The effects of dehydration during cycling on skeletal muscle metabolism in females.

INTRODUCTION: This study investigated the effects of progressive dehydration on the time course of changes to whole body substrate oxidation and skeletal muscle metabolism during 120 min of cycling in hydrated females. METHODS: Subjects (n=9) cycled for 120 min at ∼65% VO2peak on two occasions: with no fluid (DEH) and with fluid replacement to match sweat losses (HYD). Venous blood samples were taken at rest and every 20 min and muscle biopsies taken at 0, 60 and 120 min of exercise. RESULTS: DEH subjects lost 0.9% body mass (BM) from 0-60 min and 1.1% from 60-120 min (2.0% total). HR and Tc were significantly greater from 30-120 min, Pvol loss from 40-120 min, and RPE from 60-120 min in the DEH trial. There were no differences in VO2 or sweat loss between trials. RER (HYD 0.85 ± 0.01 vs. DEH 0.87 ± 0.01) and total carbohydrate (CHO) oxidation (175 ± 17 vs. 191 ± 17g) were higher in the DEH trial. Blood [La] was significantly higher in the DEH trial with no change in plasma free fatty acid and epinephrine concentrations. Muscle glycogenolysis was 31% greater in the DEH trial (252 ± 49 vs. 330 ± 33 mmol/kg dm) and muscle [La] was also higher at 60 min. CONCLUSION: Progressive dehydration significantly increased HR, Tc, RPE, Pvol loss, whole body CHO oxidation, and muscle glycogenolysis, and these changes were already apparent in the first hour of exercise when BM losses were ≤1%. The increased muscle glycogenolysis with DEH appeared to be due to increased core and muscle temperature, secondary to less efficient movement of heat from the core to the periphery.