Effect of carbohydrate ingestion on glucose kinetics during exercise in the heat

Six endurance-trained men [peak oxygen uptake (VO₂) = 4.58 ± 0.50 (SE) l/min] completed 60 min of exercise at a workload requiring 68 ± 2% peak VO₂ in an environmental chamber maintained at 35°C (<50% relative humidity) on two occasions, separated by at least 1 wk. Subjects ingested either a 6% glucose solution containing 1 µCi [3-³H]glucose/g glucose (CHO trial) or a sweet placebo (Con trial) during the trials. Rates of hepatic glucose production [HGP = glucose rate of appearance (R_a) in Con trial] and glucose disappearance (R_d), were measured using a primed, continuous infusion of [6,6-^₂H]glucose, corrected for gut-derived glucose (gut R_a) in the CHO trial. No differences in heart rate, VO₂, respiratory exchange ratio, or rectal temperature were observed between trials. Plasma glucose concentrations were similar at rest but increased (P < 0.05) to a greater extent in the CHO trial compared with the Con trial. This was due to the absorption of ingested glucose in the CHO trial, because gut R_a after 30 and 50 min (16 ± 5 µmol · kg^-1 · min^-1) was higher (P < 0.05) compared with rest, whereas HGP during exercise was not different between trials. Glucose R_d was higher (P < 0.05) in the CHO trial after 30 and 50 min (48.0 ± 6.3 vs 34.6 ± 3.8 µmol · kg^-1 · min^-1, CHO vs. Con, respectively). These results indicate that ingestion of carbohydrate, at a rate of ~1.0 g/min, increases glucose Rd but does not blunt the rise in HGP during exercise in the heat.