Effects of ingesting 6% and 12% glucose/electrolyte beverages during prolonged intermittent cycling in the heat

This study compared the effects of ingesting 6% (MC) and 12% (HC) glucose/electrolyte beverages, and a flavored water placebo (P) on markers of fluid absorption, palatability, and physiological function during prolonged intermittent cycling in the heat. On three occasions, 15 trained male cyclists performed two 60 min cycling bouts at 65% \(\dot V_{{\text{O}}_{{\text{2 max}}} } \) (E ₁ and E ₂). A brief exhaustive performance ride (∼3 min) was completed after E ₁ and E ₂, and after 20 min recovery (P ₁, P ₂, P ₃). Every 20 min, subjects consumed 275 mL of P, MC or HC. The first drink contained 20 mL of D₂O, a tracer of fluid entry into blood plasma. Plasma D₂O accumulation was slower for HC than for P and MC (P<0.001). HC caused more nausea (P<0.01) and fullness (P<0.05) than MC or P, and subjects said they would be less likely to consume HC during training or competition (P<0.10). Sweat rates, HR, T _re, T _sk, \(\dot V_{{\text{O}}_{\text{2}} } \) , and PV were similar for all drinks. Performance of P ₁, P ₂, P ₃ were not different among drinks. However, four cyclists failed to maintain the prescribed work rate during E ₂ for HC but only one failed for MC and P. These data suggest that the slow absorption of a 12% glucose/electrolyte beverage during prolonged intermittent exercise in the heat may increase the risk of gastrointestinal distress and thereby limit performance.