to Beat the Heat?
R A Robergs
Affiliations: R.A. Robergs,
PhD, Center for Exercise and Applied Human Physiology, University of New
Mexico, Albuquerque, New Mexico, USA
Acknowledgments: J. Andrew Doyle (reviewer), Michael J. Rennie (reviewer), Mary Ann Wallace (editing), Will G. Hopkins (editing)
Correspondence: rrobergs=AT=unm.edu (R.A. Robergs)
Reference: Robergs, R.A. (1998). Glycerol hyperhydration to beat the heat? Sportscience Training & Technology http://www.sportsci.org/traintech/glycerol/rar.htm
Date: Jan 1998
Summary. Extra water ingested with glycerol is held in the body for several hours and may enhance performance by a few percent in events lasting more than an hour in hot and humid conditions. Headaches, blurred vision, and stomach upsets are side effects for some athletes who take too much too quickly. More research is needed to determine whether adequate intake of a sports drink containing salt and carbohydrate gives similar gains in performance.
Exercise performed for more than an hour in a hot and humid environment can cause fluid loss in excess of 3 liters. Loss of even 1.5 liters reduces endurance performance, possibly through reduction in delivery of blood to the muscles and skin. Many athletes do not drink enough to offset this effect of dehydration during competition, even when given unlimited access to fluid (Noakes, 1993). Any strategy that helps the athlete to take on extra fluid or "hyperhydrate" is therefore likely to enhance performance in long hot endurance events.
It's possible to increase the amount of fluid in the body by drinking extra water, but the kidneys remove most of it within an hour. That's where glycerol comes in. Adding glycerol to the water can prolong the period of hyperhydration for up to four hours (Reidesel et al., 1987, Lyons et al., 1990). So what is glycerol, and how does it work?
Glycerol is a three-carbon molecule similar to alcohol. It occurs naturally in the body as a component of stored fat; a small amount is also present in body fluids as free glycerol. When glycerol is ingested, it is absorbed and increases the concentration (technical term: osmolarity or tonicity) of the fluid in the blood and tissues. The concentration of these fluids is held constant by the body, so water consumed with the glycerol is not excreted until the extra glycerol is either removed by the kidneys or broken down by the body (Freund et al., 1995).
Glycerol has been used to treat swelling of the brain (cerebral edema) or of the eyes (glaucoma) (Frank, Nahata and Hilty, 1981). Because glycerol does not easily penetrate the brain and eyes, the increased concentration of glycerol in the blood following glycerol ingestion helps to remove excess fluid from these organs by a process known as osmosis. These clinical applications of glycerol ingestion explain two of the main side effects for athletes: headaches and blurred vision, a result of shrinkage of the brain and eyes (Freund et al., 1995).
Despite the interest in the effects of glycerol on athletic performance, to date there are only six published studies on the effects of glycerol on exercise, and two published studies on the effects of glycerol ingestion on body fluids. See Robergs and Griffin (1998) for a scholarly review of this research.
Evidence for Benefit of Glycerol
Riedesel et al. (1987) were first to document that ingestion of a glycerol solution can increase the water content of the body. Similar findings have been reported by other researchers (Latzka et al.(1997, Montner et al.(1996), Freund et al.,1995). The gain in body water is typically up to a liter, depending on the amount and timing of the ingestion.
Lyons et al. (1990) investigated whether glycerol hyperhydration altered sweating, regulation of body temperature, and cardiovascular function during exercise in a hot environment (42°C and 25% relative humidity). Six subjects of average fitness (maximum oxygen uptake averaging 42 ml/kg/min) completed three trials. Each trial involved fluid ingestion, followed by running for 1.5 hours at 60% of maximum oxygen uptake in an environmental chamber. The three trials differed in the fluid ingested: limited fluid intake (3.3 ml/kg of orange juice); no glycerol (28 ml/kg of orange juice and water over 4 hours); and glycerol (1.0 g/kg at time 0 and 0.1 g/kg at hours 3 and 4, total volume = 28 ml/kg). At 2.5 hours, glycerol ingestion had resulted in 500 ml less urine and 700 ml more total body water compared to the no glycerol trial. Subjects sweated more and had a smaller increase in core temperature throughout the 90 min of exercise during the glycerol trial. Glycerol ingestion did not significantly reduce exercise heart rate. These findings indicated that glycerol hyperhydration could improve evaporative cooling of the body during exercise in a hot environment.
A group of researchers I work with modified the glycerol hyperhydration regimen to test effects on exercise endurance in two studies (Montner et al., 1996). The studies were double blind (subjects and researchers did not know what treatment was being administered), and the treatments were crossed over (subjects received both treatments in random order).
In the first study, 11 subjects of moderate to high endurance fitness (maximum oxygen uptake of 61 ml/kg/min) consumed glycerol or a placebo of colored and flavored water over a 90-min period. An hour later, the subjects cycled at 74% of maximum until they could not maintain the cycling cadence above 60 rpm. No fluid was ingested during the exercise. Glycerol intake increased pre-exercise body water by 730 ml and decreased urine volume by 670 ml. During the glycerol trial, subjects exercised significantly longer to fatigue (94 vs 73 min). There were no real differences in heart rate or core temperature.
Athletes usually consume carbohydrate during long endurance events, so in the second study we added carbohydrate to see if glycerol ingestion would still enhance performance. Seven subjects of high endurance fitness (maximum oxygen uptake of 73 ml/kg/min) completed two trials as in the first study, but in both trials the subjects consumed a 5% glucose solution at the rate of 3 ml/kg of body weight every 20 min. The difference in body water was reduced to 100 ml, and the difference in urine volume was reduced to 92 ml, but glycerol still prolonged endurance time (123 vs 99 min). Glycerol also decreased exercise heart rate, but did not reduce the increase in core temperature.
The enhancement of performance of about 20% in these studies was for time to exhaustion at a fixed work load. In a real event, the enhancement of performance (time to complete a fixed distance) is certain to be much less. Until a more realistic test is used in research with glycerol, my best guess for the enhancement in an event is no more than a few percent.
The way in which glycerol enhanced performance in these studies is also not clear. There was no evidence of an increase in blood (plasma) volume, which would improve pumping of blood either to the skin to remove heat or to the muscles to maintain power output. Nor did we see a reduction in core temperature, in contrast to Lyons et al. (1990) in their study. These differences may have been due to the increased heat stress of the Lyons study as well as the higher level of fitness and associated heat acclimation of the subjects in our studies. It's also possible that glycerol works by increasing the amount of fluid inside cells rather than the amount in the circulation, in which case any effects on core temperature and blood would be incidental.
Negative Research Findings
Some studies have shown no benefit of glycerol. The discrepancies may be due to differences in the protocols for glycerol ingestion. It's also possible that the exercise and heat stress did not produce enough dehydration to reduce performance.
Murray et al. (1991) researched whether glycerol ingestion during exercise would improve cardiovascular function and body temperature regulation during 90 min of cycle ergometer exercise at 50% of maximum oxygen uptake in a hot environment (30°C and 45% relative humidity). Nine subjects of average fitness completed four trials that involved the ingestion of four different solutions: Gatorade (a carbohydrate-electrolyte sports drink), Gatorade plus 4% glycerol, 10% glycerol, and a placebo (water). The solutions were ingested every 15 min during the first hour, providing a total of 650 ml. No differences were found between the trials for changes in heart rate, core temperature, sweat rate, and perceived exertion. These negative results are not surprising, given that there was no pre-exercise hyperhydration and that the intensity of exercise was relatively low.
Two studies showing no benefit of glycerol ingestion were presented at the annual meeting of the American College of Sports Medicine in Denver in June 1997. In one of these studies, now fully published (Latzka et al., 1997), the intensity of exercise was not sufficiently high (45% of maximum oxygen uptake) to be of interest to serious athletes. In the other study (Sawka et al., 1997), exercise intensity was also not particularly high (55% of maximum oxygen uptake), and although endurance time was greater with glycerol relative to water alone (34 vs 31 min), the difference was not statistically significant (in other words, it may have been due to chance).
US Olympic Committee's Stance on Glycerol
The International Olympic Committee bans substances that increase the flow of urine (diuretics), because of the potential harmful effects of rapid fluid loss, as well as the practice of using diuretics to decrease the concentration of markers of steroids and other banned substances in the urine. Glycerol was formerly classified as a diuretic, but it is now accepted that there is little extra urine flow at doses between 1.0 to 1.5 g/kg. In September 1997, the US Olympic Committee removed the ban on glycerol.
How to Ingest Glycerol
There is minimal research and no consensus on the best strategy for glycerol ingestion. The greatest hyperhydration occurred in the study of Montner et al. (1996). For a 70-kg athlete the total volume ingested is nearly 2 liters, which may be excessive for running or other weight-bearing activity. The protocol involved starting glycerol ingestion 2.5 hours before exercise, as follows:
If the event lasts more than 2 hours, ingesting a 5% glycerol solution at the rate of 400-800 ml/h during the event may be beneficial (Koenigsberg et al.,1995; Lyons et al., 1990). In theory, though, a sports drink like Gatorade should be just as good.
Athletes should note well that the side effects of headaches and blurred vision may occur with higher doses. You are also likely to feel sick if you take too much glycerol or take it too concentrated (Montner et al., 1996). There is no advantage in increasing the intake above 1.2 g/kg, because the extra glycerol and water are excreted in the urine (Riedesel et al., 1987).
Conclusions and Further
Drinking a glycerol solution before a long hot endurance event is probably worthwhile for those athletes who don't drink enough fluid before and during the event. More research is needed to determine whether there is any benefit for athletes who do drink well. Drinks containing salt need to be looked at, because salt can also prolong hydration. Finally, the exercise tests need to simulate more closely the demands of real endurance and ultraendurance events in conditions that clearly challenge fluid balance.
Frank, M.S.B., Nahata, M.C., Hilty, M.D. (1981). Glycerol: a review of its pharmacology, pharmacokinetics, adverse reactions, and clinical use. Pharmacotherapy, 1, 147-160.
Freund, B.J., Montain, S.J., Young, A.J., Sawka, M.N., DeLuca, J.P., Pandolf, K.B., Valeri, C.R. (1995). Glycerol hyperhydration: hormonal, renal, and vascular fluid responses. Journal of Applied Physiology, 79, 2069-2077.
Koenigsberg, P.S., Martin, K.K., Hlava, H.R., Riedesel, M.L. (1995). Sustained hyperhydration with glycerol ingestion. Life Sciences, 5, 645-653.
Latzka, W.A., Sawka, M.N., Montain, S.J., Skrinar, G.S., Fielding, R.A., Matott, R.P., and Pandolf, K.B. (1997). Thermoregulatory effects during compensable exercise-heat stress. Journal of Applied Physiology, 83, 860-866.
Lyons, T.P., Riedesel, M.L., Meuli, L.E., Chick, T.W. (1990). Effects of glycerol-induced hyperhydration prior to exercise in the heat on sweating and core temperature. Medicine and Science in Sports and Exercise, 22, 477-483.
Montner, P., Stark, D.M., Riedesel, M.L., Murata, G., Robergs, R.A., Timms, M., Chick, T.W. (1996). Pre-exercise glycerol hydration improves cycling endurance time. International Journal of Sports Medicine, 17, 27-33.
Murray, R., Eddy, D.E., Paul, G.L., Seifert, J.G., Halaby, G.A. (1991). Physiological responses to glycerol ingestion during exercise. Journal of Applied Physiology, 71, 144-149.
Noakes, T.D. (1993). Fluid replacement during exercise. Exercise Sport Science Review, 21, 297-330.
Riedesel, M.L., Allen, D.Y., Peake, G.T., Al-Qattan, K. (1987). Hyperhydration with glycerol solutions. Journal of Applied Physiology, 63, 2262-2268.
Robergs, R.A. and Griffin, S.E. (accepted for publication, November, 1997). Glycerol: biochemistry, pharmacokinetics, clinical and applied applications. Sports Medicine.
Sawka, M.N., Latzka, W.A., Montain, S.J., Skrinnar, G.S., Fielding, R.A., and Pandolf, K.B. (1997). Hyperhydration: Thermal and cardiovascular effects during uncompensable exercise-heat stress. Medicine and Science in Sports and Exercise, 29, Abstract 760.
J Andrew Doyle PhD
Assistant Professor, Kinesiology and Health, Georgia State University, Atlanta, Georgia, USA; ACSM Health Fitness Director; member of the Sportscience website team.
This review states that significant hyperhydration can be achieved prior to exercise by ingestion of glycerol and water. The author presents an excellent rationale for how this may benefit the endurance athlete exercising in hot and/or humid environments, and carefully examines the few studies published to date.
While glycerol hyperhydration may hold great promise for improvement of thermoregulation and endurance performance, in my view there does not yet appear to be sufficient evidence from published studies to conclusively make this claim. Clearly, more research is needed to further elucidate the most effective ingestion strategy.
The current weight of evidence suggests a definite effect of glycerol in sustaining body hydration during exercise in dehydrating conditions. How glycerol works is not totally clear; we know that the kidneys don't excrete glycerol rapidly, so the glycerol stays in the body and holds water with it. But research is needed to find whether glycerol works by increasing the amount of fluid inside cells rather than the amount in the circulation.
Although a metabolic explanation for performance enhancement (e.g. the use of glycerol to provide more blood glucose) seems to be ruled out by the small amounts of glycerol required for effficacy of hydration, there still remains the puzzle of how the relatively small differences in extent of hydration reported in some studies resulted in such relatively large benefits in endurance. It is worth reiterating that benefits are likely to be major only if the exercise results in substantial dehydration. Complete confidence in the technique will come only with more knowledge of exactly how it works and where the glycerol and water go in the body.
The author noted the need for performance tests that closely simulate competitive events. A member of the Sportscience website team, Dave Martin, has just written us about a glycerol study at the Australian Institute of Sport, using just such a test. His comments follow.
David T Martin PhD
Department of Physiology and Applied Nutrition, Australian Institute of Sport, Canberra, Australia; member of the Sportscience website team. 15 Jan 98
A group at the AIS (S. Hitchins, L. Burke, K. Fallon, K. Yates, A.Tatterson, G.P. Dobson and me) found that hyperhydration with glycerol improved endurance cycling performance in hot humid conditions. We used a double-blind crossover design, in which eight competitive cyclists completed a maximal 60-min laboratory time-trial with and without glycerol. Our study differed from those reported in the review by Rob Robergs in two ways. First, we used a constant-duration test rather than a constant-power test to exhaustion. Second, our protocol differed from the recommended portocol by Montner et al. (1996), in that our subjects hyperhydrated at one time, 2.5-2.0 hours prior to the start of exercise.
Each time trial was preceded by ingestion of either a glycerol solution (1.0 g/kg in 22 ml/kg of half-strength Isosport sports drink) or a placebo solution (equal volume of half-strength Isosport). The time trial involved a 30-min fixed-power output phase followed by a 30-min variable power phase. Glycerol resulted in a 600 ml increase in total body water relative to placebo prior to the time trial. Cycling performance as indicated by the total amount of work performed in 60 min was 2.4% greater in the glycerol trial. Glycerol did not appear to affect rectal temperature, sweat rate, blood lactate concentration or plasma volume during the time trial, and there were no complaints of stomach upsets.
I agree with the author and reviewers that research is needed to explain how glycerol hyperhydration improves performance in hot, humid conditions. Someone also needs to study runners, because hyperhydration for these athletes represents extra weight that may offset the gains observed for cyclists.