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1Exercise Physiology Lab, Clinical Studies Unit, Veterans Affairs Palo Alto Health Care System, Palo Alto; 2Exercise Physiology Laboratory, Department of Integrative Biology, University of California Berkeley, Berkeley, Califoria; and 3University of Colorado, Health Science Center, Division of Cardiology, Denver, Colorado
Submitted 29 March 2006 ; accepted in final form 21 July 2006
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% 
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% O2 max (45UT, 65UT)] and two posttraining trials [65% of pretraining O2 peak (ABT), and 65% of posttraining O2 peak (RLT)] were performed using [1-13C]palmitate infusion and femoral arteriovenous sampling. Training increased 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 (
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; exercise; regional metabolism
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