Cortisol as a sensitive index of heat-intolerance

The relationship between plasma cortisol levels, core temperatures, heat storage and the appearance of subjective manifestations of heat intolerance were investigated in two groups of 4 and 3 men, respectively. The first group underwent 4 randomized experimental sessions: a control session and three heat-exposure sessions (A: 48 degrees C, 34 Torr, 80 min; B: 55 degrees, 15 Torr, 120 min; C: 48 degrees C, 15 Torr, 180 min). During control periods, the subjects were maintained in a thermoneutral climate (28 degrees C, 10.5 Torr). The second group of subjects was studied before and after five successive daily exposures to hot and humid conditions (D: 43 degrees C, 32 Torr, 165 min). Signs of subjective discomfort in experiments A and B were accompanied by an increase in cortisol values over control day values, and this increase began at a mean rectal temperature of about 38 degrees C. Condition C was well tolerated, the plasma cortisol remained at basal levels and mean rectal temperature averaged 38 degrees C at the end of the exposure. Repeated exposures in condition D improved heat tolerance with a lesser effect on plasma cortisol levels and a lower body temperature at the end of exposure. Circulating cortisol is shown to be a very sensitive index of heat stress heralding the onset of poor tolerance of severe climates. The relation between concomitant levels of cortisol and rectal temperatures, in fit men, is affected by additional factors (hypotension, uncomfortable posture). These reduce tolerance time and are not reflected by body temperatures, but by rapidly increasing cortisol levels.