Muscle triglyceride utilization during exercise: effect of training

TLDR

During exercise of equal intensity, trained individuals exhibit a lower respiratory exchange ratio and reduced plasma free fatty acids and glycerol compared with untrained subjects, indicating diminished plasma FFA availability. The study aimed to determine whether muscle triglyceride lipolysis is greater in the trained state. Nine male participants performed a prolonged exercise bout at 64 % VO₂max before and after a 12‑week endurance training program. After training, plasma FFA, glycerol, and RER were lower, fat contributed 57 % of energy versus 35 % pre‑training, muscle glycogen use fell 41 %, and quadriceps triglyceride depletion doubled, supporting increased muscle TG lipolysis as the source of FFA.

Abstract

The respiratory exchange ratio (RER) is lower during exercise of the same intensity in the trained compared with the untrained state, even though plasma free fatty acids (FFA) and glycerol levels are lower, suggesting reduced availability of plasma FFA. In this context, we evaluated the possibility that lipolysis of muscle triglycerides might be higher in the trained state. Nine adult male subjects performed a prolonged bout of exercise of the same absolute intensity before and after adapting to a strenuous 12-wk program of endurance exercise. The exercise test required 64% of maximum O2 uptake before training. Plasma FFA and glycerol concentrations and RER during the exercise test were lower in the trained than in the untrained state. The proportion of the caloric expenditure derived from fat, calculated from the RER, during the exercise test increased from 35% before training to 57% after training. Muscle glycogen utilization was 41% lower, whereas the decrease in quadriceps muscle triglyceride concentration was roughly twice as great (12.7 +/- 5.5 vs. 26.1 +/- 9.3 mmol/kg dry wt, P less than 0.001) in the trained state. These results suggest that the greater utilization of FFA in the trained state is fueled by increased lipolysis of muscle triglyceride.