title: Heat stress increases muscle glycogen use but reduces the oxidation of ingested carbohydrates during exercise.
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.
authors.affiliation: Human Performance Laboratory, School of Sport and Exercise Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom.
authors.forename: Roy L P G
authors.initials: RL
authors.lastname: Jentjens
authors.forename: Anton J M
authors.initials: AJ
authors.lastname: Wagenmakers
authors.forename: Asker E
authors.initials: AE
authors.lastname: Jeukendrup
mesh_terms: D000328:Adult; D001642:Bicycling; D001786:Blood Glucose / Q000378:metabolism; D015992:Body Mass Index; D002247:Carbon Isotopes; D004040:Dietary Carbohydrates / Q000493:pharmacokinetics*; D004064:Digestive System / Q000503:physiopathology; D005947:Glucose / Q000493:pharmacokinetics; D006003:Glycogen / Q000378:metabolism*; D006339:Heart Rate; D018882:Heat Stress Disorders / Q000378:metabolism*; D006801:Humans; D019344:Lactic Acid / Q000378:metabolism; D008297:Male; D018482:Muscle, Skeletal / Q000378:metabolism*; D010084:Oxidation-Reduction; D010465:Perception; D005082:Physical Exertion / Q000502:physiology*; D010953:Plasma Volume
chemical_list: D001786:Blood Glucose; D002247:Carbon Isotopes; D004040:Dietary Carbohydrates; D019344:Lactic Acid; D006003:Glycogen; D005947:Glucose