The available scientific evidence does not support the use of glycerol to benefit hydration or thermoregulation in exercise. The abstract from the most recent study I found summarizes the investigators' findings:
They compared a 10% glycerol solution (G), a 6% carbohydrate-electrolyte solution (CE), a 6% carbohydrate-elec. plus 4% glycerol (CEG), and a water-placebo drink (WP). Subjects bicycled for 90 minutes in a 30 degree C, 45% relative humidity environment. Subjects served as their own controls. Ingestion of the glycerol solutions resulted in an increase in plasma osmolality and attenuation of the decrease in plasma volume associated with the WP treatment (P less than 0.05). Plasma renin activity was highest with WP (P less than 0.05), and G was associated with increased antidiuretic hormone levels (P less than 0.05). Ratings of perceived thirst were lowest for CEG and G, and the frequency of gastrointestinal distress was greatest for G (P less than 0.05). However, no differences among beverage treatments were observed for heart rate, esophageal temperature, sweat rate, ratings of perceived exertion, or changes in cortisol and aldosterone levels.
These data indicate that there are no substantial metabolic, hormonal, cardiovascular, or thermoregulatory advantages to the consumption of solutions containing 4 or 10% glycerol during exercise.
Medically, one use for glycerol solutions is to help a patient's mouth feel moist (eg. patients who have poorly functioning salivary glands). An effect the subjects in the above study noted -- they didn't feel as thirsty. This decreased thirst was an illusion however.
Even though it does not appear to be helpful, I am concerned that in ultra-distance races glycerol may potentially be harmful. I suggest this because of the increased anti-diuretic hormone (ADH) levels that were observed. Elevated ADH is one proposed mechanism behind the hyponatremia that is observed in athletes towards the end of an ultra-distance race. Also note the increased incidence of gastrointestinal complaints in the glycerol group. Certainly something to be avoided during a long race.
1. Physiological responses to glycerol ingestion during exercise. Source: Journal of Applied Physiology. 71(1):144-9, 1991 Jul.
May 19, 1998
Addendum: I have re-reviewed the literature on the subject. The most recent information still does not support the use of glycerol "hyperhydration" as a method of altering thermoregulation (i.e., water balance) during exercise. Glycerol, a naturally occuring carbohydrate, can be used as a fuel source by the body. The main function of glycerol in the body is as a framework onto which fatty acids are stuck, for storage. Thus the name, triglyceride. Earlier studies, which demonstrated a positive effect of glycerol on exercise performance, failed to take into account the caloric use of glycerol. Extra calories means extra performance -- the same situation applies to consumption of other types of carbohydrates. So there is no magic manipulation of water balance with the use of glycerol, but it can be used as a calorie source similar to carbo containing drinks and "energy" bars.
2. Hyperhydration: thermoregulatory effects during compensable exercise-heat stress. Journal of Applied Physiology. 83(3):860-6, 1997 Sep.
Abstract from this study:
This study examined the effects of hyperhydration on thermoregulatory responses during compensable exercise-heat stress. The general approach was to determine whether 1-h preexercise hyperhydration [29. 1 ml/kg lean body mass; with or without glycerol (1.2 g/kg lean body mass)] would improve sweating responses and reduce core temperature during exercise. During these experiments, the evaporative heat loss required (Ereq = 293 W/m2) to maintain steady-state core temperature was less than the maximal capacity (Emax = 462 W/m2) of the climate for evaporative heat loss (Ereq/Emax = 63%). Eight heat-acclimated men completed five trials: euhydration, glycerol hyperhydration, and water hyperhydration both with and without rehydration (replace sweat loss during exercise). During exercise in the heat (35 degrees C, 45% relative humidity), there was no difference between hyperhydration methods for increasing total body water (approximately 1.5 liters). Compared with euhydration, hyperhydration did not alter core temperature, skin temperature, whole body sweating rate, local sweating rate, sweating threshold temperature, sweating sensitivity, or heart rate responses. Similarly, no difference was found between water and glycerol hyperhydration for these physiological responses. These data demonstrate that hyperhydration provides no thermoregulatory advantage over the maintenance of euhydration during compensable exercise-heat stress.