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 oxidation during high-intensity exercise, we studied FFA metabolism in six endurance-trained cyclists during 20-30 min of exercise [85% of maximal O₂ uptake (V̇O₂(max))]. They were studied on two occasions: once 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 measured by indirect calorimetry, and the rates of appearance (Ra) of plasma FFA and glucose were determined by the constant infusion of [6,6-²H₂]glucose and [²H₂]palmitate. Lipid-heparin infusion did not influence the Ra or rate of disappearance of glucose. During exercise in the control trial, Ra FFA failed to increase above resting levels (11.0 ± 1.2 and 12.4 ± 1.7 μmol · kg^-1 · min^-1 for rest and exercise, respectively) and plasma FFA concentration dropped from a resting value of 0.53 ± 0.08 to 0.29 ± 0.02 mM. The restoration of plasma FFA concentration resulted in a 27% increase in total fat oxidation (26.7 ± 2.6 vs. 34.0 ± 4.4 μmol · 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 elevation of plasma FFA concentration during exercise at 85% V̇O₂(max) only partially restored fat oxidation compared with the levels observed during exercise at 65% V̇O₂(max)These findings indicate that fat oxidation is normally impaired during exercise at 85% V̇O₂(max) because of the failure of FFA mobilization to increase above resting levels, but this explains only part of the decline in fat oxidation when exercise intensity is increased from 65 to 85% V̇O₂(max).