). Kjaer et
al. (25) also found
no difference
in tracer-determined
however, Jansson and Kaijser (20) have reported that leg
glucose uptake, as determined from the arterial-femoral venous difference for glucose and estimated leg blood flow, is lower in well-trained
cyclists compared
with
glucose was infused continuously during 2 h of cycle ergometer exercise at 60% of pretraining peak O2 uptake (vo2) to deter-
untrained men during exercise at 65% VOWmax. Somewhat paradoxically, based on the known adap-
mine glucose turnover
tations
and oxidation.
Training
increased
cycle
ergometer peak Vo2 by 23% and decreased the respiratory exchange ratio during the final 30 min of exercise from 0.89 + 0.01 to 0.85 -t 0.01 (SE) (P c 0.001). Plasma glucose turnover during exercise decreased from 44.6 t 3.5 pmol. kg fat-free mass (FFM)-l min-l before training to 31.5 $- 4.3 after training (P < O.OOl), whereas plasma glucose clearance (i.e., rate of disappearance/plasma glucose concentration) fell from 9.5 t 0.6 to 6.4 t 0.8 ml kg FFM-l. min-’ (P < 0.001). Oxidation of plasma-derived glucose, which accounted for -90% of plasma l
l
glucose disappearance
in both the untrained
and trained
states,
to endurance
exercise training,
it might be rea-
sonable to expect that utilization of plasma glucose during exercise would be increased after training. During exercise,
intramuscular
glucose
6-phosphate
(G-6-P)
concentrations are lower in the trained than in the untrained state (8, 20). This decrease, along with an increase in hexokinase
(HK) activity
(30), might protect
against inhibition of glucose phosphorylation within the muscle cell and thereby increase the uptake and oxidation of plasma glucose. Exercise also increases insulin
decreased from 41.1 & 3.4 pmol kg FFM-l . min-’ before training to 27.7 * 4.8 after training (P < 0.001). This decrease could
sensitivity (24), and insulin levels tend to decrease less during exercise in the trained state (13,43). Insulin might
account
therefore play a greater role in promoting
l
for roughly
one-half
of the total
reduction
in the
amount of carbohydrate utilized during the final 30 min of exercise in the trained compared with the untrained state.
effects of training
carbohydrate metabolism; glucose kinetics; stable isotopes
ENDURANCE EXERCISE TRAINING results in a decreased
reliance
on carbohydrate
submaximal
as an energy source during
exercise, as indicated
by a decrease in the
respiratory exchange ratio (RER) (6) and a slower rate of depletion of muscle glycogen in humans (22) or rats (I, 10) during exercise performed
at the same absolute
intensity before and after training. In rats, training also decreases blood glucose turnover and oxidation during moderate intensity exercise (3), and therefore results in a slower depletion of liver glycogen during exercise in the trained state (1, 8). Consequently, trained rats are better able to maintain plasma glucose concentration during prolonged exercise (1, 3). In humans, however, the effect of training on the utilization of plasma glucose is not clear. Studies in which only one leg has been trained have not shown any significant differences in glucose uptake between the trained and untrained legs during two-legged exercise at 990
0161-7567/90
$1.50
Copyright
glucose uptake
during exercise after training. The purpose of the present study was to examine the on plasma glucose metabolism
during
exercise in man. Plasma glucose turnover and oxidation were determined by using [ UJ3C]glucose during exercise in seven men before and after adaptation to a strenuous exercise training program. The data indicate that endurance training results in a decreased reliance on plasma glucose as an energy source during exercise performed at the same absolute intensity before and after training. METHODS
Subjects. Seven young [age 26 t 1 (SE) yr] men volunteered for this study, which was approved by the Human Studies Committee at Washington University School of Medicine. All subjects were healthy, as indicated by medical history, physical examination, and standard blood and urine chemistries, and all had normal plasma glucose and insulin responses to a 75-g oral glucose challenge. None of the men had performed regular endurance exercise for at least 6 mo before the study. Preliminary testing. VO zmax was determined during a continuous treadmill test, at a constant speed with the
0 1990 the American
Physiological
Society
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (128.059.222.107) on January 17, 2019.
TRAINING
AND
GLUCOSE
KINETICS
DURING
EXERCISE
991
grade increasing 2% every 2 min. Peak O2 consumption After completion of the pretraining experiments, sub(VO,) during cycle ergometry was also determined during jects trained for 12 wk by running and cycling, using a a continuous exercise test in which power output was training program described in detail previously (15). increased by 25 W every 2. min. In all tests, subjects Briefly, on 3 days each week, the men exercised by exercised until fatigue, with Vo2 maxand peak Vo2 defined running as fast as possible for 40 min. On 3 other days by a plateau in Vo2 with increasing work rate. To2 was each week, subjects performed six work intervals of 5calculated every 30 s during exercise using a computermin duration on the cycle ergometer, at a power output based system that incorporated a Parkinson-Cowan CD- designed to elicit 90-100% peak VOW, separated by 2- to 4 dry gas meter, electronic O2 (Applied Electrochemistry 3-min recovery periods of pedalling at 40-50% peak Vo2. S3-A) and COz (Beckman LB-2) analyzers, and a &liter vozrnax and peak vos were reassessed every 2-3 wk during training, and running pace and power output on mixing chamber. Body composition was determined by hydrostatic the cycle ergometer increased to maintain a constant weighing (4), with residual lung volume determined by training stimulus. 02 dilution (41). Sample collection and analysis. Plasma was obtained Study design and exercise training program. Subjects by collecting 3 ml of blood into heparinized tubes and at 4°C. Glucose concentration was deterwere studied both before and after training during 2 h of centrifuging exercise on an electrically braked Lode cycle ergometer. mined using the glucose oxidase method (Glucose AnaExercise was performed at 60% of pretraining peak vo2, lyzer 2, Beckman Instruments, Fullerton, CA). FFA conwith the same absolute power output (124 t 9 W) em- centration was assayed using an enzymatic calorimetric technique (Wako NEFA C kit, Wako Chemicals, Dallas, ployed after training. Testing was conducted in the early afternoon -6 h after a light breakfast and, in the postTX). An additional 3 ml of blood were placed in chilled training experiments, between 48 and 72 h after the final tubes containing 7.5 mg EDTA, 2,000 kallikrein inhibitraining session. Food intake was constant in terms of tory units of aprotinin, and the separated plasma used of insulin (27) and glucagon (9). both quantity and composition for 72 h before the pre- for the determination and posttraining studies. Then 1 ml of blood was deproteinized in 2 ml of 1 M An indwelling catheter. was placed in retrograde fash- perchloric acid, centrifuged, and the supernatant fluid ion in a hand vein and the hand placed in a box heated used for the determination of lactate (12). to 65°C for the sampling of arterialized venous blood For the determination of [ U-‘“Cl glucose enrichment, plasma samples were deproteinized with (29). A second catheter was placed in an antecubital vein duplicate in the contralateral arm for the infusion of sterile, py- Ba(OH)2 and ZnS04 and passed rapidly over cation- and rogen-free [U-13C]glucose (91.6 mol%, Cambridge Iso- anion-exchange resins (Dowex AG50W-X8 and AGl-X8, Bio-Rad Laboratories) to remove charged metabolites, tope Laboratories, Woburn, MA) dissolved in saline. Preexercise blood and expired gas samples (see below) were and lyophilized (2). The butylboronate monoacetate deobtained after the subjects had been sitting quietly on rivative of glucose was prepared (2, 40) and samples the cycle ergometer for 5 min. Thereafter, subjects began analyzed using electron-impact (70 keV) gas chromatography-mass spectrometry (Finnigan 3300 GCMS system, pedalling at the required power output, with priming doses of NaH13C03 [lo0 pmol/kg fat-free mass (FFM); Sunnyvale, CA). After isothermal (200°C) separation on 90 atom%; MSD Isotopes, Rahway, NJ] and [U-‘“Cla 3% W-17 column (1 m X 0.3 mm), selected ion glucose (9 pmol/kg FFM) administered through the an- monitoring was performed at m/z 297 and 303, representing unlabeled and uniformly labeled species, respectecubital catheter at the onset of exercise. [U-13C]glucose was then infused throughout exercise at 0.3 prnol. kg tively. Standards of known enrichment ranging from 0.25 FFM-l. min-’ using a calibrated Harvard syringe pump. to 10 mol% excess were analyzed before and after each vo2 and RER were measured at rest and every 10 min set of unknowns. The relative coefficients of variation at during exercise using the previously described system, 0.5 and 1.0 mol% excess were 4.2 and 2.0%, respectively. and expired breath samples were collected for determiGlucose production determined in this manner reprenation of 13C02 enrichment (see below). Heart rate (HR) sents total glucose production because the probability of was also determined by electrocardiogram (ECG) every [ U-‘“Cl glucose recycling as a uniformly labeled species 10 min during exercise. Blood samples were obtained at is minute (38). Enrichments at m/z 298 to m/z 302 (as were also rest and every 10 min during exercise and used for the the result of recycling of the [U-‘“Clglucose) measurement of plasma glucose concentration and [Umonitored but were generally close to or below the limit 13C] glucose enrichment (see below). Additional blood of detection (-0.1 mol% excess). samples were drawn at rest and every 30 min during Expired gas samples for the determination of 13C02 exercise and used for the determination of lactate, free enrichment were collected in triplicate in 20-ml evacuated tubes (Tracer Technologies, Somerville, MA) difatty acids (FFA), insulin, and glucagon concentrations rectly from the mixing chamber by means of a needle (see below). Four of the subjects (2 when untrained, 2 when port. Samples were analyzed via isotope-ratio mass spectrained) were also studied to determine the effects of trometry, using an automated instrument similar to that exercise alone on background 13C02 production (44). described by Schoeller and Klein (35). 13C02 enrichment These experiments followed the same general procedures was determined against a known standard, after correction for mass spectrometer background, abundance senas described above, except that no isotope was infused and no blood samples were obtained. sitivity, and 170 abundance. Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (128.059.222.107) on January 17, 2019.
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Calculation of glucose turnover and oxidation. Tracer was infused only during exercise and not at rest to minimize the possibility of incorporation of the label into liver and muscle glycogen stores (36). However, a dynamic steady state was achieved with respect to 13COZ production and [U-13C]glucose enrichment during the final 30 min of exercise in both the untrained and trained states (see RESULTS). The rate of appearance (R,) of plasma glucose (in pmol . kg FFM-l. min-‘) was therefore calculated during this time period using the steady-state tracer-dilution equation modified for use with stable isotopes (2, 7)
R a = Fm[(IEinf*IE,,-l)
0 l]
(1) where F is the infusion rate of [U-13C]glucose (expressed as prnol. kg FFM-l. min-‘) and IEinf and IE,, represent the isotopic enrichment of the infusate and of plasma glucose at plateau (both in mol% excess). The rate of disappearance of plasma glucose (RJ and plasma glucose turnover (R) were assumed to be equal to Rae The rate of [ U-‘“Cl glucose infusion was calculated by multiplying the infusion pump rate by the infusate glucose concentration determined using the glucose analyzer. The clearance rate of plasma glucose (R,, in ml. kg FFM-1 min-‘) was calculated as l
R
C
= Rd'[pg]-'
(2)
where [pg] is the plasma glucose concentration (expressed as pmol/ml). Non-steady-state Rap Rd, and R, were also calculated throughout exercise (37). A three-point moving average technique was used to minimize large deviations in glucose kinetics as the result of experimental error in the determination of IE,, and [pg] (5). The rate of oxidation (R,,) of plasma-derived glucose (in pmol . kg FFM-l. min-‘) during the final 30 min of exercise was calculated as R ox = IEco, IE,g’ l
l
Vco2 6-l l
(3)
where IEco, is the isotopic enrichment in expired breath COa (in mol% excess), ho2 is the rate of CO2 production (in pmol . kg FFM-l. min-‘), and division by six accounts for the fact that six 13C02 molecules are produced for every [U-13C]glucose molecule completely oxidized. Based on the results of Wolfe et al. (45), it was assumed that all the 13C02 produced by oxidation of [U-‘“Clglucose during exercise would appear in expired breath, and therefore no correction was applied for bicarbonate retention. Similarly, results of the preliminary experiments in which no [U-13C]glucose was infused indicated that a change in background 13C02 production as the result of exercise alone (44) could have contributed Cl% to the plateau in 13C02 production observed during [U13C]glucose infusion, and no correction was therefore necessary. Rates of total carbohydrate and fat oxidation were estimated from Tjo2 and RER using the tables of Lusk (28), assuming a nonprotein RER. StatisticaL analyses. Data were analyzed by repeated measures analysis of variance (ANOVA). When statistical significance (P 5 0.05) was indicated by ANOVA,
DURING
EXERCISE
differences were isolated using Newman-Keul post hoc tests. Where appropriate, before- and after-training means were compared using a paired t test. RESULTS
Training resulted in a slight (2%) decrease in body weight but no significant changes in percent body fat or FFM (Table 1). VO zrnaxand peak VOWboth increased (P c 0.001) by 9-10 ml. kg-l. min-’ in response to training, representing increases of 19 and 23%, respectively (Table 1). Exercise at 124 t 9 W therefore demanded 61% of peak i702 before training but only 49% of peak vo2 after training (P c 0.001; Table 2). Further evidence of a training effect is provided by the significant reductions in HR and RER during submaximal exercise (Table 2) ‘Plasma glucose concentration, [U- 13C]glucoseenrichment, and l3CO2 production. At rest, plasma glucose
concentration averaged 5.08 t 0.18 and 5.14 t 0.20 mM in the untrained and trained states, respectively, and did not change significantly during exercise either before or after training (Fig. 1). [U-13C]glucose enrichment in plasma decreased steadily during exercise in the untrained state, before plateauing during the final 30 min of exercise (Fig. 1). After training, [U-13C]glucose enrichment was constant throughout exercise and significantly higher than before training from 50 min until the end of exercise (Fig. 1). In both the untrained and trained states, 13C02 production initially fell during the first 20-30 min of exercise, representing decay of the NaH13C03 prime (Fig. 1). In the untrained state, 13C02 production then rose steadily before plateauing during the final 30 min of exercise. This increase, however, appeared to be delayed by lo-20 min after training, possibly as a result of the slower turnover of the glucose pool (see below). 13C02 production was therefore significantly lower after training from 40 min until 80 min of exercise (Fig. 1). However, 13C02 production did not differ significantly with time or beTABLE
1. Physical characteristics of subjects Before Training
After Training
Weight, kg 74.9t2.4 73.3k2.4” %Fat 14.3t1.6 13.3t1.1 FFM, kg 63.9k1.5 63.5t1.6 VO 2max,ml kg-’ . min-’ 47.3k2.3 56.2+3.0? TO 2peak, IIll 0kg-’ min-’ 43.2k2.1 53.1*2.2-t Values are means t SE for 7 subjects. FFM, fat-free mass; VO, max, maximal 02 uptake measured during treadmill running; i702peak, peak O2 uptake measured during cycle ergometry. * P < 0.05; t P < 0.001. l
l
TABLE 2. vo2, HR, and RER during 90. to 120-min period of exercise Before Training
After Training
V02, l/min 1.98,tO.ll 1.90t0.11 %Peak i702 61.01kO.7 48.9*1.3* HR, beats/min 158k3 13odz3* RER 0.89kO.01 0.85kO.01' Values are means t SE for 7 subjects. \io2, O2 consumption; HR, heart rate; RER, respiratory exchange ratio. * P < 0.001.
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o-
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......... ......... ......... ......... ......... ......... ........ ......... ........ ......... ..................... ......... ..................... ......... ......... ......... ......... ........ ................. ---------1 Glucose
turnover
......... ......... ......... ......... ......... ......... ......... ......... ......... ......... ..... ..... ......... ......... ......... ......... ......... Glucose
oxidation
FIG. 2. Plasma glucose turnover and oxidation during final 30 min of exercise before and after training. FFM, fat-free mass. Bars, group means; circles and lines, individual subjects. i P c 0.001.
08. 069 .
also decreased after training, averaging 9.5 t 0.6 ml. kg FFM-1 min-’ before training and 6.4 t 0.8 ml* kg FFM-l. min-’ after training (-33%; P < 0.001; Table 3). Non-steady-state analyses indicated that Ra, Rd, and Rc were significantly lower after training from 50 min through the end of exercise (data not shown). Most of the glucose cleared from the plasma during exercise was directed to oxidation both before and after training, with -90% of the infused 13C label recovered as 13COz at isotopic plateau (Table 3). Oxidation of glucose derived from plasma (R,,) was therefore 33% lower during the final 30 min of exercise after training (Table 3). The rate of plasma glucose turnover (R) was significantly correlated with the rate of total carbohydrate oxidation as estimated from VOWand RER (Fig. 3). Similar relationships were observed when plasma glucose clearance (R,) or oxidation of plasma-derived glucose (R,,) were related to total carbohydrate oxidation (Fig. 3) . l
. 0402 . O120 100 80 60 40 20 0 11
0
11
1
20
40
II
11
60 80 TIME (min)
’
11
100
1
120
FIG. 1. Plasma glucose concentration, [U-13C]glucose enrichment, and 13C02 production before (0) and after (0) training. Values are means t SE. * P < 0.05.
Contribution of fat, carbohydrate, and glucose oxidation to total energy expenditure. VOW did not differ signifi-
cantly between the untrained and trained states at any time during exercise. Total energy expenditure estimated from vo2 and RER was also not different during the first 90 min of exercise. However, total energy expenditure TABLE 3. Plasma glucose kinetics during 90- to 120-min was slightly (4%) but significantly lower during the final period of exercise 30 min of exercise after training (Table 4) because of the Before Training After Training consistent decrease in RER. Oxidation of carbohydrate accounted for almost twoPlasma glucose, mmol/l 4.69k0.13 4.93t0.14 31.5*4.3* thirds of total energy expenditure during the final 30 min Rt, pmol . kg FFM-1 min-’ 44.6t3.5 R,, ml. kg FFM-’ .min-’ 9.5k0.6 6.4kO.8* of exercise in the untrained state (Table 4). After train92.2k2.6 85.424.5 % of R, oxidized ing, the estimated rate of total carbohydrate oxidation 41.1t3.4 27.7t4.8* R 0x,pmol . kg FFM-l. min-’ was significantly reduced, whereas that of fat was signifValues are means * SE for 7 subjects. FFM, fat-free mass; R, icantly increased (Table 4). plasma glucose turnover; R. plasma glucose clearance; R,,, oxidation Before training, oxidation of plasma-derived glucose rate. *P < 0.001. represented one-third of total carbohydrate oxidation cause of training during the final 30 min of exercise (Fig. and one-fifth of total energy expenditure during the final 30 min of exercise (Table 4). Training resulted in a 1) . smaller proportion of both total carbohyTurnover, clearance, and oxidation of plasma glucose. significantly drate oxidation and total energy expenditure being deDuring the final 30 min of exercise, steady-state plasma rived from plasma glucose (Table 4). The decrease in the glucose turnover was lower by an average of 29% after glucose could account for training (Table 3). All subjects demonstrated a slower oxidation of plasma-derived roughly one-half of the decrease in total carbohydrate turnover of the plasma glucose pool after training, with individual decreases ranging from 17 to 54% (Fig. 2). oxidation during the last 30 min of exercise (Table 4). Lactate, FFA, glucagon, and insulin responsesto exerBecause plasma glucose concentration was similar before and after training (Table 3), the rate of clearance of cise. Blood lactate concentrations were lower during the plasma glucose during the final 30 min of exercise was final 30 min of exercise after training, whereas plasma l
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5. Blood lactate and plasma FFA, insulin, and glucagon concentrations during 90to 1ZO-min period of exercise
TABLE
Before Training
Blood lactate, mM 1.98t0.15 Plasma FFA, mM 1.26kO.21 Plasma insulin, pU/ml 7.2t1.3 Plasma glucagon, pg/ml 185*31 Values are means ,t SE for 7 subjects. FFA, free 0.05; t P c 0.001.
p
