RELATIONSHIP BETWEEN FATTY-ACID DELIVERY AND FATTY-ACID OXIDATION DURING STRENUOUS EXERCISE

To evaluate the extent to which decreased plasma free fatty acid (FFA) concentration contributes to the relatively low rates of fat oxidatio n during high-intensity exercise, we studied FFA metabolism in six end urance-trained cyclists during 20-30 min of exercise [85% of maximal O -2 uptake (Vover dotO(2max))]. They were studied on two occasions: onc e during a control trial when plasma FFA concentration is normally low and again when plasma FFA concentration was maintained between 1 and 2 mM by intravenous infusion of lipid (Intralipid) and heparin. During the 20-30 min of exercise, fat and carbohydrate oxidation were measur ed by indirect calorimetry, and the rates of appearance (Ra) of plasma FFA and glucose were determined by the constant infusion of [6,6-H-2( 2)]glucose and [H-2(2)]palmitate. Lipid-heparin infusion did not influ ence the Ra or rate of disappearance of glucose. During exercise in th e control trial, Ra FFA failed to increase above resting levels (11.0 +/- 1.2 and 12.4 +/- 1.7 mu mol . kg(-1). min(-1) for rest and exercis e, respectively) and plasma FFA concentration dropped from a resting v alue of 0.53 +/- 0.08 to 0.29 +/- 0.02 mM. The restoration of plasma F FA concentration resulted in a 27% increase in total fat oxidation (26 .7 +/- 2.6 vs. 34.0 +/- 4.4 mu mol . kg(-1). min(-1), P < 0.05) with a concomitant reduction in carbohydrate oxidation, apparently due to a 15% (P < 0.05) reduction in muscle glycogen utilization However, the e levation of plasma FFA concentration during exercise at 85% Vover dotO (2max) only partially restored fat oxidation compared with the levels observed during exercise at 65% Vover dotO(2max). These findings indic ate that fat oxidation is normally impaired during exercise at 85% Vov er dotO(2max) because of the failure of FFA mobilization to increase a bove resting levels, but this explains only part of the decline in fat oxidation when exercise intensity is increased from 65 to 85% Vover d otO(2max).