Carbohydrate supplementation improves moderate and high-intensity exercise in the heat

The aim of the present study was to clarify the effect of carbohydrate (CHO) supplementation on moderate and high-intensity endurance exercise in the heat. Eight endurance-trained men [maximal oxygen uptake (VO2max) 59.5±1.6 ml kg−1 bw−1, mean±SE] cycled to exhaustion twice at 60% VO2max and twice at 73% VO2max at an ambient temperature of 35°C. Subjects ingested either a 6.4% maltodextrin solution (CHO) or an artificially flavoured and coloured placebo (PLA). Time to fatigue was significantly greater with CHO in both the 60% and 73% VO2max trials (14.5% and 13.5% improvement, respectively). Heart rate and oxygen uptake (VO2) did not differ at any point between PLA and CHO. Hypoglycaemia was not seen in any condition but plasma glucose concentrations tended to be higher at both intensities when CHO was fed. CHO oxidation rates were similar at 60% VO2max between CHO and PLA. There were no differences between PLA and CHO in the rate of rise of rectal temperatures (Trec) at either intensity but there was a trend for subjects to fatigue at a high temperature when taking CHO. Ratings of perceived exertion (RPE) tended to be lower throughout both CHO trials; this was significant at 80 min and at fatigue at 60% VO2max. It is concluded that supplementation with CHO improves exercise performance in the heat at both moderate and high endurance intensities. In the absence of a clear metabolic explanation, a central effect involving an increased tolerance of rising deep body temperature merits further investigation.