Reduced oxidation of ingested carbohydrates during exercise in the heat

The aim of the present study was to establish whether the oxidation rate of ingested carbohydrate (CHO) is impaired during exercise in the heat compared with a cool environment. For this purpose 9 trained cyclists performed 2 exercise trials in random order and separated by at least one week. Each trial consisted of 90 min of cycling at 67 ± 1% VO2max at an ambient temperature of either 16.4 ± 0.2°C (Cool) or 35.4 ± 0.1°C (Heat). Subjects received 8 ml·kg bw-1 of an 8% glucose solution at the onset of exercise followed by 3 ml·kg bw-1 every 15 min thereafter. [U13C]-glucose was added to the glucose solution for measurements of exogenous glucose (EXO), plasma glucose (PGO), hepatic glucose (HGO) and muscle glycogen oxidation (MGO). Blood and breath samples were collected at rest and every 15 min during exercise. Heart rate (HR) and rectal temperature (Trec) were continuously recorded. HR and Trec were significantly (p < 0.05) higher in the heat compared with Cool (164 ± 5 vs. 140 ± 4 bpm and 38.95 ± 0.19 vs. 38.26 ± 0.08°C, resp.). EGO was significantly (p < 0.05) lower in the heat compared with Cool (0.76 ± 0.06 vs. 0.84 ± 0.05 g·min-1). There was a trend towards a higher total CHO oxidation and a lower PGO in the heat although this did not reach statistical significance (p = 0.087 and p = 0.082, resp.). MGO was increased by 25% in the heat (2.07 ± 0.16 vs. 1.66 ± 0.09 g·min-1). No difference was observed in HGO. It is concluded that muscle glycogen utilization is increased and the oxidation rate of ingested CHO is reduced during exercise in the heat compared with a cool environment. Supported by a grant of SmithKline Beecham