EFFECTS OF ACCLIMATIZATION TO COLD BATHS ON MENS RESPONSES TO WHOLE-BODY COOLING IN AIR

The purpose of this study was to investigate the thermoregulatory mech anisms underlying artificial acclimatization to cold and to compare th em with those of naturally acclimatized men. Six white men were cooled , nude, in air at 10 degrees C for 2 h before and after they had been acclimatized by ten daily cold (15 degrees C) baths of 30-60 min follo wed by rapid rewarming in hot (38-42 degrees C) water, and again 4 mon ths later after acclimatization had decayed. Six control subjects also underwent the same tests, providing an opportunity to discriminate be tween changes caused by the immersions and those caused by extraneous influences. Acclimatization significantly reduced heat production and heat loss (P< 0.05) but did not change heat debt. The reduced heat pro duction was accompanied by reductions in shivering (P< 0.10) and in co ld-induced muscle tenseness; no evidence of nonshivering thermogenesis or active brown fat was found. These findings are attributed to incre ased tissue insulation, mediated by an enhanced vascular response to c old that did not involve the cutaneous circulation and was probably lo cated in skeletal muscle. Thermal sensation and discomfort did not cha nge, although perceived strain tended to increase (P=0.08). Acclimatiz ation was accompanied by, but was unrelated to, slower cooling of the finger and toe. The main conclusions, and many specific findings, agre e with those of two previous studies made by the same techniques in na turally acclimatized men wintering in Antarctica. Other significant fi ndings included changes - in particular reduced thermoneutral rectal t emperature and a delayed onset of shivering - that are commonly regard ed as evidence of acclimatization but were in fact unrelated to it as they also occurred in the control group. They are attributed to extran eous influences, in particular the relaxation of heightened arousal (' first-time effects') found in the baseline tests.