Endurance exercise training increases fat oxidation during large muscle mass exercise. Although the source of this fat has been thought to be plasma free fatty acids (FFA) released from adipose tissue, the training-induced decrease in lipolytic hormonal responses to exercise is not consistent with this concept. The purpose of this communication is to review findings from our laboratory indicating that, in young healthy subjects, endurance exercise training reduces plasma FFA turnover and oxidation during moderate intensity prolonged 2-leg cycling while simultaneously enhancing depletion of triglycerides from the active musculature. Evidence is presented that metabolism of intramuscular triglycerides can explain the increase in total fat oxidation observed in the trained state during large muscle mass exercise. However, these results may not be applicable to exercise involving small muscle groups, a distinction that is likely to be important in explaining the apparent conflict between our findings and those from other laboratories where 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 supply the major portion of the increase in oxidized fatty acids.
Department of Medicine, Washington University School of Medicine, St. Louis, MO 63110
Submitted for publication January 1996.
Accepted for publication March 1996.
This work was supported by National Institutes of Health Resource for Biomedical Mass Spectrometry Grant RR-00954; General Clinical Research Center Grant RR-00036; Diabetes Research and Training Center Grant AM-20579; National Institute of Aging Institutional National Research Service Award AG-00078; National Heart, Lung, and Blood Institute (NHLBI) Institutional National Research Service Award HL-07456; and NHLBI Grant HL-41290.
Address for correspondence: Wade H. Martin, III, M.D., Division of Cardiology, John Cochran VA Hospital 111A-JC, 915 North Grand, St. Louis, MO 63106.