Heat stress increases muscle glycogen use but reduces the oxidation of ingested carbohydrates during exercise

J Appl Physiol (1985). 2002 Apr;92(4):1562-72. doi: 10.1152/japplphysiol.00482.2001.

Abstract

The aim of the present study was to test the hypothesis that the oxidation rate of ingested carbohydrate (CHO) is impaired during exercise in the heat compared with a cool environment. Nine trained cyclists (maximal oxygen consumption 65 +/- 1 ml x kg body wt(-1) x min(-1)) exercised on two different occasions for 90 min at 55% maximum power ouptput at an ambient temperature of either 16.4 +/- 0.2 degrees C (cool trial) or 35.4 +/- 0.1 degrees C (heat trial). Subjects received 8% glucose solutions that were enriched with [U-13C]glucose for measurements of exogenous glucose, plasma glucose, liver-derived glucose and muscle glycogen oxidation. Exogenous glucose oxidation during the final 30 min of exercise was significantly (P < 0.05) lower in the heat compared with the cool trial (0.76 +/- 0.06 vs. 0.84 +/- 0.05 g/min). Muscle glycogen oxidation during the final 30 min of exercise was increased by 25% in the heat (2.07 +/- 0.16 vs. 1.66 +/- 0.09 g/min; P < 0.05), and liver-derived glucose oxidation was not different. There was a trend toward a higher total CHO oxidation and a lower plasma glucose oxidation in the heat although this did not reach statistical significance (P = 0.087 and P = 0.082, respectively). These results demonstrate that the oxidation rate of ingested CHO is reduced and muscle glycogen utilization is increased during exercise in the heat compared with a cool environment.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Bicycling
  • Blood Glucose / metabolism
  • Body Mass Index
  • Carbon Isotopes
  • Dietary Carbohydrates / pharmacokinetics*
  • Digestive System / physiopathology
  • Glucose / pharmacokinetics
  • Glycogen / metabolism*
  • Heart Rate
  • Heat Stress Disorders / metabolism*
  • Humans
  • Lactic Acid / metabolism
  • Male
  • Muscle, Skeletal / metabolism*
  • Oxidation-Reduction
  • Perception
  • Physical Exertion / physiology*
  • Plasma Volume

Substances

  • Blood Glucose
  • Carbon Isotopes
  • Dietary Carbohydrates
  • Lactic Acid
  • Glycogen
  • Glucose