Heat acclimation improves the neural control of body temperature during heat stress in older adults.

The mechanisms underlying impaired temperature regulation in older age, and the ability of older adults to physiologically adapt to heat exposure remain understudied. This study determined the effects of older age on the neural control of body temperature during heat stress and its plasticity in response to passive heat acclimation. The neural control of body temperature during heat stress was quantified with measurements of skin sympathetic nerve activity (SSNA), local sweat rate (LSR) and cutaneous vascular conductance (CVC) performed on older (n = 15, 60-78 years) and younger (n = 16, 21-40 years) adults before and after seven consecutive days of hot water immersion. Pre-acclimation, the onset threshold of SSNA (mean difference [95% CI]: -0.12°C [-0.38, 0.15], P = 0.518), LSR (-0.14°C [-0.37, 0.08], P = 0.265) and CVC (-0.09°C [-0.33, 0.14], P = 0.600) did not differ between older and younger adults. Heat acclimation reduced resting core temperature and increased sweating in both groups. Post-acclimation, the onset threshold of SSNA (-0.19°C [-0.36, -0.02], P = 0.024), LSR (-0.15°C [-0.29, 0.00], P = 0.036), and CVC (-0.23°C [-0.41, -0.06], P = 0.009) was reduced in older adults and these reductions did not differ from those observed in younger adults (P ≥ 0.703). These results show that older age does not alter the neural control of body temperature during heat stress, nor its plasticity to heat acclimation. The implications of these results are that passive heat acclimation may be a practical strategy to improve the physiological resilience of older adults to extreme heat events. KEY POINTS: It remains unclear how older age modulates the neural control of body temperature during heat stress and its plasticity to heat acclimation. This study determined the effect of older age on the neural control of body temperature during heat stress before and after 7 days of heat acclimation. The results show that older age does not alter the neural control of body temperature during heat stress, nor its improvement with heat acclimation.