Abstract
The purpose of this study was to investigate what effect the size and location of skeletal muscle mass have on the source of glucose oxidised with a specific emphasis on plasma glucose, and to find out how pre-exercise diet might influence these results. Indirect calorimetry and stable isotope methodology using orally ingested [13C]glucose were used to examine the effect of muscle mass and exercise mode on plasma glucose and muscle glycogen oxidation (PGO and MGO, resp.).
Muscle mass: On separate occasions, eight healthy male cyclists (35–55 y) completed 75 min of arm cranking (Arms), one-legged cycling (1-leg) and two-legged cycling (2-leg) at 50% mean maximal power output (VO2peak, 33 ± 5, 41 ± 4 and 56 ± 5 mL/kg/min, resp.) and one-legged cycling at the same absolute intensity as 2-leg (2-legA; 94 ± 10 W). PGO in 2-leg was similar to that in Arms and greater than in 1-leg (p = 0.13, p = 0.002, resp.), while PGO in 1-leg was greater than in 2-legA (p = 0.03), (0.67 ± 0.17, 0.49 ± 0.16, 0.39 ± 0.10, and 0.27 ± 0.09 g/min, resp.). The percentage energy utilisation [%En] from plasma glucose in 2-leg was similar to that in 1-leg but less than in Arms (p = 0.26, p = 0.03, resp.), [13 ± 3, 15 ± 3 and 21 ± 5%En, resp.]. MGO was similar between 2-leg and 1-leg while both were greater than in Arms (p = 0.14, p ≤ 0.05, resp.) (1.15 ± 0.39, 0.89 ± 0.34, and 0.55 ± 0.23 g/min, resp.).
Breakfast: Eight healthy male runners (18–22 y; VO2max, 67 ± 5 mL/kg/min) completed 75 min of treadmill running (59% VO2max) in both an overnight fasted state and 150 min after consuming a liquid CHO-rich breakfast. Greater CHO oxidation in the fed state was attributed to greater MGO (1.21 ± 0.63 vs. 0.79 ± 0.25 g/min [33 ± 15 vs. 23 ± 8%En], resp.) (p = 0.01) and while fat oxidation was enhanced in the fasted state (~35%), PGO was unaffected by dietary status (0.73 ± 0.16 vs. 0.71 ± 0.17 g/min [20 ± 4 vs. 20 ± 3%En], resp.) (p ≥ 0.23).
PGO was related to the mode of exercise and both the size and location of the active muscle mass. As the lower limbs are generally better trained than the arms, specific training adaptations that reduce the reliance on CHO as an energy yielding substrate also reduce the reliance on plasma glucose during lower limb exercise. Muscle mass differentials and %En utilisation from plasma glucose indicate that PGO per kg is greater in arm than in lower limb muscle, i.e., PGO is inversely related to muscle mass, however, in lower limb muscle, PGO is proportional to the muscle mass. Breakfast feeding increases plasma insulin and glucose above fasting concentrations and during the early stages of exercise these conditions enhance PGO, and when combined with an increase in MGO, a transient decline in plasma glucose concentration is observed.