Journal of Applied Physiology, Vol 63, Issue 1 31-35, Copyright © 1987 by American Physiological Society
Substrate utilization in leg muscle of men after heat acclimation
J. P. Kirwan, D. L. Costill, H. Kuipers, M. J. Burrell, W. J. Fink, J. E. Kovaleski and R. A. Fielding
Eight men were heat acclimated (39.6 degrees C and 29.2% rh) for 8 days to
examine changes in substrate utilization. A heat exercise test (HET),
(cycling for 60 min; 50% maximal O2 consumption) was performed before
(UN-HET) and after (ACC-HET) the acclimation period. Muscle glycogen
utilization (67.0 vs. 37.6 mmol/kg wet wt), respiratory exchange ratio
(0.85 +/- 0.002 vs. 0.83 +/- 0.001), and calculated rate of carbohydrate
oxidation (75.15 +/- 1.38 vs. 64.80 +/- 1.52 g/h) were significantly
reduced (P less than 0.05) during the ACC-HET. Significantly lower (P less
than 0.05) femoral venous glucose (15, 30, and 45 min) and lactate (15 min)
levels were observed during the ACC-HET. No differences were observed in
plasma free fatty acid (FFA) and glycerol concentrations or glucose,
lactate and glycerol arteriovenous uptake/release between tests. A small
but significant increase (P less than 0.05) above resting levels in FFA
uptake was observed during the ACC-HET. Leg blood flow was slightly greater
(P greater than 0.05) during the ACC-HET (4.64 +/- 0.13 vs. 4.80 +/- 0.13
l/min). These findings indicate a reduced use of muscle glycogen following
heat acclimation. However, the decrease is not completely explained by a
shift toward greater lipid oxidation or increased blood flow.
