Exercise intensity regulates the effect of heat stress on substrate oxidation rates during exercise

Abstract Hyperthermia stimulates endogenous carbohydrate metabolism during exercise; however, it is not known if exercise intensity impacts the metabolic effect of heat stress. In the first study of this two‐part investigation, endurance‐trained male cyclists performed incremental exercise assessments in 18 and 35°C (60% rH). The stimulatory effect of heat stress on carbohydrate oxidation rates was greater at high vs. moderate vs. low relative intensity ( P < 0.05). In agreement, no effects of heat stress on carbohydrate oxidation rates were observed during 60‐min of subsequent low‐intensity cycling. In study two, endurance‐trained male cyclists performed 20‐min of moderate‐intensity (power at the first ventilatory threshold) and 5‐min of high‐intensity (power at the second ventilatory threshold) cycling in 18, 28, 34, and 40°C (60% rH). At moderate‐intensity, carbohydrate oxidation rates were significantly elevated by heat stress in 40°C ( P < 0.05), whereas at high‐intensity carbohydrate oxidation rates were significantly elevated by heat stress in 34 and 40°C ( P < 0.05). This exercise intensity‐mediated regulation of the effect of heat stress on carbohydrate oxidation may be partially attributable to observed plasma adrenaline responses. Our data suggest that under moderate environmental heat stress (34–35°C, 60% rH), heat stress‐induced changes in CHO oxidation rates are unlikely to occur unless the relative exercise intensity is high (81 ± 8%⩒O 2max ), whereas under more extreme environmental heat stress (40°C, 60% rH), these changes occur at lower relative intensities (69 ± 8%⩒O 2max ). This provides indication of when heat stress‐induced metabolic changes during exercise are likely to occur.