Euhydration, hyperhydration, and the use of glycerol for improving body water status

2.19.01, by Eric Goulet (

Eric Goulet practices triathlon in his free time. He has completed several extreme triathlons. He also acts as a consultant in exercise physiology and coaches ultra-endurance athletes from his office in Quebec. He will soon launch a Web site specializing in the coaching of ultra-endurance athletes. He can be reached for questions by e-mail at

Most experienced triathletes are aware that dehydration can impair exercise capacity, cardiovascular and thermoregulatory functions, and increase the risk of heat injury. Paradoxically, though, rare is the triathlete who consumes enough fluid during exercise to compensate for his or her sweat loss. In fact, Dr. Tim Noakes, a physiology professor at the University of Cape Town in South Africa, has found that triathletes, on average, allow themselves to become dehydrated by up to 3% of their body weight during competitions. Interestingly, weight loss during triathlons appears to be relatively constant-and independent of the race's duration (from Olympic to Ironman). "It is as if total weight loss during exercise is a regulated variable," Dr. Noakes says.

So one might ask whether there's a hydration strategy out there that could prolong the period of time during which our body would perform with optimal water reserves. Yes, there is: It's called pre-exercise hyperhydration (a state of extra body water).

The advantage of beginning an exercise hyperhydrated is that it will delay or eliminate the onset of hypohydration if one fails to completely replace its sweat losses during exercise. Nevertheless––and I think it is unfortunate––the American College of Sports Medicine does not promote such a hydration strategy. Instead, it recommends that athletes begin any exercise euhydrated (a state of normal body water (see table at left) in the hope that they will avoid dehydration completely during exercise by consuming "fluids at a rate sufficient to replace all the water lost through sweating."

But given that most triathletes do not prevent dehydration during exercise, would it be possible or appropriate to think that, for certain athletic situations, it could be more advantageous for them to begin an exercise hyperhydrated? Probably yes! In fact, many studies have demonstrated that when athletes failed to adequately replace their sweat losses, those who began the exercise hyperhydrated, compared to euhydrated, improved at least one measure of cardiovascular function and/or temperature regulation.

So what kind of situations call for hyperhydration? Athletes should consider hyperhydration any time they "suspect" the exercise they intend to do could produce a loss of more than 1.5-2% of their body weight, or 2 to 3 pounds for a 150-pound person. Then, the decision to go forward with the hyperhydration procedure is made based on whether the advantages of hyperhydration are considered superior to its temporary disadvantages, which are an increased body weight, urine production and incidence of gastrointestinal discomfort.

Traditionally, the fluid of choice for hyperhydration has been water. But in the last five years, an increasing number of athletes are choosing to hyperhydrate exclusively with a glycerol solution (see table 2). Why? The kidneys are extremely efficient at rapidly excreting the excess water in water-induced hyperhydration, so the increase in total body water typically is transient. But when an athlete hyperhydrates with glycerol, its osmotic (soaking) property significantly reduces urine production and allows, during rest, to make the hyperhydration period last roughly twice as long. Proponents of glycerol argue that because its use allows an athlete to maintain an enhanced fluid reservoir for a longer period of time, cardiovascular and thermoregulatory functions will be better preserved during exercise, thereby improving exercise capacity. All this is fine, but what is glycerol exactly?

Glycerol is a three-carbon alcohol metabolite that the human body produces naturally. It is a safe, clear, syrupy, and exceptionally sweet liquid classified as legal by the International Olympic Committee and the U.S. Olympic Committee. It is available commercially from Advance Kinetics, InterNutria Sports and Twinlab. In certain individuals, regrettably, its ingestion can produce side effects such as nausea, vomiting, gastrointestinal distress, blurred vision, headaches and light-headedness. Additionally, women who are pregnant and individuals with diabetes, kidney, liver and cardiovascular disorders must consult their family physician before using it. To date, no research has evaluated the long-term effects of glycerol ingestion.

The ingestion of 20-26 milliliters of water per kilogram of body weight within 90 or 30 minutes, 1 and 2 hours before the onset of exercise, respectively, will typically increase total body water by about 450 milliliters. In comparison, when glycerol at a dose of 1-1.5 grams per kilogram of body weight is added to the water, total body water is enhanced by about another 500 milliliters.

Remarkably such a hydration advantage over plain water seems to have little or no impact on parameters such as plasma volume during rest or exercise and sweat rates, core temperature, heart rate or perceived exertion during exercise. More precisely, using both aforementioned methods of ingestion, none study out of 2 (0/2), 1/4, 1/4, 2/4, and 0/3 reported that glycerol hyperhydration, compared to water hyperhydration, significantly improved plasma volume, sweat rates, core temperature, heart rate or perceived exertion, respectively. Based on how hydrating with glycerol affected those parameters, we would imagine a similar result regarding its effect on exercise capacity, i.e., equivocal at best. Surprisingly, that is not the case! In fact, the only three studies that measured exercise capacity all reported a significant increase in performance (5, 22, and 24 %) with glycerol.

So should athletes be encouraged to use this particular substance? For the moment, my answer leans towards the negative. Here's why:

First, none of those three studies was able to identify the mechanism by which glycerol hyperhydration improved exercise capacity. In truth, except for a lower heart rate response (a mean of three beats per minute) in two studies, no other change in any single variable was found that could explain the connection between the water retention responses and enhanced exercise capacities. Such small decreases in heart rate can, at best, explain a small part (through a better maintained cardiac output), but certainly not all, of the whopping increases in exercise capacity.

Though it can't be completely ruled out that glycerol improved exercise capacity because of a yet-to-be-identified mechanism, it's important to mention that each of those studies has methodological flaws that might explain their confounding results. For example, no studies reported that the distinctive taste of glycerol was efficiently/totally masked. One study did not control for or standardize pre-exercise hydration, diet, and levels of activity, whereas another compared, at the onset of exercise, already dehydrated athletes with hyperhydrated athletes. Finally, a last study has shown that the increase in exercise capacity with glycerol was mainly due to 2 athletes-out of 11-who increased their exercise capacities by about 60 minutes, compared to about 6 minutes for the rest of the group. A closer examination of the data reveals that two subjects actually showed a decrease in exercise capacity. Therefore, a lack of reliability (reproducibility) of the exercise tests, not glycerol's effect, could well account for the jumps in exercise capacity. Unfortunately, the public is generally unaware of these small but essential facts.

Second, because glycerol hydration may produce significant side effects, exercise capacity may be negatively affected.

Third, in contrast with water hyperhydration, glycerol hyperhydration increases plasma osmolality by more than 10 mOsmol/kg, which could delay the onset of sweating, attenuate cutaneous blood flow, and reduce sweat rate. For instance, a team of researchers from the University of Idaho recently reported that, during a one-hour run at 65 percent of VO2 max, sweat rates were reduced by 7 milliliters/minute (420 milliliters/hour) with glycerol, compared to water. The body's inability to dissipate heat at an optimal speed will force the athlete to reduce her/his pace and thus reduce exercise capacity.

Fourth, with comparable increases in body weight immediately before exercise, researchers from the United States Army Research Institute of Environmental Medicine have demonstrated that glycerol and water were equally effective in decreasing urine production during exercise conducted in hot conditions. So the often used argument that glycerol considerably improves the retention of fluid during exercise-and therefore better protects exercise capacity, cardiovascular and thermoregulatory functions while decreasing the probability of needing to stop to urinate-is probably not a robust one anymore.

Finally, it would be doubtful that a large part of the athletic population could easily tolerate the necessary volume of water to be taken with glycerol before key competitions when stress and anxiety are at their peak.

Instead of inducing hyperhydration with glycerol, a less expensive, safer and more practical approach would be to consume about 500 milliliters of water two hours before going to bed the night before an exercise or competition and another 500 milliliters of fluid (juice, milk or water) first thing the next morning. Then, one should drink 1) just enough water to maintain euhydration, 2) 300-500 milliliters of diluted sports drink (3-4%) one hour before exercise and then 3) take in an additional 300-500 milliliters (1-2.5%) 25 minutes before exercise.

Before even thinking about hyperhydrating or calculating the amount of fluid needed to match sweat loss during exercise, athletes must first be sure to induce, and more importantly maintain, a state euhydration prior to undertaking an exercise. Hyperhydration is unnecessary when euhydration is maintained during exercise. Obviously, the hyperhydration procedure to be followed before key races must first have been thoroughly tested during specific training and low-key races.

* United States Army Research Institute of Environmental Medicine



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