The oxidation of glucose and fructose ingested during moderate exercise performed on a cycle ergometer (120 min, 52% VO2max) was compared in ten young males fasted (n = 5) or fed (n = 5) before exercise. The subjects ingested randomly 1.33 g/kg body weight (approximately 96 +/- 9 g) of either enriched 13C-glucose (G), 13C-fructose (F), or water only (W); the solutions were evenly distributed over the exercise period. The fasted subjects began the three exercises with a lower blood glucose (P less than or equal to 0.05 for F only) and insulin (P less than or equal to 0.05) levels and a higher free fatty acid (FFA) concentration (P less than or equal to 0.05) than the fed ones. Throughout the exercise period, blood glucose level was maintained in fasted as well as in fed group for G and F ingestions, while it decreased (P less than or equal to 0.05 at the 100th min in fasted subjects) with water ingestion. Insulin level was similar in both fed and fasted conditions with F and W ingestions and lower than G trials for the fed subjects. For the three ingestions, FFA was lower (P less than or equal to 0.05) in the fasted than in the fed group over the exercise period. Over the 2-h period of exercise, a greater (P less than or equal to 0.05) amount of exogenous F was oxidized in the fasted (49 +/- 6 g) than in the fed (36 +/- 5 g) group, which represent 31% and 20% of the total carbohydrate energy supply, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)