Effects of training duration on substrate turnover and oxidation
during exercise.
Phillips, S. M., H. J. Green, M. A. Tarnopolsky, G. J. F. Heigenhause,
R. E. Hill, and S. M. Grant.
Department of Kinesiology, University of Waterloo, Waterloo, ON,
N2L 3G1, Canada and Departments of Kinesiology, Cardiorespiratory
Medicine, and Pathology, McMaster University, Hamilton, ON, L8N 3Z5,
Canada
APStracts 3:0356A, 1996.
Adaptations in fat and carbohydrate metabolism following a prolonged
endurance training program, were examined using stable isotope
tracers of glucose ([6,6-2H2]glucose), glycerol ([2H5]glycerol), and
palmitate ([2H2]palmitate). Active, but untrained males exercised on
a cycle for 2h/day (60% pre-training O2 peak=44.3+/-2.4 ml/kg/min)
for a total of 31 days. Three cycle tests (90 min at 60% pre-training
O2 peak) were administered, pre-training (PRE), and after 5 (5D) and
31 (31D) days of training. Exercise increased the rate of glucose
production (Ra) and utilization (Rd) as well as the rate of lipolysis
(glycerol Ra) and free fatty-acid turnover (FFA Ra/Rd). At 5D,
training induced a 10% (P<0.05) increase in total fat oxidation,
due to an increase in intramuscular triglyceride oxidation (+63%,
P<0.05) and a decreased glycogen oxidation (-16%, P<0.05).
Following 31D, total fat oxidation during exercise increased a
further 58% (P<0.01). The pattern of fat utilization during
exercise at 31D, showed a reduced reliance on plasma FFA oxidation
(FFA Rd), and a greater dependence on oxidation of intramuscular
triglyceride, which increased >2-fold (P<0.001). In
addition, glucose Ra and Rd were reduced at all time points during
exercise at 31D versus both PRE and 5D. We conclude that long-term
training induces a progressive increase in fat utilization, mediated
by a greater oxidation of fats from intramuscular sources and a
reduction in glucose oxidation. Initial changes are present after
only 5D of training, and occur prior to increases in muscle maximal
mitochondrial enzyme activity (29).
Received 19 March 1996; accepted in final form 16 July 1996.
APS Manuscript Number A277-6.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 4 August 1996