Seasonal variation in sweating responses of older and younger men

Eight older (60–65 years) and six younger (20–25 years) men were exposed to a standard heat stress for 60 min in summer, autumn, winter, and spring. The test consisted of placing the lower legs and feet in a 42°C water bath while sitting in constant environmental conditions (30°C and 45% relative humidity). The increase of rectal temperature (ΔT _re) was significantly greater (P < 0.05) in autumn, winter, and spring than in summer for the older group, but significantly greater only in winter than in summer for the younger group (P < 0.05). The ΔT _re was greater for the older group in all seasons, but of significance only in autumn and spring (P < 0.01). There were no significant season-related differences for metabolic heat production (m) and mean skin temperature ( \(\overline T \) _sk) during the heat test in the respective groups, although the m and \(\overline T \) _sk were lower for the older group in all seasons (P < 0.01). In the older group total body sweating rate (m_sw) divided by ΔT _re (total m_sw/ΔT _re) decreased from summer to winter (P < 0.02) and did not differ between winter and spring, whereas total m_sw/ΔT _re in the younger group increased in spring after decreasing from autumn to winter (P < 0.03). The variations of the value, local sweating rate on the back and thigh divided by ΔT _re (back m_sw/ΔT _re and thigh m_sw/ΔT _re), were similar to those of the total m_sw/ΔT _re in each group, except for back m_sw/ΔT _re in the younger group, which did not increase from winter to spring. The total m_sw/ΔT _re, back m_sw/ΔT _re and thigh m_sw/ΔT _re were significantly less for the older group in summer, autumn and spring (P < 0.05). The range of seasonal variations was significantly less for the older group (P < 0.001). The results indicated that, compared with younger men in older men, the enhancement of sweating function toward summer occurred later and its reduction toward winter occurred earlier despite a smaller range of seasonal variation and that older men had a somewhat lesser capability to maintainT _re when challenged by heat stress in all seasons.