To evaluate the contribution of working muscle to whole body lipid oxidation, we examined the effects of exercise intensity and endurance training (9 wk, 5 days/wk, 1 h, 75% V̇o2 peak) on whole body and leg free fatty acid (FFA) kinetics in eight male subjects (26 ± 1 yr, means ± SE). Two pretraining trials [45 and 65% V̇o2 max (45UT, 65UT)] and two posttraining trials [65% of pretraining V̇o2 peak (ABT), and 65% of posttraining V̇o2 peak (RLT)] were performed using [1-13C]palmitate infusion and femoral arteriovenous sampling. Training increased V̇o2 peak by 15% (45.2 ± 1.2 to 52.0 ± 1.8 ml·kg−1·min−1, P < 0.05). Muscle FFA fractional extraction was lower during exercise (EX) compared with rest regardless of workload or training status (≈20 vs. 48%, P < 0.05). Two-leg net FFA balance increased from net release at rest (≈−36 μmol/min) to net uptake during EX for 45UT (179 ± 75), ABT (236 ± 63), and RLT (136 ± 110) (P < 0.05), but not 65UT (51 ± 127). Leg FFA tracer measured uptake was higher during EX than rest for all trials and greater during posttraining in RLT (716 ± 173 μmol/min) compared with pretraining (45UT 450 ± 80, 65UT 461 ± 72, P < 0.05). Leg muscle lipid oxidation increased with training in ABT (730 ± 163 μmol/min) vs. 65UT (187 ± 94, P < 0.05). Leg muscle lipid oxidation represented ∼62 and 30% of whole body lipid oxidation at lower and higher relative intensities, respectively. In summary, training can increase working muscle tracer measured FFA uptake and lipid oxidation for a given power output, but both before and after training the association between whole body and leg lipid metabolism is reduced as exercise intensity increases.
- crossover concept
- free fatty acids
- substrate partitioning
- regional metabolism
- Copyright © 2007 by American Physiological Society