Effects of glycerol-induced hyperhydration prior to exercise in the heat on sweating and core temperature.

Hypohydration reduces exercise performance and thermoregulatory capacity in the heat. Hyperhydration prior to exercise may decrease, delay, or eliminate the detrimental effects of hypohydration. The rapid clearance of excess fluid makes hyperhydration of subjects with common beverages difficult. Glycerol, a natural metabolite which is rapidly absorbed, has osmotic action, and is evenly distributed within the body fluid compartments, was tested as a possible hyperhydrating agent. In six subjects, the following fluid regimens at time 0 were randomly administered on three separate days: in trial 1, glycerol (1 g.kg-1 body weight) plus water (21.4 ml.kg-1 body weight); in trial 2, water (21.4 ml.kg-1); and in trial 3, water (3.3 ml.kg-1) was ingested at time 0. The subjects performed moderate exercise (equivalent to 60% VO2max in a comfortable environment) in a hot dry environment. The exercise started at 2.5 h after the fluids were ingested. The urine volume prior to exercise was decreased when glycerol was ingested, thus resulting in glycerol-induced hyperhydration. During the exercise following the glycerol-induced hyperhydration, there was elevated sweat rate and lower rectal temperature during the moderate exercise in the heat. There were no changes in hemoglobin, hematocrit, or serum electrolyte concentrations following glycerol intake. These data support the hypothesis that glycerol-induced hyperhydration reduces the thermal burden of moderate exercise in the heat.