Hyperthermia, but not muscle water deficit, increases glycogen use during intense exercise

We determined if dehydration alone or in combination with hyperthermia accelerates muscle glycogen use during intense exercise. Seven endurance‐trained cyclists ( VO 2max = 54.4 ± 1.05 mL/kg/min) dehydrated 4.6% of body mass ( BM ) during exercise in the heat (150 min at 33 ± 1 ° C , 25 ± 2% humidity). During recovery (4 h), subjects remained dehydrated ( HYPO trial) or recovered all fluid losses ( REH trials). Finally, subjects exercised intensely (75% VO 2max ) for 40 min in a neutral (25 ± 1 ° C ; HYPO and REH trials) or in a hot environment (36 ± 1 ° C ; REH HOT ). Before the final exercise bout vastus lateralis glycogen concentration was similar in all three trials (434 ± 57 mmol/kg of dry muscle (dm)) but muscle water content was lower in the HYPO (357 ± 14 mL/100 g dm) than in REH trials (389 ± 25 and 386 ± 25 mL/100 g dm; P < 0.05). After 40 min of intense exercise, intestinal temperature was similar between the HYPO and REH HOT trials (39.2 ± 0.5 and 39.2 ± 0.4 ° C , respectively) and glycogen use was similar (172 ± 86 and 185 ± 97 mmol/kg dm, respectively) despite large differences in muscle water content. In contrast, during REH , intestinal temperature (38.5 ± 0.4 °C) and glycogen use (117 ± 52 mmol/kg dm) were significantly lower than during HYPO and REH HOT . Our data suggest that hyperthermia stimulates glycogen use during intense exercise while muscle water deficit has a minor role.