Muscle metabolism during prolonged exercise in humans: influence of
carbohydrate availability.
McConell, G., R. J. Snow and M. Hargreaves.
1Department of Physiology, Monash University, Clayton, Victoria,
3168, 2School of Health Sciences, Deakin University, Burwood,
Victoria, 3125 and 3Department of Physiology, The University of
Melbourne, Parkville, Victoria, 3052, Australia.
APStracts 6:0258A, 1999.
Eight endurance-trained men cycled to volitional exhaustion at 69 +/-
1% VO2 peak on two occasions to examine the effect of carbohydrate
supplementation during exercise on muscle energy metabolism. Subjects
ingested an 8% carbohydrate solution (CHO) or a sweet placebo (CON)
in a double-blind, randomized order with vastus lateralis muscle
biopsies (n=7) obtained prior to and immediately following exercise.
No differences in oxygen uptake, heart rate or respiratory exchange
ratio during exercise were observed between the trials. Exercise time
to exhaustion was increased by [sim]30% when carbohydrate was
ingested (199 +/- 21 vs 152 +/- 9 min, mean +/- SE, P<0.05). Plasma
glucose and insulin levels during exercise were higher and plasma
free fatty acids lower in CHO. No differences between trials were
observed in the decreases in muscle glycogen and phosphocreatine or
the increases in muscle lactate due to exercise. Muscle ATP levels
were not altered by exercise in either trial. There was a small but
significant increase in muscle inosine monophosphate (IMP) levels at
the point of exhaustion in both trials and despite the subjects
cycling 47 min longer in CHO, muscle IMP was significantly lower than
in CON (CHO: 0.16 +/- 0.08, CON: 0.23 +/- 0.09 mmol.kg-1 dry muscle).
These data suggest that carbohydrate ingestion may increase endurance
capacity, at least in part, by improving muscle energy balance.
Received 28 December 1998; accepted in final form 24 May 1999.
APS Manuscript Number A1179-8.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1999 The American Physiological Society.
Published in APStracts on 14 June 1999