Fiber type-specific muscle glycogen sparing due to carbohydrate intake before and during exercise.

The effect of carbohydrate intake before and during exercise on muscle glycogen content was investigated. According to a randomized crossover study design, eight young healthy volunteers (n = 8) participated in two experimental sessions with an interval of 3 wk. In each session subjects performed 2 h of constant-load bicycle exercise (∼75% maximal oxygen uptake). On one occasion (CHO), they received carbohydrates before (∼150 g) and during (1 g·kg body weight^-1·h^-1) exercise. On the other occasion they exercised after an overnight fast (F). Fiber type-specific relative glycogen content was determined by periodic acid Schiff staining combined with immunofluorescence in needle biopsies from the vastus lateralis muscle before and immediately after exercise. Preexercise glycogen content was higher in type IIa fibers [9.1 ± 1 × 10^-2 optical density (OD)/μm²] than in type I fibers (8.0 ± 1 × 10^-2OD/μm²; P < 0.0001). Type IIa fiber glycogen content decreased during F from 9.6 ± 1 × 10^-2OD/μm² to 4.5 ± 1 × 10^-2OD/μm² (p = 0.001), but it did not significantly change during CHO (P = 0.29). Conversely, in type I fibers during CHO and F the exercise bout decreased glycogen content to the same degree. We conclude that the combination of carbohydrate intake both before and during moderate-to high-intensity endurance exercise results in glycogen sparing in type Ha muscle fibers. [ABSTRACT FROM AUTHOR]