Thermoregulation in surfers and nonsurfers immersed in cold water.

During immersion to the neck in 19 +/- 0.6 degrees C water, surfers maintained higher toe temperatures than nonsurfers. The Lewis hunting response was seen only in the toes of the surfers. The shivering response of the surfers occurred later and was of lesser intensity. Threefold increases in metabolic heat production (140 kcal/m2-h) above rest level were found in both groups. Rectal temperature changes were similar in both groups, with a mean decline of 1.2 degrees C over the 1-h exposure period. Individual changes in rectal temperature were negatively correlated to percent of body fat. An initial hyperventilation upon immersion was followed first by a decline and then by a rise to three times that of rest. A transient rise in heart rate (35 beats/min) occurred in the initial stage of immersion in both groups and subsequently fell to basal levels, rising slightly thereafter. Sinus arrhythmias were observed during the first few minutes of immersion. In nonsurfers, plasma cortisol approximated the decrease anticipated because of the circadian cycle but was elevated in the surfers. Plasma volume decreased 12.2% (surfers) and 17.6% (nonsurfers). Diuresis was observed in both groups, and was 3.2 and 5.0 ml/min for surfers and nonsurfers, respectively.