Hot Water Immersion as a Heat Acclimation Strategy in Older Adults

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

Aging is associated with impairments in heat loss responses of skin blood flow and sweating leading to reductions in whole-body heat loss. Consequently, older adults store more body heat and experience greater elevations in core temperature during heat exposure at rest and during exercise. This maladaptive response occurs in adults as young as 40 years of age. Recently, heat acclimation associated with repeated bouts of exercise in the heat performed over 7 successive days has been shown to enhance whole-body heat loss in older adults, leading to a reduction in body heat storage. However, performing exercise in the heat may not be well tolerated or feasible for many older adults. Passive heat acclimation, such as the use of warm-water immersion may be an effective, alternative method to enhance heat-loss capacity in older adults. Thus, the following study aims to assess the effectiveness of a 7-day warm-water immersion (~40°C) protocol in enhancing whole-body heat loss in older adults. Warm-water immersion will consist of a one-hour immersion in warm water with core temperature clamped at 38.5°C. Improvements in whole-body heat loss will be assessed during an incremental exercise protocol performed in dry heat (i.e., 40°C, ~15% relative humidity) prior to and following the 7-day passive heat acclimation protocol. The incremental exercise protocol will consist of three 30 minute exercise bouts performed at increasing fixed rates of metabolic heat production (i.e., 150, 200, and 250 W/m2), each separated by 15-minutes of recovery, with exception final recovery will be 1-hour in duration) performed in a direct calorimeter (a device that provides a precise measurement of the heat dissipated by the human body).

Each participant will complete seven consecutive experimental sessions consisting of a passive heat acclimation involving a one hour immersion in warm water (~40°C) with core temperature clamped at 38.5°C. Prior to and following the passive heat acclimation protocol, study participants will perform an exercise-heat stress test (Days 0 and 8) to assess whole-body heat loss in a hot-dry environment (40°C, 15% relative humidity). The exercise protocol will consist of three successive 30-minute bouts of exercise performed at increasing rates of metabolic heat production (i.e., 150, 200 and 250 W/m2), each separated by a 15-minute rest break, with the final recovery 1-hour in duration.

Participants will undergo an exercise heat stress test prior to and following seven consecutive days of warm-water immersion (~40°C) of 1-hour duration with core temperature clamped at 38.5°C. During the exercise-heat stress test participants will perform three, successive 30-minute bouts of semi-recumbent cycling performed at increasing fixed loads of metabolic heat production of 150, 200 and 250 W/m2 (i.e., exercise bout 1, exercise bout 2 and exercise bout 3, respectively), each separated by 15-minute of rest break with the final recovery extended to 1-hour.

Participants will complete a 7-day passive heat acclimation protocol consisting of immersion in warm water (~40°C) for 1 hour over 7 consecutive days.

The sum of evaporative and dry heat exchange will be quantified during each exercise period (i.e., 150, 200 and 250 W/m2).

The sum of evaporative and dry heat exchange will be quantified during each exercise period (i.e., 150, 200 and 250 W/m2).

The sum of evaporative and dry heat exchange will be quantified during each exercise period (i.e., 150, 200 and 250 W/m2).

Cumulative amount of heat stored within the body calculated as the temporal summation of metabolic heat production and net heat loss.

Cumulative amount of heat stored within the body calculated as the temporal summation of metabolic heat production and net heat loss.

Cumulative amount of heat stored within the body calculated as the temporal summation of metabolic heat production and net heat loss.

Cumulative amount of heat stored within the body calculated as the temporal summation of metabolic heat production and net heat loss.

Skin temperature measured at 4-sites (chest, upper arm, thigh, calf) with mean value calculated as weighted value of 4 sites - upper arm, 30%; chest, 30%; thigh, 20%; and calf, 20%.

Skin temperature measured at 4-sites (chest, upper arm, thigh, calf) with mean value calculated as weighted value of 4 sites - upper arm, 30%; chest, 30%; thigh, 20%; and calf, 20%.

Skin temperature measured at 4-sites (chest, upper arm, thigh, calf) with mean value calculated as weighted value of 4 sites - upper arm, 30%; chest, 30%; thigh, 20%; and calf, 20%.

Skin temperature measured at 4-sites (chest, upper arm, thigh, calf) with mean value calculated as weighted value of 4 sites - upper arm, 30%; chest, 30%; thigh, 20%; and calf, 20%.

Measures of variability computed from the time, frequency, time-frequency, scale-invariant, entropy, and other nonlinear domains

Measures of variability computed from the time, frequency, time-frequency, scale-invariant, entropy, and other nonlinear domains

Measures of variability computed from the time, frequency, time-frequency, scale-invariant, entropy, and other nonlinear domains

Thermal comfort assessed via a questionnaire using the ASHRAE 4-point scale ("1=comfortable to 4=very uncomfortable")

Thermal comfort assessed via a questionnaire using the ASHRAE 4-point scale ("1=comfortable to 4=very uncomfortable")

Thermal comfort assessed via a questionnaire using the ASHRAE 4-point scale ("1=comfortable to 4=very uncomfortable")

Thirst sensation will be quantified using a 9-point scale ("1=not thirsty at all to 9=very, very thirsty")

Thirst sensation will be quantified using a 9-point scale ("1=not thirsty at all to 9=very, very thirsty")

Thirst sensation will be quantified using a 9-point scale ("1=not thirsty at all to 9=very, very thirsty")

Rating of perceived exertion will be quantified using a 14-point scale ("6=no exertion at all to 20=maximal exertion")

Rating of perceived exertion will be quantified using a 14-point scale ("6=no exertion at all to 20=maximal exertion")

Rating of perceived exertion will be quantified using a 14-point scale ("6=no exertion at all to 20=maximal exertion")

Inclusion Criteria: Elderly (60-80 years), non-smoking adults. Not engaged in regular physical activity (>2 sessions/week for ≥20 minutes per session). Willing to provide informed consent. Healthy, no diagnosed health conditions. Body Mass index (BMI) <35 kg/m2. Exclusion Criteria: Heat adapted due to repeated exposure to hot environments within the last 3 weeks (use sauna, recent travel to hot climates, other).

Deidentified participant data will be made available with approved analysis plan and signed access agreement