Relationship between fatty acid delivery and fatty acid oxidation
during strenuous exercise.
Romijn, J. A., E. F. Coyle, L. S. Sidossis, X-J. Zhang, R. R. Wolfe.
Metabolism Unit, Shriners Burns Institute, Galveston, TX 77550,
Human Performance Laboratory, Department of Kinesiology and Health
Education, The University of Texas at Austin, Austin, TX 78712, and
Departments of Anesthesiology, Biochemistry, and Surgery, University
of Texas Medical Branch, Galveston, TX 77550
APStracts 2:0346A, 1995.
To evaluate the extent to which decreased plasma free fatty acid (FFA)
concentration contributes to the relatively low rates of fat
oxidation during high intensity exercise, we studied FFA metabolism
in six endurance-trained cyclists during 20-30 min of exercise (85%
VO2max). They were studied on two occasions, once during a control
trial when plasma FFA concentration is normally low, and again to
when plasma FFA concentration was maintained between 1-2 mM by
intravenous infusion of lipid (Intralipid_) and heparin. During the
20-30 min period of exercise, fat and carbohydrate oxidation were
measured by indirect calorimetry and the rates of appearance (Ra) of
plasma FFA and glucose were determined by the constant infusion of
(6, 6-2H2)glucose and (2H2)palmitate. Lipid/heparin infusion did not
influence the Ra or rate of disappearance (Rd) of glucose. During
exercise in the control trial, Ra FFA failed to increase above
resting levels (i.e.; rest: 11.0+/-1.2 vs exercise: 12.4+/-1.7
[mu]mol. kg-1. min-1) and plasma FFA concentration dropped from a
resting value of 0.53+/-.08 to 0.29+/-.02 mM. The restoration of
plasma FFA concentration resulted in a 27% increase in total fat
oxidation (26.7+/-2.6 vs. 34.0+/-4.4 [mu]mol. kg-1. min-1;
P<0.05) with a concomitant reduction in carbohydrate oxidation,
apparently due to a 15% (P<0.05) reduction of muscle glycogen
utilization. However, the elevation of plasma FFA concentration
during exercise at 85% VO2max only partially restored fat oxidation
compared with the levels observed during exercise at 65% VO2max.
These findings indicate that fat oxidation is normally impaired
during exercise at 85% VO2max due to the failure of FFA mobilization
to increase above resting levels, but this explains only part of the
decline in fat oxidation when exercise intensity is increased from
65% to 85% VO2max.
Received 18 April 1994; accepted in final form 19 July 1995.
APS Manuscript Number A357-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 14 August 1995.