EFFECT OF ENDURANCE TRAINING ON FATTY-ACID METABOLISM DURING WHOLE-BODY EXERCISE

Endurance exercise training increases fat oxidation during large muscl e mass exercise. Although the source of this fat has been thought to b e plasma free fatty acids (FFA) released from adipose tissue, the trai ning-induced decrease in lipolytic hormonal responses to exercise is n ot consistent with this concept. The purpose of this communication is to review findings from our laboratory indicating that, in young healt hy subjects, endurance exercise training reduces plasma FFA turnover a nd oxidation during moderate intensity prolonged 2-leg cycling while s imultaneously enhancing depletion of triglycerides from the active mus culature. Evidence is presented that metabolism of intramuscular trigl ycerides can explain the increase in total fat oxidation observed in t he trained state during large muscle mass exercise. However, these res ults may not be applicable to exercise involving small muscle groups, a distinction that is likely to be important in explaining the apparen t conflict between our findings and those from other laboratories wher e experimental conditions were different. In summary, for large muscle mass exercise up to 2 h in duration, plasma FFA are a less important fuel source in the trained state, and intramuscular triglycerides supp ly the major portion of the increase in oxidized fatty acids.