Six well-trained men (peak pulmonary oxygen uptake = 5.03 +/- 0.11 1/m
in) were studied during 2 h of exercise at 69 +/- 1% peak pulmonary ox
ygen uptake to examine the effect of carbohydrate (CHO) ingestion on g
lucose kinetics. Subjects ingested 250 ml of either a 10% glucose solu
tion containing 6-[H-3]glucose (CHO) or a sweet placebo every 15 min d
uring exercise. Glucose kinetics were assessed by 6,6-[H-2]glucose inf
usion corrected for gut-derived glucose in CHO. Plasma glucose was hig
her (P < 0.05) in CHO from 20 min. Total glucose appearance was higher
in CHO due to glucose delivery from the gut (68 +/- 7 g), since hepat
ic glucose production was reduced by 51% (29 +/- 5 vs. 59 +/- 5 g). Gl
ucose uptake was higher in CHO (96 +/- 7 vs. 60 +/- 6 g) with the inge
sted glucose supplying 67 +/- 4 g and, with the assumption that it was
fully oxidized, accounted for 14 +/- 1% of total energy expenditure.
In conclusion, CHO ingestion during prolonged exercise results in supp
ression of hepatic glucose production and increased glucose uptake. Th
ese effects appear to be mediated mainly by increased plasma glucose a
nd insulin levels.