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Acclimatization

Persons chronically exposed to cold experience adjustments in thermoregulation (Young, 1988). Habituation is, by far, the most commonly observed adjustment to chronic cold exposure. Blunting of both shivering and cold-induced vasoconstriction are the hallmarks of habituation (Young, 1988). These adjustments enable skin to be kept warmer during cold exposure, but they can contribute to a greater heat loss and more pronounced fall in core temperature.

Besides habituation, cold acclimatization and cold acclimation can heighten responses to cold or induce responses not apparent in the unacclimatized state. These adjustments follow two patterns. First, metabolic acclimatization-acclimation is characterized by a more pronounced thermogenic response to cold (Young, 1988). An exaggerated shivering response may develop because of chronic cold exposure, and the possibility that humans develop a nonshivering thermogenesis cannot be completely ruled out. In contrast, enhanced heat conservation mechanisms characterize the insulative acclimatization-acclimation pattern (Young, 1988). More rapid cutaneous vasoconstriction develops in some chronically cold-exposed persons, an adjustment that may reflect an enhanced sympathetic nervous response (Young, 1988).

Compared to chronic heat stress, physiological adjustments to chronic cold exposure appear less practical in terms of relieving thermal strain, defending body temperature, and preventing thermal illness and injury. Nonetheless, changes in shivering response to cold resulting from habituation or metabolic acclimatization may have some nutritional implications.

AUTHORS' CONCLUSIONS AND RECOMMENDATIONS

Humans exhibit two major physiological responses to cold exposure. Peripheral vasoconstriction limits heat loss. Shivering, physical activity, or both increase heat production. Thus, heat balance in the cold and the requirement for shivering are dependent on the severity of environmental stress and the effectiveness of the vasoconstriction for conserving heat, as well as the intensity and mode of activity or exercise. There are nutritional implications of the physiological responses, particularly the thermogenic response. Increasing metabolic heat production requires increased energy intake. Carbohydrate metabolism may contribute more to total energy metabolism in cold than in temperate environments. Gender, aging, and acclimatization all affect thermoregulatory responses to cold, but these effects probably have little nutritional significance. Body composition is probably the most important physiological determinant of thermoregulatory tolerance in cold environments. Behavioral responses, such as taking shelter from the cold and wearing adequate protective clothing, can greatly reduce the physiological strain of cold exposure and obviate the need for nutritional interventions.



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