Influence of heat stress and carbohydrate availability on substrate metabolism and exercise tolerance time in humans
Whilst the effects of environmental heat stress on the physiological responses of humans during exercise have been investigated for over half a century, the mechanisms responsible for fatigue during exercise in the heat are not well understood. There is increasing evidence that heat stress increases the reliance on carbohydrate (CHO), particularly muscle glycogen, as a fuel for prolonged exercise. The provision of CHO during exercise and during short-term recovery from exercise in the heat may theoretically offer some benefit. However, the literature available on the efficacy of CHO feedings during prolonged running in the heat is scarce. The aim of the experiments that are reported in this thesis were to investigate the effects of heat stress and CHO feeding regimens on substrate metabolism and exercise tolerance during prolonged running. An initial investigation revealed that the heat stress imposed by wearing a military protective clothing ensemble during prolonged running impaired exercise tolerance time and increased the reliance on CHO as a fuel. This response was associated with increases in circulating adrenaline and lactate concentrations, which may be indicative of an enhanced ß-adrenergic receptor stimulation of muscle glycogenolysis. Thus, further studies into the efficacy of CHO supplementation regimens during exercise and recovery from exercise in the heat were performed. Rehydration with a carbohydrate-electrolytes olution (CES) during a 4-h recovery period markedly increased total CHO utilisation and exercise tolerance during subsequent exercise in the heat (35°C) compared to a sweetened placebo. Whilst there was no difference in post-recovery exercise tolerance time after ingesting 55-g or 220-g of CHO within a CES, ingesting 220-g lead to a five-fold increase in estimated glycogen synthesis during recovery, which increased CHO availability and utilisation during subsequent exercise. Ingesting a 12.5% glucose solution attenuated the increased reliance on endogenous CHO stores during exercise in the heat, but the associated increases in thermal and cardiovascular strain and gastric discomfort may have been responsible for the impairment of exercise capacity. These findings suggest that increases in endogenous CHO metabolism occur in response to exogenous heat stress during prolonged running. Whilst CHO ingestion during short-term recovery periods are associated with favourable changes in glycogen synthesis and tolerance to subsequent exercise in the heat, ingestion of a hypertonic glucose solution during exercise in the heat, may impair exercise capacity.