Benefits
Supports pre-exercise hydration by retaining more fluid than water alone
Glycerol taken with a large drink before exercise lowers urine output so more fluid stays in the body. A pooled analysis of 14 comparisons found about 7.7 mL more fluid retained per kg of body weight than with the same fluid alone, roughly 540 mL at 70 kg. These data come from plain glycerol, not HydroMax® itself.
Endurance performance support
Results are mixed. A pooled analysis of 4 trials found about 2.6% better endurance performance, a marginal result its authors said needs more study, and adding glycerol to rehydration fluids improved a later 40-km cycling time trial. Other trials, including a 60-minute cycling test and a 5-km run in the heat, found no change.
Hydration during exercise in the heat
In a mountain-bike race in the heat, glycerol before the race left riders less dehydrated, with less thirst and fewer heat-strain symptoms than water; the final loop was about 5 minutes faster, but this was not statistically significant. In other heat trials, glycerol did not lower core temperature compared with water.
Mechanism of action
Osmotic water retention
Glycerol drinks create an osmotic gradient in the circulation that favours fluid retention, so less of a large drink is lost as urine and athletes can start exercise with extra body water.
Less urine from the kidneys
In a resting study, glycerol cut urine flow and free water clearance compared with water alone. Changes in antidiuretic hormone (ADH) were small and near the limits of the test, and the authors suggested glycerol may also raise the concentration gradient in the kidney so more water is reabsorbed.
Clinical trials
Meta-analysis of studies comparing glycerol hyperhydration (on average about 1.1 g/kg with 24 mL/kg fluid) with the same fluid without glycerol (Goulet et al. 2007, Int J Sport Nutr Exerc Metab)
Pooled studies: 14 comparisons for fluid retention, 4 for endurance performance.
Glycerol increased fluid retention by about 7.7 mL per kg body weight, roughly 540 mL at 70 kg. Endurance performance improved by about 2.6%, a marginal result (P = 0.047) from only 4 studies, and the authors said more research is needed before drawing firm conclusions about performance.
Randomized, double-blind crossover of a pre-race glycerol drink versus water before a 30-mile mountain-bike race in the heat, with a third trial of water before but none during the race (Wingo et al. 2004, J Athl Train)
12 heat-acclimated male mountain bikers.
With glycerol, riders were less dehydrated after the race and had less pre-race urine, less thirst and lower heat-symptom questionnaire scores. The final loop was about 5 minutes faster with glycerol, but this was not statistically significant.
Randomized crossover study of four rehydration methods (oral fluids, oral fluids plus glycerol, intravenous saline, intravenous saline plus oral glycerol) after 4% dehydration, followed by a 40-km time trial in the heat (Van Rosendal et al. 2012, Med Sci Sports Exerc)
9 endurance-trained men.
Compared with oral fluids alone, adding oral glycerol improved 40-km time by 3.7%. Intravenous fluid plus oral glycerol retained the most fluid but did not improve performance more than either method alone. Heart rate, temperature and perceived exertion did not differ.
Crossover trial of glycerol 1.2 g/kg with 21 mL/kg flavoured water versus the same volume of water, 2.5 hours before a self-paced 60-minute cycling test in hot, humid conditions (Marino et al. 2003, Pflugers Arch)
7 moderately to well-trained adults.
Null result: distance cycled (28.9 km glycerol vs 29.7 km water), power output and final rectal temperature did not differ. Heart rate was higher with glycerol during the high-intensity efforts.