AJP - Endocrinology and Metabolism, Vol 267, Issue 6 E934-E940, Copyright © 1994 by American Physiological Society
Fat metabolism during low-intensity exercise in endurance-trained and untrained men
S. Klein, E. F. Coyle and R. R. Wolfe
Department of Internal Medicine, University of Texas Medical Branch, Galveston 77555.
Whole body lipid kinetics were evaluated during basal resting conditions, 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-trained men. Glycerol
and free fatty acid (FFA) rate of appearance (Ra) values in plasma were
determined by infusing [2H5]glycerol and [1-13C]palmitate, respectively,
and lipid oxidation was determined by indirect calorimetry. The lipolytic
response to 4 h of exercise, expressed as the average glycerol and FFA Ra
values, was similar in both trained (9.85 +/- 1.02 and 24.64 +/- 3.76
mumol.kg-1.min-1, respectively) and untrained subjects (11.29 +/- 0.99 and
24.13 +/- 0.39 mumol.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 mumol.kg-1.min-1, respectively; P
< 0.001). During recovery, glycerol and FFA Ra values decreased more
rapidly in trained than in untrained subjects. We conclude that highly
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 fat oxidation must
be related to an increased percentage of FFA uptake oxidized, a greater
contribution from intramuscular triglyceride stores, or both. Additionally,
lipid kinetics return to baseline more rapidly in trained than in untrained
subjects after completing an exercise bout of the same absolute intensity.
