Oxidation of Exogenous Carbohydrate During Prolonged Exercise in Fed and Fasted Conditions*

Abstract

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 (=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 ≤ 0.05 for F only) and insulin (P ≤ 0.05) levels and a higher free fatty acid (FFA) concentration (P ≤ 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 ≤ 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 ≤ 0.05) in the fasted than in the fed group over the exercise period. Over the 2-h period of exercise, a greater (P ≤ 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. There was, however, a similar utilization of ingested G in the two groups, while a slightly greater amount of fat was oxidized in the fasted as compared to the fed group. As a consequence, endogenous carbohydrate utilization was lower (P ≤ 0.05) in the fasted than in the fed subjects with either F or G ingestions.

Key words

Fasting - glucose - fructose - stable isotope¹³C - substrate utilization - insulin