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Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations
significant, the magnitude of the effects was reported to be smaller for treadmill and cycle-ergometer exercise.
Skeletal Muscle Metabolism
Several investigations examined the effects of environmental heat stress on skeletal muscle metabolism during exercise. Fink et al. (1975) had six subjects perform 45 minutes of cycle exercise (70 to 85 percent of ) in a cold (9°C) and a hot (41°C) environment. They found greater plasma lactate levels and increased muscle glycogen utilization during exercise in the heat. Also, muscle triglyceride utilization was reduced during exercise in the heat as compared to the cold. In addition, serum glucose concentration increased, and serum triglyceride concentration decreased during exercise in the heat, compared to the opposite responses during exercise in the cold. During exercise in the heat, the increased muscle glycogen utilization was attributed to an increased anaerobic glycolysis resulting from local muscle hypoxia, caused by a reduced muscle blood flow. Because these investigators (Fink et al., 1975) did not perform control experiments in a temperate environment, it is not known if the differences reported are due partially to the effects of the cold exposure.
Young et al. (1985) had 13 subjects perform 30 minutes of cycle exercise (70 percent of ) in a temperate (20°C) and a hot (49°C) environment. They found skeletal muscle and plasma lactate concentrations were greater during exercise in the heat. There was no difference in muscle glycogen utilization between the two experimental conditions. Young et al. (1985) speculated that during exercise in the heat, an alternative glycolytic substrate might have been utilized, such as blood glucose. Rowell et al. (1968) demonstrated a dramatic increase in hepatic glucose release into the blood during exercise in a hot compared to a temperate environment. Such an increased release of hepatic glucose could account for the elevated serum glucose concentration reported in the hot environment by Fink et al. (1975).
Data from Dimri et al. (1980) and Young et al. (1985) support the concept of increased anaerobic metabolism during submaximal exercise in the heat. Much of the other support for this concept is based on the findings that, during submaximal exercise, the plasma lactate accumulation is greater in a hot than in a comfortable environment. However, any inference about metabolic effects within the skeletal muscle from changes in plasma lactate is open to debate. Plasma lactate concentration reflects the balance between muscular production, efflux into the blood, and removal from the blood. Rowell et al. (1968) have shown that during exercise in the heat the splanchnic vasoconstriction reduced hepatic removal of plasma lactate. Therefore, the greater blood lactate accumulation during submaximal exercise in the heat