[Physiologic basis of human adaptation to cold]

Intermittent exposures to cold (15 head out immersions into 14 degrees C water, for 1 h, within 3 weeks) induce physiological changes leading to the restriction of heat loss from the body, due to hypothermia and vasoconstriction. These adaptational changes may save more than 25% of energy during 1 hour cold test. Insulative adaptation due to increased amount of the subcutaneous fat is less prominent. Winter swimmers show similar mechanisms of cold adaptationn as humans adapted by intermittent cold exposures in the laboratory. Additionally, winter swimmers exert a greater capacity of nonshivering thermogenesis than nonadapted controls. Thus, adaptation of humans to cold involves hypothermic, metabolic and insulative mechanisms. Time courses of development of individual mechanisms of adaptation differ. Intermittent local cold exposures (20 immersions of legs into 12 degrees C water, for 45 min, within 4 weeks lower sympathetic tone and induce redistribution of cardiac output, in order to ensure preferential warming of central parts of the body. Basal metabolism and cold induced thermogenesis are increased, but the threshold for induction of cold thermogenesis is not influenced. In contrast to changes induced by repeated whole body immersions, these changes may not ensure greater resistance to the cold stress.