Heat transfer and loss by whole-body hyperthermia during severe lower-body heating are impaired in healthy older men

Most studies demonstrate that aging is associated with a weakened thermoregulation. However, it remains unclear whether heat transfer (for heat loss) from the lower (uncompensable) to the upper (compensable) body during passively-induced severe lower-body heating is delayed or attenuated with aging. Therefore, the main purpose of this study was to investigate heat transfer from uncompensable to compensable body areas in young men and healthy older men during passively-induced whole-body hyperthermia with a demonstrated post-heating change in core body (rectal; Tre) temperature. Nine healthy older men and eleven healthy young men (69 ± 6 vs. 21 ± 1 years old, mean ± SD, P < 0.05) participated in passively-induced severe lower-body heating in water at approximately 43 °C. Despite a similar increment in Tre (approximately 2.5 °C) in both groups, the heating rate was significantly lower in older men than in young men (1.69 ± 0.12 vs. 2.47 ± 0.29 °C/h, respectively; P < 0.05). The temperature increase in calf muscle and calf skin (uncompensable areas) was significantly higher in older men than in young men (5.10 ± 0.18 vs. 3.99 ± 0.14 °C; P < 0.05 and 9.92 ± 0.22 vs. 7.65 ± 0.33 °C; P < 0.05, respectively). However, the temperature increase in back skin and forearm skin (compensable areas) was significantly lower in older men than in young men (0.76 ± 0.63 vs. 2.83 ± 0.68 °C; P < 0.05 and 0.39 ± 0.76 vs. 2.73 ± 0.5 °C; P < 0.05, respectively). Furthermore, a post-warming increase in Tre of approximately 0.2 °C was observed only in older men (P < 0.05). In conclusion, older men whose lower extremities were immersed showed greater accumulation and storage of heat in the skin and deep muscles than young men, and this was associated with a greater heat-transfer delay and subsequent inertia in the increased core body (Tre) temperature.