Turnover rate and oxidation of free fatty acids of blood plasma in man during exercise: studies during continuous infusion of palmitate-1-C14.

Previous studies have shown that the free fatty acids (FFA) of blood plasma are renewed much more rapidly than the other plasma lipids (1-3). It has been calculated that, if oxidized, they could satisfy the requirements of oxidative metabolism in the postabsorptive state in man at rest (3). Measurements of arteriovenous (A-V) differences have shown that sufficient FFA are taken up by the heart in the postabsorptive state to account for much of its oxidative metabolism, provided they are all oxidized (4-5). Similar observations related to skeletal muscle have been difficult to evaluate because of the multiple sources of venous blood draining limbs (6). Measurements of radioactivity in expired CO2 after injection of C14-labeled fatty acids suggest that, in man (3) and in dogs (7), oxidation of circulating FFA accounts for 25 to 50% of energy metabolism in the postabsorptive state at rest. Such quantitative evaluations have been complicated by recycling of FFA between blood and tissues, recycling of triglyceride fatty acids from the liver, and slow equilibration of labeled CO2 derived from oxidation of labeled FFA with the body pool of CO2. We thought that many of these problems could be minimized if such studies were done during exercise , since blood flow would be directed primarily to workinig muscle. Thus, steady-state conditions might be reached earlier and permit more reliable calculation of the contribution of plasma FFA to total body metabolism. Furthermore , since energy metabolism during exercise can be considered essentially that of skeletal muscle, such studies would measure utilization of FFA by muscle in vivo. The efflux of FFA from plasma is increased during exercise (8-11), presumably a result of augmented blood flow to working muscles (10-11). Basu, Passmore, and Strong (12) have shown that the concentration of FFA in plasma from antecubital veins is elevated when men walk 3 to 4 miles per hour for more than 30 to 60 minutes. This suggests that the turnover rate of FFA is considerably increased during such exercise. We have chosen this model for our studies. The results show that the turnover rate of FFA is indeed increased and strongly suggest that FFA are the major circulating metabolites burned by working muscle in the postabsorptive state. METHODS Experimental subjects and procedures. Six members of a wrestling team, aged 23 to 30, volunteered as subjects. All were participating in a strenuous program of training and were in excellent physical …