Physiological responses to glycerol ingestion during exercise ROBERT MURRAY, DENNIS E. EDDY, GREGORY L. PAUL, JOHN G. SEIFERT, AND GEORGE A. HALABY Exercise Physiology Laboratory, John Stuart Research Laboratories, The Quaker Outs Company, Barrington, Illinois 60010 MURRAY, ROBERT, DENNIS E. EDDY, GREGORY L. PAUL, JOHN G. SEIFERT, AND GEORGE A. HALABY. Physiologkal responses to glycerol ingestion during exercise. J. Appl. Physiol. 71(l): 144-149, 1991.-To study selectedcardiovascular, thermoregulatory, and hormonal responsesto the consumption of glycerol solutions during exercise, nine subjectscycled for 90 min at 50% peak 0, uptake in a 30’C, 45% relative humidity environment. Beveragestested included a 10% glycerol solution (G), a 6% carbohydrate-electrolyte beverage (CE), the 6% carbohydrate-electrolyte beverageplus 4% glycerol (CEG), and a water placebo (WP) ingested at regular intervals during the first 60 min of exercise. The beverageswere administered in counterbalanced order with subjectsserving as their own controls. Ingestion of the glycerol solutions resulted in an increase in plasmaosmolality and attenuation of the decreasein plasma volume associatedwith the WP treatment (P < 0.05). Plasma renin activity was highestwith WP (P < 0.05), and G wasassociated with increasedantidiuretic hormone levels (P < 0.05). Ratings of perceivedthirst were lowest for CEG and G, and the frequency of gastrointestinal distresswas greatest for G (P c 0.05). However, no differences among beverage treatments were observed for heart rate, esophagealtemperature, sweat rate, ratings of perceived exertion, or changesin cortisol and aldosteronelevels. These data indicate that there are no substantial metabolic, hormonal, cardiovascular, or thermoregulatory advantagesto the consumption of solutions containing 4 or 10% glycerol during exercise.
tained the plasma volume of exercising subjects at resting levels (4) or 8-9% above resting levels (3, 5) with infusions of albumin and saline. Under these conditions, heart rate and esophageal temperature during exercise were significantly lower, and there were tendencies for increased skin blood flow and reduced sweat rate. Such reductions in cardiovascular and thermoregulatory strain would ostensibly be of benefit to athletes, workers, and military personnel who exercise in the heat. Similar positive cardiovascular and thermoregulatory responses might also be expected to result from the ingestion of glycerol solutions during exercise. Glycerol is an osmotically active solute that, after ingestion, is eventually distributed throughout the body water and is removed slowly from the intravascular space via hepatic and renal metabolism (15, 16). Consequently, ingestion of glycerol results in increased plasma osmolality, reduced urine volume, and expanded plasma volume (6, 13). Furthermore, glycerol ingestion before exercise has been reported to be associated with reduced core temperature and increased sweat rate during exercise in the heat (8). For these reasons, we studied the effects of the consumption of glycerol during steady-state cycling exercise to determine if positive thermoregulatory, cardiovascular, and hormonal responses are indeed associated with carbohydrate-electrolyte beverages;plasmavolume expansion; glycerol ingestion. To our knowledge, this is the first rehyperhydration port of the effects of glycerol ingestion during exercise. THE ATTENUATION of the normal
decrease in plasma volume during exercise presumably helps maintain central blood volume and allows for reduced heart rate and increased skin blood flow. When the normal decrease in plasma volume associated with cycling exercise is attenuated by the experimental infusion of albumin and saline, cure temperature, heart rate, and sweat rate are reduced, while stroke volume and skin blood flow increase (4). The rise in skin blood flow is postulated to promote an increase in radiative heat loss and decreases in sweat rate and esophageal temperature. For example, Deschamps et al. (1) recently reported that when the plasma volume of exercising subjects was maintained near resting levels by infusion of isotonic saline, mean heart rate and esophageal temperature were significantly lower than in the control (no-infusion) trial. Similar evidence is provided by the work of Fortney et al. (3-9, who main144
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METHODS
Five women and four men served as subjects in this study. All subjects gave informed consent before participating in testing. The experimental protocol was reviewed and approved by an institutional human subjects review committee. The subjects’ age, weight, lean body mass (LBM), and peak 0, uptake (VO,~& values were 30.2 t 6.4 (SD) yr, 68.6 t 14.5 kg, 54.1 t 11.4 kg, and44.2 t 7.2 ml kg-’ min? All subjects were nonathletes and engaged in modest physical activity two to three times per week. To familiarize the subjects with the demands of the test procedure, each subject completed at least three orientation sessions and one sham test. All exercise sessions were conducted in the afternoon during March and April 1989. In an effort to minimize training and acclimatization responses, at least 7 days (range 7-14 days) separated each test. Subjects reported l
l
Copyright 0 1991 the American Physiological Society
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to the laboratory 3-4 h after a light lunch. To help ensure an adequate and consistent state of hydration before each exercise session, subjects were encouraged to drink water before reporting to the laboratory. After 20 min of seated rest in a climate chamber, the subjects performed 90 min of cycling exercise at 51.8 t 0.8% VOZpeak in a warm (30°C and 45% relative humidity). . environment VO 2peakwas determined during a progressive-intensity exercise test on a cycle ergometer using standard opencircuit spirometric techniques. LBM was calculated using conventional hydrostatic weighing procedures. The four experimental beverages were given in counterbalanced order with subjects serving as their own controls. The beverages were a water placebo (WP; 27 mosmol/kgH,O), a carbohydrate-electrolyte beverage (CE; 6% glucose, 20 mM Na+, 3 mM K’, 11 mM Cl-, 3 mM H,PO;, 9 mM citrate-; 405 mosmol/kgH,O), the carbohydrate-electrolyte beverage plus 4% glycerol (CEG; 928 mosmol/kgH,0), and a 10% glycerol solution (G; 1,275 mosmol/kgH,O). Subjects consumed 3.0 ml beverage/kg LBM (162 t 8 ml) at 15,30,45, and 60 min during exercise, providing a total fluid intake of 647 t 33.0 (SD) ml. Beverages were of similar sweetness (aspartame, in WP), color, and flavor, and were served chilled (1OOC) in plastic squeeze bottles. The total glycerol dose provided by the G solution was 1.2 g glycerol/kg LBM (0.94 g glycerol/kg body wt). Expired air was collected in 12O-liter meteorological balloons at 20,50, and 80 min during exercise; 600 ml of each sample were analyzed for 0, and CO2 content, and ventilatory volume was measured by passing the expired air through an air flowmeter. Blood samples were collected from indwelling 18, gauge forearm venous catheters after 15 min of seated rest and at 13,28,43,58,68,78, and 88 min during exercise. The catheters were kept patent with periodic injections of 10 U/ml heparin in saline. Aliquots of each blood sample were analyzed in duplicate for lactate (model 27 Analyzer, Yellow Springs Instruments), hematocrit (microhematocrit technique), and hemoglobin (model 482 Co-Oximeter, Instrumentation Laboratories). Hematocrit was measured using a digital micrometer to ensure accuracy. Changes in plasma volume were calculated from hemoglobin and hematocrit values by use of the equations of Dill and Costill (2). Plasma was collected from aliquots of each blood sample by centrifugation at 1,000 g for 15 min at 4°C and was subsequently analyzed for glucose (Ames Seralyzer reflectance spectrophotometer), osmolality (Advanced Instruments 3M0 MicroOsmometer), glycerol (Boehringer-Mannheim test kit), free fatty acids (FFA; NEFA C kit, Wako Pure Chemical Industries), reniniangiotensin I [plasma renin activity radioimmunoassay (RIA) kit, Dade Baxter Travenol Diagnostics], and vasopressin (vasopressin RIA kit, Euro-Diagnostics). Serum samples were assayed for aldosterone and cortisol (RIA kits, Diagnostic Products). Spectrophotometric measures were conducted on a Perkin-Elmer model 55 spectrophotometer; immunoreactivity assays were completed using a Packard Minaxi 5000 Auto-Gamma Counter. Esophageal temperature (model 402 thermister and model 47 telethermometer, Yellow Springs Instruments)
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FIG. 1. Changes in plasma volume and plasma osmolality during exercise. Values are means t SE. #Treatment G > CE, WP. ##Treatments G, CEG > CE, WP. +-Treatment G > CEG, CE, WP. *Treatment CEG > CE, WP. **Treatment CEG > WP. All differences, P < 0.05.
was measured at the level of the left atrium. Heart rates (Vantage heart rate monitor) and ratings of perceived exertion (Borg scale) were recorded at rest and at lo- to 15-min intervals during exercise. Ratings of perceived thirst were recorded on a O-100 point scale at 15- to 20min intervals. Subject complaints (e.g., bloated feeling, nausea, lightheadedness) were also recorded. Immediately after exercise, subjects were encouraged to urinate; urine volume was subsequently measured. Nude dry body weight (Mettler ID2 Multirange scale) was recorded to the nearest 2 g before and after exercise. Total body sweat rates were calculated from changes in body weight corrected for urine production, blood sampling volume, fluid volume consumed, body surface area, and expiratory water loss. Data were analyzed for treatment effects by repeatedmeasures two-way analysis of variance. Duncan’s multiple range post hoc tests were conducted when significant (P < 0.05) F ratios were obtained. A nonparametric oneway analysis of variance ranking procedure (SAS, Cary, NC) was used to evaluate the frequency of the subjective complaints recorded in response to the beverage treatments. All data are reported as means t SE unless otherwise indicated. RESULTS
The changes in plasma volume associated with beverage treatment are illustrated in Fig. 1. During the initial 30 min of exercise, plasma volume decreased 7-10%. At 43 min, plasma volume increased toward baseline in all beverage treatments, with the greatest changes in
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plasma volume associated with ingestion of the two:glycerol beverages. This plasma volume change likely occurred as a result of ingestion of fluid at 15 and 30 min. By 60 min, plasma volume had decreased somewhat in all treatments. The final fluid ingestion occurred at 60 min, and the subsequent increases in plasma volume at 68 and 78 min ostensibly reflect the entry of that ingested fluid into the vascular space. The decrease in plasma volume noted at 88 min demonstrates the transient nature of the influence that ingesting fluid has on plasma volume. Plasma osmolality during exercise is also shown in Fig. 1. We encouraged all subjects to consume water ad libiturn before reporting to the laboratory in an attempt to keep preexercise plasma osmolality values below 290 musmul/kgH,O. Despite this precaution, preexercise plasma usmolalities averaged between 293 and 296 mosmul/kgH,O fur all treatments. However, there were no differences am .ung treatments. Plasma osmolalities changed similarly for all beverage treatments throughout the first 40 min of exercise. Thereafter, plasma osmulality ruse markedly in treatment G, reaching an end-exercise peak of 312 musmol/kgH,O. Mean exercise heart rates ranged between 135 and 145 t U-2.0 beats/min in each trial and were not affected by the alterations in plasma volume that occurred during exercise. Ratings of perceived exertion rose progressively throughout exercise to final values of 3.5 to 4.5 but were not influenced by beverage treatment. Perceived thirst increased thruughout exercise to a value of 37.8 t 2.4 units (O-100 scale; 100 = extremely thirsty) at 86 min of exercise . Although no statistically significant interactions were noted among the ratings of perceived thirst, the overall mean ratings of perceived thirst (Fig. 2) were lower with treatments CEG and G than with WP or CE (significant treatment effect, P < 0.05). The frequency of subjective complaints was significantly greater with the G treatment; four of the nine subjects reported symptoms of nausea, bloated feeling in the abdomen, and lightheadedness. After 90 min of exercise, esophageal temperature (Fig. 2) had increased -0.6YZ (to 37.5”C) from resting values in all treatments. Although mean esophageal temperature tended to be mO.l-0.2'C higher during exercise with WP than with the uther treatments, the differences were not statistically significant. Total body sweat rates were not influenced by beverage treatment: WP = 292.2 t 41.4, CE = 380.7 t 48.5, CEG = 302.3 t 2L0, G = 307.6 $- 28.8 mm2 qh-l. Postexercise urine volumes averaged 220 t 30 ml and did not differ among treatments. Changes in plasma glucose, glycerol, and FFA values are illustrated in Fig. 3. As expected, the CE and CEG treatments resulted in significant increases in plasma glucose, whereas the plasma glucose values in treatments G and WP did not change appreciably from rest. Also as expected, plasma glycerol increased significantly in both the G and CEG trials, with glycerol rising to the highest levels (P < 0.05) with treatment G. Urine glycerol concentration (nut shown) was 6.0 t 1.0 mmulI1 for G, 0.5 t 0.2 mmul/l for CEG, and remained at near-zero levels in CE and WP. FFA levels were greater in G and WP, the two treatments that did not contain carbohydrate, g
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FIG. 2. Changes in ratings of perceived thirst and esophageal temDerature. Values are means t SE. Esophageal temperatures did not &ffer among treatments. *Overall me& perceived- thirst ratings of treatments CEG and G were significantly less than treatments WP and CE (i.e., significant treatment effect, P < 0.05).
than in the CE and CEG trials after 60 min of exercise (P < 0.05). The CE treatment resulted in higher respiratory exchange ratio (RER; Fig. 4), greater carbohydrate uxidation, and lower fat oxidation values than for the other three treatments (significant treatment effects, P < 0.05). Blood lactate levels (not shown) increased during exercise to ~2 mM and did not differ among treatments. Plasma renin activity (Fig. 5) increased during exercise in all trials but more so in the WP trial (significant treatment effect, P < 0.05), Serum aldusterune (Fig. 5) also increased during exercise with somewhat greater, but nonsignificant, increases in the WP and CE trials. Serum curtisol (nut shown) did nut change significantly during exercise fur any beverage treatment. Plasma antidiuretic hormone (ADH) values (Fig. 5) increased slightly with exercise, with the greatest increases with the G treatment (significant treatment effect, P < 0.05). Resting ADH values averaged 3.2 t 0.4 pg/ml. DISCUSSION
The present study was designed to determine whether ingestion of glycerol solutions during exercise results in an expansion of plasma volume, a reduction in heart rate and esophageal temperature, and an increase in sweat rate. A second purpose was to characterize the response of fluid-regulatory hormones to glycerol ingestion during exercise.
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GLYCEROL
INGESTION
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FIG. 3. Alterations in plasma glucose, free fatty acids (FFA), and glycerol. Values are means t SE. *Treatments GE, CEG > WP, G. ++Treatment CEG > WP, G. **Treatments WP, G > CE, CEG. +Treatment G > CEG, CE, WP. #Treatment CEG > WP, CE. All differences, P < 0.05.
When glycerol solutions are ingested at rest, a transient state of osmotically induced hyperhydration has been observed. After the ingestion of 1 g glycerol/kg body wt with 1.5 liters of water, Riedesel et al. (13) observed a decrease in urine output and the retention of -400 ml of fluid for at least 4 h. Using a similar beverage administration protocol, Lyons et al. (8) reported that hyperhydration with glycerol 2.5 h before exercise in the heat resulted in a reduced urine output, lower rectal temperature, and higher sweat rate than when the subjects ingested a similar amount of water. Although neither Riedesel et al. (13) nor Lyons et al. (8) observed glycerol-induced alterations in plasma volume, such changes were noted by Gleeson et al. (6), who studied the effects of ingestion of 400 ml of a glycerol solution (1 g glycerol/kg body wt), glucose (same dose), or water dacebo 45 min before cycling exercise to exhaus-
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tion. The glycerol feeding increased preexercise plasma osmolality and plasma volume. During exercise, both glycerol and glucose ingestion resulted in lesser changes in plasma volume than did the water placebo. Heart rate, sweat rate, and core temperature data were not reported. The extent to which glycerol ingestion induces alterations in plasma volume may be influenced by the timing of the glycerol feeding. Riedesel et al. (13) and Lyons et al. (8) fed glycerol to resting subjects 1 or 2 h before the first plasma volume measurements were made, allowing ample time for the glycerol to equilibrate across fluid compartments. Such hyperhydration with glycerol before exercise likely increases the absolute plasma volume, in which case plasma volume changes during exercise would be unaltered, whereas glycerol ingestion soon before (6) or during exercise (present study) provides a transient osmotic impetus to temporarily retain fluid in the vascular space, prompting the plasma volume changes observed in the present study. Ingestion of WP was associated with small changes in plasma volume that paralleled the changes provoked by consumption of the CE and glycerol beverages. This observation infers that a portion of the ingested water was temporarily retained in the vascular space. The comparatively greater plasma volume changes associated with ingestion of the other beverages were likely a result of the movement of fluid from the extravascular space to the vascular space in response to the entry of osmotically active solutes (i.e., glycerol, sodium, glucose). Further work is needed to examine the relative contributions of ingested and endogenous fluids to the maintenance of plasma volume during exercise. Our subjects showed no indication of the hyperhydration observed by Riedesel et al. (13) or Lyons et al. (8); i.e., there were no differences in urine volume or in changes in body weight among treatments. The absence of hyperhydration in our study is not surprising because our subjects consumed small volumes of fluid and glycerol at intervals during exercise rather than one large dose. In addition, the comparatively brief (900min) protocol employed in our study may not have been sufficient to observe the changes in urine production seen 2-8 h after glycerol ingestion (8, 13). Although plasma volume responses may vary accord-
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4. Respiratory exchange ratio (RER). Values are means -t SE. *Overall mean RER for treatment CE was significantly greater than for all other treatments (i.e., significant treatment effect, P < 0.05). FIG.
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148
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FIG. 5. Mean values for plasma renin activity (PRA), aldosterone, and antidiuretic hormone (ADH). Values are means k SE. *Overall mean PRA for treatment WP was significantly greater than for all other treatments (i.e., significant treatment effect, P < 0.05). Changes in aldosterone did not differ among beverage treatments. **Overall mean ADH concentration for treatment G was significantly greater than for all other treatments (i.e., significant treatment effect, P < 0.05).
ing to the method and timing of the experimental intervention, it is clear from the present study that the alterations in plasma volume that accompany glycerol feeding during exercise do not demonstrably influence cardiovascular or thermoregulatory function. Specifically, heart rate and sweat rate were unaltered by beverage treatment, and, although the increases in esophageal temperature tended to be mO.l-0.2’C less with the glycerol treatments than with WP, the same tendency occurred with ingestion of the CE beverage. The lower overall mean ratings of perceived thirst associated with consumption of the glycerol solutions may have been related to the comparatively greater plasma volumes (and therefore less of a volume-related impetus for stimulating thirst) or perhaps bv a central mecha-
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nism associated with decreased cerebrospinal fluid sodium (11,12). For whatever reason, it can be argued that reduced thirst is a negative attribute for those laboring or exercising in the heat when a diminished drive to consume fluids will exacerbate dehydration. The progressive rise in plasma glycerol seen with ingestion of the G beverage lends additional support to the contention that glycerol is not rapidly cleared from the blood. Unlike the rat (14), the human liver does not possess the gluconeogenic capacity to rapidly convert glycerol to glucose (7,10). For this reason, and because glycerol cannot be directly metabolized by human muscle (7, lo), glycerol is not a viable exogenous substrate for consumption during exercise. Accordingly, exercise performance is not improved with glycerol feedings in humans (6, 9, 10). Fuel homeostasis was largely unaltered by glycerol feeding. Compared with ingestion of the WP, glycerol feedings did not affect changes in plasma glucose, FFA, lactate, RER, or serum cortisol. These results are similar to the responses reported when glycerol is fed 30-45 min before exercise. That is, plasma glucose, insulin, lactate, P-hydroxybutyrate, and FFA remain either unchanged (10) or vary slightly from the WP treatment (6,9). However, it is interesting to note that although the CEG beverage maintained blood glucose and prevented the rise in FFA, it was ineffective in raising RER or carbohydrate oxidation values in comparison to the CE beverage. How the presence of glycerol in the CEG and G beverages served to decrease carbohydrate oxidation and/or increase fat oxidation values remains unclear. Glycerol ingestion was not associated with large changes in fluid-regulatory hormones. The renin-angiotensin-aldosterone system was not differentially influenced by the glycerol treatments, and increases in ADH levels with ingestion of the G beverage appear to parallel similar increases in plasma osmolality. It is possible that the experimental protocol was not of sufficient duration for the alterations in ADH to be reflected by changes in urine production. The argument could be made that ingestion of larger amounts or concentrations of glycerol might result in hormonal and plasma volume changes of a magnitude sufficient to alter cardiovascular and thermoregulatory response, It could also be argued that feeding glycerol in a single dose before exercise might produce larger and more sustainable plasma volume changes than those produced by feeding comparatively small doses of glycerol during exercise. However, ingestion of large amounts of glycerol increases the risk of inducing cerebral and intraocular dehydration. Because glycerol enters the brain, cerebrospinal fluid, and aqueous humor at extremely slow rates, ingestion of glycerol has been used clinically to reduce cerebral edema and intraocular pressure (7,15). Headaches, dizziness, nausea, and vomiting are common side effects of glycerol ingestion (6,15); the increased incidence of nausea, bloated feeling, and light-headedness with ingestion of the glycerol solutions in this study corroborates the likelihood of at least some modest negative side effects with glycerol ingestion. However, none of the subjects complained of headaches with
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GLYCEROL
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the . glycerol treatments, possibly because the glycerol solut1 .ons were relatively dilute. In summary, although the ingestion of hypertonic glycerol solutions du ring exercise attenuates the normal decrease in plasma volume associated with cycling exercise, no demonstrably significant cardiovascular or thermoregulatory responses were observed. While the use of glycerol as a hyperhydrating and plasma-expanding agent warrants further investigation, ingestion of glycerol solutions is not without noticeable side effects in some subjects, the severity of which is known to increase with the consumption of larger doses. The authors thank Sue Pruden for medical and technical assistance throughout this project. They also thank David R. Lamb for excellent suggestions in manuscript preparation. Address for reprint requests: R. Murray, Exercise Physiology Laboratory, The Quaker Oats Co., 617 W. Main St., Barrington, IL 60010. Received 12 July 1990; accepted in final form 4 March 1991. REFERENCES 1. DESCHAMPS, A.,R. D. LEVY, M.G.Cosro,E.B. MARLISS,AND~. MADGER. Effect of saline infusion on body temperature and endurance during heavy exercise. j. Appl. Physid. 66: 2799-2804, 1989. 2. DILL, D. B., AND D, L. COSTILL. Calculation of percentage changes in volumes of blood, plasma, and red cells in dehydration. J. Appl. Physiol.
37: 247-248,1974.
3. FURTNEY, S. M., E. R. NADEL, C. B. WENGER, AND J. R. BOVE. Effect of blood volume on sweating rate and body fluids in exercising humans. J. Appl. PhysioL 51: 1594-1600, 1981. 4. FORTNEY,~. M., N. B. VRUMAN, W.S. BECKETT,~. PERMUTT, AND N. D. LAFRANCE. Effect of exercise hemoconcentration and hyperosmolality on exercise responses, J. Appl. Physiol. 65: 519524, 1988. 5. FURTNEY, S. M., C. B. WENGER, J. R. BOVE, AND E. R. NADEL.
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Effect of blood volume on forearm venous and cardiac stroke volume during exercise. J. Appl. Physiol. 55: 884-890, 1983. 6. GLEESON, M.,R.J. MAUGHAN, ANDP.L. GREEN-. Comparison of the effects of pre-exercise feeding of glucose, glycerol, and placebo on endurance and fuel homeostasis in man. Eur. J. Appl. Physid. Occup. Physiol. 55: 645-653, 1986. 7. LIN, E. C. C. Glycerol utilization and its regulation in mammals. Annu.
Rev. Biochem.
46: 765-795,1977.
8. LYONS, M.P.,M. L. RIEDESEL, L. MEULI,AND T. W. CHICK. Effects of glycerol-induced hyperhydration prior to exercise in the heat on sweating and core temperature. 1Med. Sci. Sports Exercise 22: 477-483,199O. 9. MAUGHAN, R. J., AND M. GLEESON. Influence of a 36 h fast followed by refeeding with glucose, glycerol, or placebo on metabolism and performance during prolonged exercise in man. Eur. J. Appl. Physiol. Occup. Physiol. 57: 570-576, 1988. 10. MILLER, J.M.,E.F.COYLE, W.M. SHERMAN, J.M. HAGBERG, D. L. COSTILL, W. J. FINK, S. E. TERBLANCHE, AND J. 0. HOLLOSZY. Effect of glycerol feeding on endurance and metabolism during prolonged exercise in man. 1Med. Sci. Sports Exercise 15: 237-242,1983. 11. OLSSON, K.,F. FYHRQUIST, B. LARSSON,AND L. ERIKSON. Inhibition of vasopressin-release during developing hypernatremia and plasma hyperosmolality: an effect of intracerebroventricular glycerol. Acta Physiol. Scand. 102: 399-409, 1978. 12. OLSSON, K., B. LARSSON, AND E. LUEKVIST. Intracerebroventricular glycerol: a potent inhibitor of ADH-release and thirst, Acta Physiol.
Scar&
98: 47O-477,1976.
13. RIEDESEL, M.L.,D.Y. ALLEN,G. T. PEAKE,AND K. AL-QATTAN. Hyperhydration with glycerol solutions. J. AppE. Physiol. 63: 22622268,1987. 14. TERBLANCHE,~. E.,R.D. FELL, A.C. JUHLIN-DANNFELT, B. W. CRAIG, AND J. 0. HOLLOSZY. Effects of glycerol feeding before and after exhausting exercise in rats. J. Appl. Physicd. 50: 94-101,1981. 15. TOURTELLOTTE, W. W., R. L. REINGLASS, AND T. A, NEWKIRK. Cerebral dehydration action of glycerol. Clin. Pharmacol. Z’her. 13: 159-171,1972. 16. WINKLER, B.,R. STEEL,ANDN.ALTSZULER. Relationshipofglycerol uptake to plasma glycerol concentration in the normal dog. Am. J. Physiol. 216: 191-196, 1969.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (130.212.018.096) on January 2, 2019.
tained the plasma volume of exercising subjects at resting levels (4) or 8-9% above resting levels (3, 5) with infusions of albumin and saline. Under these conditions, heart rate and esophageal temperature during exercise were significantly lower, and there were tendencies for increased skin blood flow and reduced sweat rate. Such reductions in cardiovascular and thermoregulatory strain would ostensibly be of benefit to athletes, workers, and military personnel who exercise in the heat. Similar positive cardiovascular and thermoregulatory responses might also be expected to result from the ingestion of glycerol solutions during exercise. Glycerol is an osmotically active solute that, after ingestion, is eventually distributed throughout the body water and is removed slowly from the intravascular space via hepatic and renal metabolism (15, 16). Consequently, ingestion of glycerol results in increased plasma osmolality, reduced urine volume, and expanded plasma volume (6, 13). Furthermore, glycerol ingestion before exercise has been reported to be associated with reduced core temperature and increased sweat rate during exercise in the heat (8). For these reasons, we studied the effects of the consumption of glycerol during steady-state cycling exercise to determine if positive thermoregulatory, cardiovascular, and hormonal responses are indeed associated with carbohydrate-electrolyte beverages;plasmavolume expansion; glycerol ingestion. To our knowledge, this is the first rehyperhydration port of the effects of glycerol ingestion during exercise. THE ATTENUATION of the normal
decrease in plasma volume during exercise presumably helps maintain central blood volume and allows for reduced heart rate and increased skin blood flow. When the normal decrease in plasma volume associated with cycling exercise is attenuated by the experimental infusion of albumin and saline, cure temperature, heart rate, and sweat rate are reduced, while stroke volume and skin blood flow increase (4). The rise in skin blood flow is postulated to promote an increase in radiative heat loss and decreases in sweat rate and esophageal temperature. For example, Deschamps et al. (1) recently reported that when the plasma volume of exercising subjects was maintained near resting levels by infusion of isotonic saline, mean heart rate and esophageal temperature were significantly lower than in the control (no-infusion) trial. Similar evidence is provided by the work of Fortney et al. (3-9, who main144
0161-7567/91$1.50
METHODS
Five women and four men served as subjects in this study. All subjects gave informed consent before participating in testing. The experimental protocol was reviewed and approved by an institutional human subjects review committee. The subjects’ age, weight, lean body mass (LBM), and peak 0, uptake (VO,~& values were 30.2 t 6.4 (SD) yr, 68.6 t 14.5 kg, 54.1 t 11.4 kg, and44.2 t 7.2 ml kg-’ min? All subjects were nonathletes and engaged in modest physical activity two to three times per week. To familiarize the subjects with the demands of the test procedure, each subject completed at least three orientation sessions and one sham test. All exercise sessions were conducted in the afternoon during March and April 1989. In an effort to minimize training and acclimatization responses, at least 7 days (range 7-14 days) separated each test. Subjects reported l
l
Copyright 0 1991 the American Physiological Society
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GLYCEROL
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to the laboratory 3-4 h after a light lunch. To help ensure an adequate and consistent state of hydration before each exercise session, subjects were encouraged to drink water before reporting to the laboratory. After 20 min of seated rest in a climate chamber, the subjects performed 90 min of cycling exercise at 51.8 t 0.8% VOZpeak in a warm (30°C and 45% relative humidity). . environment VO 2peakwas determined during a progressive-intensity exercise test on a cycle ergometer using standard opencircuit spirometric techniques. LBM was calculated using conventional hydrostatic weighing procedures. The four experimental beverages were given in counterbalanced order with subjects serving as their own controls. The beverages were a water placebo (WP; 27 mosmol/kgH,O), a carbohydrate-electrolyte beverage (CE; 6% glucose, 20 mM Na+, 3 mM K’, 11 mM Cl-, 3 mM H,PO;, 9 mM citrate-; 405 mosmol/kgH,O), the carbohydrate-electrolyte beverage plus 4% glycerol (CEG; 928 mosmol/kgH,0), and a 10% glycerol solution (G; 1,275 mosmol/kgH,O). Subjects consumed 3.0 ml beverage/kg LBM (162 t 8 ml) at 15,30,45, and 60 min during exercise, providing a total fluid intake of 647 t 33.0 (SD) ml. Beverages were of similar sweetness (aspartame, in WP), color, and flavor, and were served chilled (1OOC) in plastic squeeze bottles. The total glycerol dose provided by the G solution was 1.2 g glycerol/kg LBM (0.94 g glycerol/kg body wt). Expired air was collected in 12O-liter meteorological balloons at 20,50, and 80 min during exercise; 600 ml of each sample were analyzed for 0, and CO2 content, and ventilatory volume was measured by passing the expired air through an air flowmeter. Blood samples were collected from indwelling 18, gauge forearm venous catheters after 15 min of seated rest and at 13,28,43,58,68,78, and 88 min during exercise. The catheters were kept patent with periodic injections of 10 U/ml heparin in saline. Aliquots of each blood sample were analyzed in duplicate for lactate (model 27 Analyzer, Yellow Springs Instruments), hematocrit (microhematocrit technique), and hemoglobin (model 482 Co-Oximeter, Instrumentation Laboratories). Hematocrit was measured using a digital micrometer to ensure accuracy. Changes in plasma volume were calculated from hemoglobin and hematocrit values by use of the equations of Dill and Costill (2). Plasma was collected from aliquots of each blood sample by centrifugation at 1,000 g for 15 min at 4°C and was subsequently analyzed for glucose (Ames Seralyzer reflectance spectrophotometer), osmolality (Advanced Instruments 3M0 MicroOsmometer), glycerol (Boehringer-Mannheim test kit), free fatty acids (FFA; NEFA C kit, Wako Pure Chemical Industries), reniniangiotensin I [plasma renin activity radioimmunoassay (RIA) kit, Dade Baxter Travenol Diagnostics], and vasopressin (vasopressin RIA kit, Euro-Diagnostics). Serum samples were assayed for aldosterone and cortisol (RIA kits, Diagnostic Products). Spectrophotometric measures were conducted on a Perkin-Elmer model 55 spectrophotometer; immunoreactivity assays were completed using a Packard Minaxi 5000 Auto-Gamma Counter. Esophageal temperature (model 402 thermister and model 47 telethermometer, Yellow Springs Instruments)
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FIG. 1. Changes in plasma volume and plasma osmolality during exercise. Values are means t SE. #Treatment G > CE, WP. ##Treatments G, CEG > CE, WP. +-Treatment G > CEG, CE, WP. *Treatment CEG > CE, WP. **Treatment CEG > WP. All differences, P < 0.05.
was measured at the level of the left atrium. Heart rates (Vantage heart rate monitor) and ratings of perceived exertion (Borg scale) were recorded at rest and at lo- to 15-min intervals during exercise. Ratings of perceived thirst were recorded on a O-100 point scale at 15- to 20min intervals. Subject complaints (e.g., bloated feeling, nausea, lightheadedness) were also recorded. Immediately after exercise, subjects were encouraged to urinate; urine volume was subsequently measured. Nude dry body weight (Mettler ID2 Multirange scale) was recorded to the nearest 2 g before and after exercise. Total body sweat rates were calculated from changes in body weight corrected for urine production, blood sampling volume, fluid volume consumed, body surface area, and expiratory water loss. Data were analyzed for treatment effects by repeatedmeasures two-way analysis of variance. Duncan’s multiple range post hoc tests were conducted when significant (P < 0.05) F ratios were obtained. A nonparametric oneway analysis of variance ranking procedure (SAS, Cary, NC) was used to evaluate the frequency of the subjective complaints recorded in response to the beverage treatments. All data are reported as means t SE unless otherwise indicated. RESULTS
The changes in plasma volume associated with beverage treatment are illustrated in Fig. 1. During the initial 30 min of exercise, plasma volume decreased 7-10%. At 43 min, plasma volume increased toward baseline in all beverage treatments, with the greatest changes in
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plasma volume associated with ingestion of the two:glycerol beverages. This plasma volume change likely occurred as a result of ingestion of fluid at 15 and 30 min. By 60 min, plasma volume had decreased somewhat in all treatments. The final fluid ingestion occurred at 60 min, and the subsequent increases in plasma volume at 68 and 78 min ostensibly reflect the entry of that ingested fluid into the vascular space. The decrease in plasma volume noted at 88 min demonstrates the transient nature of the influence that ingesting fluid has on plasma volume. Plasma osmolality during exercise is also shown in Fig. 1. We encouraged all subjects to consume water ad libiturn before reporting to the laboratory in an attempt to keep preexercise plasma osmolality values below 290 musmul/kgH,O. Despite this precaution, preexercise plasma usmolalities averaged between 293 and 296 mosmul/kgH,O fur all treatments. However, there were no differences am .ung treatments. Plasma osmolalities changed similarly for all beverage treatments throughout the first 40 min of exercise. Thereafter, plasma osmulality ruse markedly in treatment G, reaching an end-exercise peak of 312 musmol/kgH,O. Mean exercise heart rates ranged between 135 and 145 t U-2.0 beats/min in each trial and were not affected by the alterations in plasma volume that occurred during exercise. Ratings of perceived exertion rose progressively throughout exercise to final values of 3.5 to 4.5 but were not influenced by beverage treatment. Perceived thirst increased thruughout exercise to a value of 37.8 t 2.4 units (O-100 scale; 100 = extremely thirsty) at 86 min of exercise . Although no statistically significant interactions were noted among the ratings of perceived thirst, the overall mean ratings of perceived thirst (Fig. 2) were lower with treatments CEG and G than with WP or CE (significant treatment effect, P < 0.05). The frequency of subjective complaints was significantly greater with the G treatment; four of the nine subjects reported symptoms of nausea, bloated feeling in the abdomen, and lightheadedness. After 90 min of exercise, esophageal temperature (Fig. 2) had increased -0.6YZ (to 37.5”C) from resting values in all treatments. Although mean esophageal temperature tended to be mO.l-0.2'C higher during exercise with WP than with the uther treatments, the differences were not statistically significant. Total body sweat rates were not influenced by beverage treatment: WP = 292.2 t 41.4, CE = 380.7 t 48.5, CEG = 302.3 t 2L0, G = 307.6 $- 28.8 mm2 qh-l. Postexercise urine volumes averaged 220 t 30 ml and did not differ among treatments. Changes in plasma glucose, glycerol, and FFA values are illustrated in Fig. 3. As expected, the CE and CEG treatments resulted in significant increases in plasma glucose, whereas the plasma glucose values in treatments G and WP did not change appreciably from rest. Also as expected, plasma glycerol increased significantly in both the G and CEG trials, with glycerol rising to the highest levels (P < 0.05) with treatment G. Urine glycerol concentration (nut shown) was 6.0 t 1.0 mmulI1 for G, 0.5 t 0.2 mmul/l for CEG, and remained at near-zero levels in CE and WP. FFA levels were greater in G and WP, the two treatments that did not contain carbohydrate, g
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DURING 3 k? l -
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FIG. 2. Changes in ratings of perceived thirst and esophageal temDerature. Values are means t SE. Esophageal temperatures did not &ffer among treatments. *Overall me& perceived- thirst ratings of treatments CEG and G were significantly less than treatments WP and CE (i.e., significant treatment effect, P < 0.05).
than in the CE and CEG trials after 60 min of exercise (P < 0.05). The CE treatment resulted in higher respiratory exchange ratio (RER; Fig. 4), greater carbohydrate uxidation, and lower fat oxidation values than for the other three treatments (significant treatment effects, P < 0.05). Blood lactate levels (not shown) increased during exercise to ~2 mM and did not differ among treatments. Plasma renin activity (Fig. 5) increased during exercise in all trials but more so in the WP trial (significant treatment effect, P < 0.05), Serum aldusterune (Fig. 5) also increased during exercise with somewhat greater, but nonsignificant, increases in the WP and CE trials. Serum curtisol (nut shown) did nut change significantly during exercise fur any beverage treatment. Plasma antidiuretic hormone (ADH) values (Fig. 5) increased slightly with exercise, with the greatest increases with the G treatment (significant treatment effect, P < 0.05). Resting ADH values averaged 3.2 t 0.4 pg/ml. DISCUSSION
The present study was designed to determine whether ingestion of glycerol solutions during exercise results in an expansion of plasma volume, a reduction in heart rate and esophageal temperature, and an increase in sweat rate. A second purpose was to characterize the response of fluid-regulatory hormones to glycerol ingestion during exercise.
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GLYCEROL
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FIG. 3. Alterations in plasma glucose, free fatty acids (FFA), and glycerol. Values are means t SE. *Treatments GE, CEG > WP, G. ++Treatment CEG > WP, G. **Treatments WP, G > CE, CEG. +Treatment G > CEG, CE, WP. #Treatment CEG > WP, CE. All differences, P < 0.05.
When glycerol solutions are ingested at rest, a transient state of osmotically induced hyperhydration has been observed. After the ingestion of 1 g glycerol/kg body wt with 1.5 liters of water, Riedesel et al. (13) observed a decrease in urine output and the retention of -400 ml of fluid for at least 4 h. Using a similar beverage administration protocol, Lyons et al. (8) reported that hyperhydration with glycerol 2.5 h before exercise in the heat resulted in a reduced urine output, lower rectal temperature, and higher sweat rate than when the subjects ingested a similar amount of water. Although neither Riedesel et al. (13) nor Lyons et al. (8) observed glycerol-induced alterations in plasma volume, such changes were noted by Gleeson et al. (6), who studied the effects of ingestion of 400 ml of a glycerol solution (1 g glycerol/kg body wt), glucose (same dose), or water dacebo 45 min before cycling exercise to exhaus-
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tion. The glycerol feeding increased preexercise plasma osmolality and plasma volume. During exercise, both glycerol and glucose ingestion resulted in lesser changes in plasma volume than did the water placebo. Heart rate, sweat rate, and core temperature data were not reported. The extent to which glycerol ingestion induces alterations in plasma volume may be influenced by the timing of the glycerol feeding. Riedesel et al. (13) and Lyons et al. (8) fed glycerol to resting subjects 1 or 2 h before the first plasma volume measurements were made, allowing ample time for the glycerol to equilibrate across fluid compartments. Such hyperhydration with glycerol before exercise likely increases the absolute plasma volume, in which case plasma volume changes during exercise would be unaltered, whereas glycerol ingestion soon before (6) or during exercise (present study) provides a transient osmotic impetus to temporarily retain fluid in the vascular space, prompting the plasma volume changes observed in the present study. Ingestion of WP was associated with small changes in plasma volume that paralleled the changes provoked by consumption of the CE and glycerol beverages. This observation infers that a portion of the ingested water was temporarily retained in the vascular space. The comparatively greater plasma volume changes associated with ingestion of the other beverages were likely a result of the movement of fluid from the extravascular space to the vascular space in response to the entry of osmotically active solutes (i.e., glycerol, sodium, glucose). Further work is needed to examine the relative contributions of ingested and endogenous fluids to the maintenance of plasma volume during exercise. Our subjects showed no indication of the hyperhydration observed by Riedesel et al. (13) or Lyons et al. (8); i.e., there were no differences in urine volume or in changes in body weight among treatments. The absence of hyperhydration in our study is not surprising because our subjects consumed small volumes of fluid and glycerol at intervals during exercise rather than one large dose. In addition, the comparatively brief (900min) protocol employed in our study may not have been sufficient to observe the changes in urine production seen 2-8 h after glycerol ingestion (8, 13). Although plasma volume responses may vary accord-
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4. Respiratory exchange ratio (RER). Values are means -t SE. *Overall mean RER for treatment CE was significantly greater than for all other treatments (i.e., significant treatment effect, P < 0.05). FIG.
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148
GLYCEROL
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FIG. 5. Mean values for plasma renin activity (PRA), aldosterone, and antidiuretic hormone (ADH). Values are means k SE. *Overall mean PRA for treatment WP was significantly greater than for all other treatments (i.e., significant treatment effect, P < 0.05). Changes in aldosterone did not differ among beverage treatments. **Overall mean ADH concentration for treatment G was significantly greater than for all other treatments (i.e., significant treatment effect, P < 0.05).
ing to the method and timing of the experimental intervention, it is clear from the present study that the alterations in plasma volume that accompany glycerol feeding during exercise do not demonstrably influence cardiovascular or thermoregulatory function. Specifically, heart rate and sweat rate were unaltered by beverage treatment, and, although the increases in esophageal temperature tended to be mO.l-0.2’C less with the glycerol treatments than with WP, the same tendency occurred with ingestion of the CE beverage. The lower overall mean ratings of perceived thirst associated with consumption of the glycerol solutions may have been related to the comparatively greater plasma volumes (and therefore less of a volume-related impetus for stimulating thirst) or perhaps bv a central mecha-
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nism associated with decreased cerebrospinal fluid sodium (11,12). For whatever reason, it can be argued that reduced thirst is a negative attribute for those laboring or exercising in the heat when a diminished drive to consume fluids will exacerbate dehydration. The progressive rise in plasma glycerol seen with ingestion of the G beverage lends additional support to the contention that glycerol is not rapidly cleared from the blood. Unlike the rat (14), the human liver does not possess the gluconeogenic capacity to rapidly convert glycerol to glucose (7,10). For this reason, and because glycerol cannot be directly metabolized by human muscle (7, lo), glycerol is not a viable exogenous substrate for consumption during exercise. Accordingly, exercise performance is not improved with glycerol feedings in humans (6, 9, 10). Fuel homeostasis was largely unaltered by glycerol feeding. Compared with ingestion of the WP, glycerol feedings did not affect changes in plasma glucose, FFA, lactate, RER, or serum cortisol. These results are similar to the responses reported when glycerol is fed 30-45 min before exercise. That is, plasma glucose, insulin, lactate, P-hydroxybutyrate, and FFA remain either unchanged (10) or vary slightly from the WP treatment (6,9). However, it is interesting to note that although the CEG beverage maintained blood glucose and prevented the rise in FFA, it was ineffective in raising RER or carbohydrate oxidation values in comparison to the CE beverage. How the presence of glycerol in the CEG and G beverages served to decrease carbohydrate oxidation and/or increase fat oxidation values remains unclear. Glycerol ingestion was not associated with large changes in fluid-regulatory hormones. The renin-angiotensin-aldosterone system was not differentially influenced by the glycerol treatments, and increases in ADH levels with ingestion of the G beverage appear to parallel similar increases in plasma osmolality. It is possible that the experimental protocol was not of sufficient duration for the alterations in ADH to be reflected by changes in urine production. The argument could be made that ingestion of larger amounts or concentrations of glycerol might result in hormonal and plasma volume changes of a magnitude sufficient to alter cardiovascular and thermoregulatory response, It could also be argued that feeding glycerol in a single dose before exercise might produce larger and more sustainable plasma volume changes than those produced by feeding comparatively small doses of glycerol during exercise. However, ingestion of large amounts of glycerol increases the risk of inducing cerebral and intraocular dehydration. Because glycerol enters the brain, cerebrospinal fluid, and aqueous humor at extremely slow rates, ingestion of glycerol has been used clinically to reduce cerebral edema and intraocular pressure (7,15). Headaches, dizziness, nausea, and vomiting are common side effects of glycerol ingestion (6,15); the increased incidence of nausea, bloated feeling, and light-headedness with ingestion of the glycerol solutions in this study corroborates the likelihood of at least some modest negative side effects with glycerol ingestion. However, none of the subjects complained of headaches with
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GLYCEROL
INGESTION
the . glycerol treatments, possibly because the glycerol solut1 .ons were relatively dilute. In summary, although the ingestion of hypertonic glycerol solutions du ring exercise attenuates the normal decrease in plasma volume associated with cycling exercise, no demonstrably significant cardiovascular or thermoregulatory responses were observed. While the use of glycerol as a hyperhydrating and plasma-expanding agent warrants further investigation, ingestion of glycerol solutions is not without noticeable side effects in some subjects, the severity of which is known to increase with the consumption of larger doses. The authors thank Sue Pruden for medical and technical assistance throughout this project. They also thank David R. Lamb for excellent suggestions in manuscript preparation. Address for reprint requests: R. Murray, Exercise Physiology Laboratory, The Quaker Oats Co., 617 W. Main St., Barrington, IL 60010. Received 12 July 1990; accepted in final form 4 March 1991. REFERENCES 1. DESCHAMPS, A.,R. D. LEVY, M.G.Cosro,E.B. MARLISS,AND~. MADGER. Effect of saline infusion on body temperature and endurance during heavy exercise. j. Appl. Physid. 66: 2799-2804, 1989. 2. DILL, D. B., AND D, L. COSTILL. Calculation of percentage changes in volumes of blood, plasma, and red cells in dehydration. J. Appl. Physiol.
37: 247-248,1974.
3. FURTNEY, S. M., E. R. NADEL, C. B. WENGER, AND J. R. BOVE. Effect of blood volume on sweating rate and body fluids in exercising humans. J. Appl. PhysioL 51: 1594-1600, 1981. 4. FORTNEY,~. M., N. B. VRUMAN, W.S. BECKETT,~. PERMUTT, AND N. D. LAFRANCE. Effect of exercise hemoconcentration and hyperosmolality on exercise responses, J. Appl. Physiol. 65: 519524, 1988. 5. FURTNEY, S. M., C. B. WENGER, J. R. BOVE, AND E. R. NADEL.
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Effect of blood volume on forearm venous and cardiac stroke volume during exercise. J. Appl. Physiol. 55: 884-890, 1983. 6. GLEESON, M.,R.J. MAUGHAN, ANDP.L. GREEN-. Comparison of the effects of pre-exercise feeding of glucose, glycerol, and placebo on endurance and fuel homeostasis in man. Eur. J. Appl. Physid. Occup. Physiol. 55: 645-653, 1986. 7. LIN, E. C. C. Glycerol utilization and its regulation in mammals. Annu.
Rev. Biochem.
46: 765-795,1977.
8. LYONS, M.P.,M. L. RIEDESEL, L. MEULI,AND T. W. CHICK. Effects of glycerol-induced hyperhydration prior to exercise in the heat on sweating and core temperature. 1Med. Sci. Sports Exercise 22: 477-483,199O. 9. MAUGHAN, R. J., AND M. GLEESON. Influence of a 36 h fast followed by refeeding with glucose, glycerol, or placebo on metabolism and performance during prolonged exercise in man. Eur. J. Appl. Physiol. Occup. Physiol. 57: 570-576, 1988. 10. MILLER, J.M.,E.F.COYLE, W.M. SHERMAN, J.M. HAGBERG, D. L. COSTILL, W. J. FINK, S. E. TERBLANCHE, AND J. 0. HOLLOSZY. Effect of glycerol feeding on endurance and metabolism during prolonged exercise in man. 1Med. Sci. Sports Exercise 15: 237-242,1983. 11. OLSSON, K.,F. FYHRQUIST, B. LARSSON,AND L. ERIKSON. Inhibition of vasopressin-release during developing hypernatremia and plasma hyperosmolality: an effect of intracerebroventricular glycerol. Acta Physiol. Scand. 102: 399-409, 1978. 12. OLSSON, K., B. LARSSON, AND E. LUEKVIST. Intracerebroventricular glycerol: a potent inhibitor of ADH-release and thirst, Acta Physiol.
Scar&
98: 47O-477,1976.
13. RIEDESEL, M.L.,D.Y. ALLEN,G. T. PEAKE,AND K. AL-QATTAN. Hyperhydration with glycerol solutions. J. AppE. Physiol. 63: 22622268,1987. 14. TERBLANCHE,~. E.,R.D. FELL, A.C. JUHLIN-DANNFELT, B. W. CRAIG, AND J. 0. HOLLOSZY. Effects of glycerol feeding before and after exhausting exercise in rats. J. Appl. Physicd. 50: 94-101,1981. 15. TOURTELLOTTE, W. W., R. L. REINGLASS, AND T. A, NEWKIRK. Cerebral dehydration action of glycerol. Clin. Pharmacol. Z’her. 13: 159-171,1972. 16. WINKLER, B.,R. STEEL,ANDN.ALTSZULER. Relationshipofglycerol uptake to plasma glycerol concentration in the normal dog. Am. J. Physiol. 216: 191-196, 1969.
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