Hot Water Immersion as a Heat Acclimation Strategy in Older Adults

NCT05838612 | Completed

• 1 other identifier: HEPRU-2023-02
• Study Type: interventional
• Target: 12
• Locations: 1 country
• Sites: 1

Brief Summary

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).

Conditions

Hyperthermia; Heat Exposure; Heat Stress; Thermoregulation; Aging

Keywords

Thermoregulation; Warm-water immersion; Heat acclimation; Aging

Outcome Measures

Primary Outcomes (6)

• Evaporative heat loss

  Evaporative heat loss as assessed using a direct air calorimeter.

  End of 30 minute exercise bout 1 (average of last 5 minutes).

• Evaporative heat loss

  Evaporative heat loss as assessed using a direct air calorimeter.

  End of 30 minute exercise bout 2 (average of last 5 minutes).

• Evaporative heat loss

  Evaporative heat loss as assessed using a direct air calorimeter.

  End of 30 minute bout 3 (average of last 5 minutes).

• Whole-body heat loss

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

  End of 30 minute exercise bout 1 (average of last 5 minutes).

• Whole-body heat loss

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

  End of 30 minute exercise bout 2 (average of last 5 minutes).

• Whole-body heat loss

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

  End of 30 minute exercise bout 3 (average of last 5 minutes).

Secondary Outcomes (33)

• Body heat storage

  Change from start to end of 30 minute exercise bout 1

• Body heat content

  Change from start to end of 30 minute exercise bout 2

• Body heat storage

  Change from start to end of 30 minute exercise bout 3

• Cumulative body heat storage

  Change from baseline resting to end of exercise bout 3

• Dry heat loss

  End of 30 minute exercise bout 1 (average of last 5 minutes).

Study Arms (1)

Heat Acclimation

EXPERIMENTAL

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.

Other: Heat acclimation

Interventions

Heat acclimation OTHER

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.

Heat Acclimation

Eligibility Criteria

• Age: 60 Years - 80 Years
• Sex: male
• Healthy Volunteers: Yes
• Age Groups: Adult (18-64), Older Adult (65+)

You may qualify if:

• 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.

You may not qualify if:

• Heat adapted due to repeated exposure to hot environments within the last 3 weeks (use sauna, recent travel to hot climates, other).

Sponsors & Collaborators

• University of Ottawa: lead

Study Sites (1)

Univerisity of Ottawa

Ottawa, Ontario, K1N6N5, Canada

Location

Related Publications (2)

• Janetos KT, O'Connor F, Morris N, Kenny GP. The influence of heat acclimation on the relation and agreement between perceptual and physiological strain in older males during exercise-heat stress. Appl Physiol Nutr Metab. 2026 Jan 16. doi: 10.1139/apnm-2025-0360. Online ahead of print.

  PMID: 41549363 DERIVED

• Janetos KT, O'Connor FK, Meade RD, Richards BJ, Koetje NJ, Kirby NV, McCormick JJ, Flouris AD, Kenny GP. Short-Term Warm Water Immersion for Improving Whole-Body Heat Loss in Older Men. Med Sci Sports Exerc. 2025 Jun 1;57(6):1137-1147. doi: 10.1249/MSS.0000000000003649. Epub 2025 Jan 17.

  PMID: 39820413 DERIVED

MeSH Terms

Conditions

Hyperthermia; Heat Stress Disorders

Condition Hierarchy (Ancestors)

Body Temperature Changes; Signs and Symptoms; Pathological Conditions, Signs and Symptoms; Wounds and Injuries

Study Officials

• Glen P Kenny, PhD.

  University of Ottawa

  PRINCIPAL INVESTIGATOR

Study Design

• Study Type: interventional
• Phase: not applicable
• Allocation: NA
• Masking: NONE
• Masking Details: Data will be blinded prior to analysis.
• Purpose: BASIC SCIENCE
• Intervention Model: SINGLE GROUP
• Sponsor Type: OTHER
• Responsible Party: PRINCIPAL INVESTIGATOR
• PI Title: Full Professor, University Research Chair, Principal Investigator

Model Details: 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.

Study Record Dates

• First Submitted: April 19, 2023
• First Posted: May 1, 2023
• Study Start: April 29, 2022
• Primary Completion: February 16, 2023
• Study Completion: February 16, 2023
• Last Updated: July 11, 2024

Record last verified: 2024-07

Data Sharing

• IPD Sharing: Will share
• Shared Documents: STUDY PROTOCOL, SAP, ANALYTIC CODE
• Time Frame: Following publication of the main study report(s)
• Access Criteria: Approved analysis plan and signed access agreement

Locations

CA (1)