Sex- and menstrual cycle-related differences in sweating and cutaneous blood flow in response to passive heat exposure

To examine sex- and menstrual cycle-related differences in thermoregulatory responses to heat exposure, ten young women and six young men were heated passively by immersing their legs in water heated to 42°C for 60 min (in ambient conditions of 30°C and 45% relative humidity). The women underwent heat exposure during the mid-follicular (F) and mid-luteal (L) phases of the menstrual cycle, which were confirmed by assaying plasma female reproductive hormones. The rectal and mean body \((\bar T_{\text{b}} )\) temperatures of women in the L phase were significantly greater than those of women in the F phase and of men during a pre-heating equilibration period (28°C) and during heat exposure. During heat exposure, the local sweat rates (\(\dot m_{{\text{sw}}} \)) on the forehead, chest, back, and forearm of women in either phase were significantly lower than those of men, but the thigh \(\dot m_{{\text{sw}}} \) was similar to that of men. The \(\dot m_{{\text{sw}}} \) did not change at any site during the different phases of the menstrual cycle. The cutaneous blood flow (%LDF) was significantly greater on the thigh for women in either phase compared with men, but no difference was found at any other site (forehead, chest, back, and forearm). The %LDF on the back was significantly greater for women in the L phase than in the F phase, but those at other sites were similar in both phases. We conclude that, compared with men, heat loss from women depends more on cutaneous vasodilation (especially on the thigh) than on sweating, irrespective of the phase of the menstrual cycle. This phenomenon was due to peripheral mechanisms, as reflected in the greater slope of the relationship between %LDF and \(\bar T_{\text{b}},\) lower slope of the relationship between \(\dot m_{{\text{sw}}} \) and frequency of sweat expulsion, and lower sweat output per gland. The menstrual cycle modified the \(\bar T_{\text{b}} \) threshold for vasodilation and sweat onset in women. Therefore, the sex difference in the \(\bar T_{\text{b}} \) threshold was more marked for women during the L phase than during the F phase. Moreover, the menstrual cycle modified the slope of the relationship between %LDF on the back and \(\bar T_{\text{b}} .\)