Regulation of plasma fatty acid oxidation during low- and high-intensity exercise

In the present study we examined the hypothesis that fatty acid oxidation is less during high-intensity exercise than during moderate- intensity exercise because of inhibition of longchain fatty acid entry into the mitochondria. Six volunteers exercised at 40% peak oxygen consumption (V̇O₂(peak)) for 60 min and at 80% V̇O₂(peak) for 30 min on two different occasions. [1-¹³C]oleate, a long-chain fatty acid, and [1-¹⁴C]octanoate, a medium-chain fatty acid, were infused for the duration of the studies. Lipids and heparin were infused during exercise at 80% V̇O₂(peak) to prevent the expected decrease in plasma free fatty acid (FFA) concentration. Plasma oleate and total FFA availability were similar in the two experiments. Oleate oxidation decreased from 2.8 ± 0.6 (40% V̇O2(peak)) to 1.8 ± 0.2 μmol · kg^-1 · min^-1 (80% V̇O₂(peak), P < 0.05), whereas octanoate oxidation increased from 1.0e^-05 ± 1.0e^-06 (40% V̇O₂(peak)) to 1.3e^-05 + 5.1e^-06 μmol · kg^-1 · min^-1 (80% V̇O₂(peak), P < 0.05). Furthermore, the percentage of oleate uptake oxidized decreased from 67.7 ± 2.8% (40% V̇O₂(peak)) to 51.8 ± 4.6% (80% V̇O₂(peak), P < 0.05), whereas the percentage of octanoate oxidized was similar during exercise at 40 and 80% V̇O₂(peak) (84.8 ± 2.7 vs. 89.3 ± 2.7%, respectively). Our data suggest that, in addition to suboptimal FFA availability, fatty acid oxidation is likely limited during high-intensity exercise because of direct inhibition of long-chain fatty acid entry into mitochondria.