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

Six endurance-trained men (peak oxy gen uptake ((V) over dotO(2)) = 4.58 +/ - 0.50 (SE) l/min] completed 60 min of exercise at a workload requiring 68 +/- 2% peak (V) over dotO(2) in an environmental chamber maintained at 35 d egreesC (<50% relative humidity) on two occasions, separated by at least 1 wk. Subjects ingested either a 6% glucose solution containing 1 <mu>Ci [3-H -3]glucose/g glucose (CHO trial) or a sweet placebo (Con trial) during the trials. Rates of hepatic glucose production [HGP = glucose rate of appearan ce (R-a) in Con trial] and glucose disappearance (R-d), were measured using a primed, continuous infusion of [6,6-H-2]glucose, corrected for gut-deriv ed glucose (gut R-a) in the CHO trial. No differences in heart rate, (V) ov er dotO(2), respiratory exchange ratio, or rectal temper ature were observe d between trials. Plasma glucose concentrations were similar at rest but in creased (P < 0.05) to a greater extent in the CHO trial compared with the C on trial. This was due to the absorption of ingested glucose in the CHO tri al, because gut R-a after 30 and 50 min (16 +/- 5 <mu>mol.kg(-1).min(-1)) w as higher (P < 0.05) compared with rest, whereas HGP during exercise was no t different between trials. Glucose R-d was higher (P < 0.05) in the CHO tr ial after 30 and 50 min (48.0 +/- 6.3 vs 34.6 +/- 3.8 mu mol.kg(-1).min(-1) , CHO vs. Con, respectively). These results indicate that ingestion of carb ohydrate, at a rate of similar to1.0 g/min, increases glucose R-d but does not blunt the rise in HGP during exercise in the heat.