Exercise-induced dehydration decreases exogenous glucose oxidation during prolonged cycling in a temperate...

This study investigated the effect of exercise-induced dehydration on exogenous glucose oxidation during cycling in a temperate environment. Nine trained male cyclists/triathletes (22 ± 4 yr; 75.44 ± 5.03 kg; 59 ± 6 mL/kg/min) completed two trials of 160-min cycling at 50% peak power output (W_peak) with low [dehydrated (DEH)] or high [euhydrated (EUH)] fluid intake. Participants ingested 60 g/h glucose (20 g every 20 min as a 27% mass:mass solution, followed by 15 mL water), enriched with 0.2% [U-¹³C₆]glucose, in both trials. In EUH, additional water (2,260 ± 477 mL) was provided. Subjective measures, skin and gastrointestinal (GI) temperature, capillary blood, and expired breath samples were collected every 20 min, with venous blood collected pre- and postexercise. Peak (EUH: 0.79 ± 0.12 g/min; DEH: 0.70 ± 0.14 g/min; P = 0.041; d_z = 0.81) and mean 40- to 160-min (EUH: 0.61 ± 0.09 g/min; DEH: 0.51 ± 0.11 g/min; P = 0.010; d_z = 1.12) exogenous glucose oxidation were reduced in DEH. The largest between-trial difference occurred at 60 min (EUH: 0.49 ± 0.11 g/min; DEH: 0.33 ± 0.13 g/min; P = 0.015; d_z = 1.52). DEH, but not EUH, produced significant body mass loss (DEH: -2.8 ± 0.4%; EUH: -0.2 ± 0.4%; P < 0.001; d_z = 4.77), decreased plasma volume (P = 0.034), and increased plasma osmolality (EUH: 290 ± 3 mosmol/kg; DEH: 302 ± 5 mosmol/kg; P < 0.001; d_z = 3.21) postexercise. Thermal sensation, skin/GI temperature, GI discomfort, intestinal fatty acid binding protein, and other measures were not different between trials (P ≥ 0.05). As exercise-induced dehydration decreases exogenous glucose oxidation, adequate fluid intake should be considered for athletes to optimize carbohydrate supplementation.NEW & NOTEWORTHY This study showed that dehydration/low fluid intake during exercise decreases exogenous glucose oxidation by ∼16% during endurance cycling in a temperate environment. These findings are important as endurance athlete vary substantially in their sweat rate and hydration responses to prolonged exercise, which could influence the efficacy of carbohydrate supplementation during exercise.