Glycerol hyperhydration improves cycle time trial performance in hot humid conditions

Eight competitive cyclists [mean peak oxygen consumption, ((V) over dot O-2peak) = 65 ml . min(-1) . kg(-1)] undertook two 60-min cycle ergometer time trials at 32 degrees C and 60% relative humidity. The time trials were split into two 30-min phases: a fixed-workload phase and a variable-workload phase. Each trial was preceded by ingestion of either a glycerol solution [1 g . kg(-1) body mass (BM) in a diluted carbohydrate (CHO)-electrolyte drink] or a placebo of equal volume (the diluted CHO-electrolyte drink). The total fluid intake in each trial was 22, ml . kg(-1) BM. A repeated-measures, double blind, cross over design with respect to glycerol was employed. Glycerol ingestion expanded body water by approximate to 600 mi over the placebo treatment. Glycerol treatment significantly increased performance by 5% compared with the placebo group, as assessed by total work in the variable-workload phase (P < 0.04). There were no significant differences in rectal temperature, sweat rate or cardiac frequency between trials. Data indicate that the glycerol-induced performance increase did not result from plasma volume expansion and subsequently lower core temperature or lower cardiac frequencies at a given power output as previously proposed. However, during the glycerol trial, subjects maintained a higher power output without increased perception of effort or thermal strain.