FAT-METABOLISM DURING LOW-INTENSITY EXERCISE IN ENDURANCE-TRAINED ANDUNTRAINED MEN

Whole body lipid kinetics were evaluated during basal resting conditio ns, 4 h of treadmill exercise eliciting an oxygen uptake of 20 ml.kg(- 1).min(-1), and 1 h of recovery in five untrained and five endurance-t rained men. Glycerol and free fatty acid (FFA) rate of appearance (R(a )) values in plasma were determined by infusing [H-2(5)]glycerol and [ 1-C-13]palmitate, respectively, and lipid oxidation was determined by indirect calorimetry. The lipolytic response to 4 h of exercise, expre ssed as the average glycerol and FFA R, values, was similar in both tr ained (9.85 +/- 1.02 and 24.64 +/- 3.76 mu mol.kg(-1).min(-1), respect ively) and untrained subjects (11.29 +/- 0.99 and 24.13 +/- 0.39 mu mo l.kg(-1).min(-1), respectively). However, mean triglyceride oxidation was greater during exercise in the trained than in the untrained group (7.51 +/- 0.26 and 5.67 +/- 0.51 mu mol.kg(-1).min(-1), respectively; P < 0.001). During recovery, glycerol and FFA R(a) values decreased m ore rapidly in trained than in untrained subjects. We conclude that hi ghly trained male endurance runners use more fat as a fuel during low- intensity exercise than do untrained healthy men despite similar rates of lipolysis and FFA uptake from plasma. Therefore, the increase in f at oxidation must be related to an increased percentage df FFA uptake oxidized, a greater contribution from intramuscular triglyceride store s, or both. Additionally, lipid kinetics return to baseline more rapid ly in trained than in untrained subjects after completing an exercise bout of the same absolute intensity.