Short-Term Warm-Water Immersion as a Heat Acclimation Strategy to Enhance Heat Dissipation in Older Adults: An Exploratory Study

Abstract

Heat acclimation mediated by repeated daily bouts of exercise in hot environments, can induce marked improvements in physiological and thermoregulatory function. However, recurrent bouts of strenuous exercise in the heat may be untenable for many heat-vulnerable older adults. While heat acclimation induced via warm-water immersion has been shown to provide near comparable improvements in thermoregulatory function in young adults, it remains unclear if a similar response would be observed in older adults. Warm-water immersion would represent a safe and practical approach to improve heat tolerance for many older populations. Thus, the present study was directed at evaluating the effectiveness of a seven-day passive warm-water (~40°C) immersion protocol (60-min at a fixed core temperature of ~38.5°C) in enhancing whole-body heat loss (WBHL) in twelve habitually active older males (mean (SD): 69 (6) years; peak aerobic power [V̇O2peak]: 32.7 (3.8) mL·kg-1·min-1). Before (Day 0) and after (Day 8) a 7-day heat acclimation protocol, whole-body (dry and evaporative) heat exchange was measured using a direct air calorimeter during an incremental intermittent exercise test. The exercise heat-stress test consisted of three, 30-min bouts of cycling performed at increasing fixed rates of metabolic heat production (150 (Low), 200 (Moderate), and 250 W·m-2 (Vigorous); equivalent to 30 (5), 42 (7) and 51 (9) % of their pre-determined V̇O2peak), each separated by 15-min of seated recovery, in a hot-dry environment (40°C, ~13% relative humidity). WHBL increased by 29 W·m-2 [95% Confidence Interval: 19, 39] at the highest metabolic heat load (P<0.01) on Day 8 relative to Day 0. This was paralleled by a reduction in body heat storage (-82 [-116, -48] kJ) and rectal temperature (-0.3 [-0.5, -0.2] °C; both P<0.01), as well as a reduction in cardiovascular strain indexed by a decrease in heart rate (-12 [-7, -17] beats·min-1; P<0.01). Altogether, we showed that a short-term passive warm-water immersion protocol is associated with marked improvements in the body’s physiological capacity to dissipate heat in older adults. The increase in heat loss capacity may help safeguard older populations when exposed to hot environments or performing physical activity by mitigating potentially dangerous increases in body temperatures.