Alterations in energy metabolism during exercise and heat stress

Much of the research that has examined the interaction between metabolism a nd exercise has been conducted in comfortable ambient conditions. It is cle ar, however, that environmental temperature, particularly extreme heat, is a major practical issue one must consider when examining muscle energy meta bolism. When exercise is conducted in very high ambient temperatures, the g radient for heat dissipation is significantly reduced which results in chan ges to thermoregulatory mechanisms designed to promote body heat loss. This can ultimately impact upon hormonal and metabolic responses to exercise wh ich act to alter substrate utilisation. In general, the literature examinin g metabolic responses to exercise and heat stress has demonstrated a shift towards increased carbohydrate use and decreased fat use. Although glucose production appears to be augmented during exercise in the heat, glucose dis posal and utilisation appears to be unaltered. In contrast, glycogen use ha s been consistently demonstrated to be augmented during exercise in the hea t. This increase in glycogenolysis is observed via both aerobic and anaerob ic pathways. Although several hypotheses have been proposed as mechanisms f or the substrate shift towards greater carbohydrate metabolism during exerc ise and heat stress, recent work suggests that an augmented sympatho-adrena l response and intramuscular temperature may be responsible for such a phen omenon.