Glycerol-Hyperhydration or Hype?

Over the past few years glycerol - a colorless, syrupy liquid - has been added to the ever-growing list of sports supplements that tout increased performance.

Fluid balance in the body is well maintained when you are sitting around on your couch. But when you begin to exercise, water loss is increased dramatically. Two to three liters can be lost in an hour, but rehydration is usually less than 500 ml per hour during competition.(1,2)

Glycerol's claim to fame is that is appears to be capable of creating an increased state of hydration in the human body. It helps your body retain water. This is of great benefit to any athlete, as dehydration of as little as 1% of body weight can decrease "work capacity."

When we examine the most recent research on glycerol's role in increased hydration, we must consider if the positive, athletic benefits hold water themselves.

Out of 10 studies we reviewed at the Colgan Institute, the first four all indicated that urine flow rates and/or urine volumes were decreased. This appears good for glycerol use in athletic endeavors. But in three of the tests, the subjects did not exercise and in the fourth, the decrease in urine volume was measured prior to exercise.(3-6) You can't use these results to justify adding glycerol to your marathon program.

In the next four studies, the test subjects used treadmills or cycles at anywhere from 50% maximum oxygen intake (VO2) to exhaustion, testing a glycerol solution versus rehydration with either water or carbohydrate solution. (7-10) All the results showed no added benefit of hydrating with glycerol.

The ninth study reported an ergogenic effect from glycerol hydration in a two study design.(11) In the first stage, 11 subjects were given either a 20% glycerol solution or a placebo, one hour before cycling at 60% of maximum workload. No extra water was given and although there was no change in rectal temperatures or sweat rates between the two groups, there was an increase in endurance and a reduction in heart rate in the glycerol group.

In the second stage of the study, a 5% dextrose solution was given every 20 minutes, and a slight increase in endurance over the control group was once again recorded in the test subjects using glycerol. The last study showed no cardiovascular benefits or thermoregulatory effects, though fluid retention was indicated.(12)

What are we to make of these contradictory results? In practical terms, it appears that proper hydration prior to, and during exercise, with either carbohydrate solution or just plain water, will protect you just as well as trying to hydrate yourself with glycerol supplement.
Until more conclusive research can show consistent, positive results of the use of glycerol, it appears you should keep your money in your pocket. Any possible benefit that glycerol will enhance your athletic prowess is, at best, short lived, and completely unneccessary if you replenish yourself with water frequently. If you decide to try glycerol anyway, note that although it is deemed safe at dosages of 1 g/kg of body weight, some studies have indicated you may feel nausea and suffer from vomiting, bloating or lightheadedness.(5,8,9) These are not exactly the feelings most folk would look forward to during the heat of battle.

For rehydrartion, we recommend nothing stronger than 7% carbohydrate solution. Carbohydrate drinks that contain higher than 7% solution don't take into account that digestion stops during exercise. As the demand for oxygen by the muscles increases, blood flow is directed towards the extremities, away from the stomach. Water is readily absorbed by the body but most sports drinks remain in the stomach, sloshing around and causing nausea.

Rehydrate continuously while exercising, running or playing your favourite sport. Colgan Institute CRUX is an ideal carbohydrate powder that was originally developed for Mt. Everest expeditions, and can be added to water at different concentrations.

References

1. Noakes TD. Fluid replacement during exercise. Exer Sports Sci Rev, 1993;21:297-300.
2. Noakes TD, et al. The importance of volume in regulating gastric emptying. Med
Sci Sports Exer, 1991;23:307-313
3. Freund BJ, et al. Glycerol hyperhydration: hormonal, renal, and vascular fluid responses. J Appl Physiol, 1995;79:2069-2077
4. Riedesel ML, it al. Hyperhydration with glycerol solutions. J appl Physiol, 1987;63:2262-2077
5. Koenigsberg PS; et al. Sustained hyperhydration with glycerol ingestion. Life Sci, 1995;57:645-653
6. Lyons TP, et al. Effects of glycerol induced hyperhydration prior to exercise in the heat on sweating and core temperature. Med Sci Sports Exer, 1990;22:447-483.

Find more information at www.colganchronicles.com