Contributions of working muscle to whole body lipid metabolism are altered by exercise intensity and training

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% V-O2 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% V-O2 (max) ( 45UT, 65UT)] and two posttraining trials [ 65% of pretraining V-O2 peak ( ABT), and 65% of posttraining V-O2 peak ( RLT)] were performed using [ 1-C-13] palmitate infusion and femoral arteriovenous sampling. Training increased V-O2 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 ( approximate to 20 vs. 48%, P < 0.05). Two-leg net FFA balance increased from net release at rest ( approximate to - 36 mu mol/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 mu mol/min) compared with pretraining ( 45UT 450 +/- 80, 65UT 461 +/- 72, P < 0.05). Leg muscle lipid oxidation increased with training in ABT ( 730 +/- 163 mu mol/min) vs. 65UT ( 187 +/- 94, P < 0.05). Leg muscle lipid oxidation represented similar to 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.