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

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 y; 75.44 ± 5.03 kg; 59 ± 6 mL/kg/min) completed two trials of 160 min cycling at 50% Wpeak with low (DEH) or high (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- 13 C 6 ]-glucose, in both trials. In EUH, additional water (2260 ± 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 post-exercise. Peak (EUH: 0.79 ± 0.12 g/min; DEH: 0.70 ± 0.14 g/min; P=0.041; dz=0.81) and mean 40-160 min (EUH: 0.61 ± 0.09 g/min; DEH: 0.51 ± 0.11 g/min; P=0.010; dz=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; dz=1.52). DEH, but not EUH, produced significant body mass loss (DEH: -2.8 ± 0.4%; EUH: -0.2 ± 0.4%; P<0.001; dz=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; dz=3.21) post-exercise. 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 decreased exogenous glucose oxidation, adequate fluid intake should be considered for athletes to optimise carbohydrate supplementation.