The Effect of Daily Brief Heat Exposures on Heat Acclimation

NCT07198334 | Completed

• 1 other identifier: LithuanianSportsU-26
• Study Type: interventional
• Target: 26
• Locations: 1 country
• Sites: 1

Brief Summary

This study is the first to examine the effects of short passive heat exposure on heat acclimation (HA). Unlike traditional protocols requiring prolonged heat exposure, this study tests whether a daily 5-minute whole-body immersions at 45°C water repeated over two weeks can trigger adaptive responses, offering a potentially more accessible and time-efficient method for heat acclimation. This study aims to investigate whether brief, repeated passive heat exposure can induce HA in both female and male participants. It evaluates changes across physiological, biological, and psychological markers to determine the efficacy of this minimal-exposure protocol. It is hypothesized that such short, repeated immersions may be sufficient to elicit significant adaptive responses across key HA indicators.

Conditions

Healthy Participants

Outcome Measures

Primary Outcomes (17)

• Dehydration (kg)

  Dehydration (in kg) was assessed by calculating body mass loss (Tanita Body Composition Analyzer; USA) during heat exposure.

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Heart rate (bpm)

  Heart rate (in bpm) was recorded using a heart rate sensor with a chest strap (Polar, Finland).

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Blood pressure (mmHg)

  Systolic and diastolic blood pressure (in mmHg) was measured using with an automatic cuff monitor (Gentle+, Microlife, Switzerland).

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Pulse pressure (mmHg)

  Pulse pressure (in mmHg) was calculated by subtracting diastolic blood pressure from systolic blood pressure.

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Rate of pressure production (AU)

  Rate of pressure production (in AU) was calculated by multiplying heart rate , by systolic blood pressure.

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Shivering/sweating (points)

  The shivering/sweating rate will be evaluated using 7-point scale. The rating of shivering/sweating range from 1 (heavily sweating) to 7 (vigorously shivering), with 4 being neutral.

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Thermal comfort (points)

  Thermal comfort will be evaluated using 4-point scale. The rating of thermal comfort range from 0 (neutral) to 3 (very uncomfortable).

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Body temperature (°C)

  Rectal temperature (in °C) was measured using a thermocouple (Rectal Probe, Ellab, Denmark) inserted to a depth of 12 cm past the anal sphincter, skin temperature (in °C) was measured with thermistors (Skin/Surface Probe, DM852, Ellab), and muscle temperature (in °C) was measured using a needle microprobe (MKA; Ellab).

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Physiological strain index

  Physiological strain index (PSI) was used to indicate heat strain. PSI = 5 x (Tret - Tre0) x (39.5 - Tre0)\^-1+ 5 x (HRt - HR0) x (180 - HR0)\^-1, where rectal temperature (Tre) t and heart rate (HR) t are simultaneous measurements taken at the end of the heat exposure and Tre0 and HR0 are the initial measurements.

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Oxygen consumption (mL/min/kg)

  Oxygen consumption (in mL/min/kg) was measured using Cortex METALYZR® 3B, Leipcig, Germany).

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Carbon dioxide output (mL/min/kg)

  Carbon dioxide output (in mL/min/kg) was measured using Cortex METALYZR® 3B, Leipcig, Germany).

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Carbon dioxide output (mL/min)

  Carbon dioxide output (in mL/min) was measured using Cortex METALYZR® 3B, Leipcig, Germany).

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Oxygen consumption (mL/min)

  Oxygen consumption (in mL/min) was measured using Cortex METALYZR® 3B, Leipcig, Germany).

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Respiratory quotient

  The respiratory quotient was calculated by dividing the carbon dioxide output by the oxygen consumption. This ratio was used to assess substrate utilization. The values for fat is assumed as 0.7, for protein is assumed as 0.8 and for carbohydrate is assumed as 1.0.

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Ventilation (L/min)

  Ventilation (in L/min) was measured using Cortex METALYZR® 3B, Germany).

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Tidal volume (L)

  Tidal volume (in L) was measured using Cortex METALYZR® 3B, Germany).

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Breathing frequency (t/min)

  Breathing frequency (t/min) was measured using Cortex METALYZR® 3B, Germany).

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

Secondary Outcomes (8)

• Height (cm)

  During enrollment

• Body mass and body composition (kg)

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Body mass index (kg/m^2)

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Body composition (%)

  From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).

• Maximal oxygen consumption (in mL/min/kg)

  During enrollment

Study Arms (1)

Heat acclimation

EXPERIMENTAL

Other: Heat acclimation

Interventions

Heat acclimation OTHER

Passive heat acclimation was designed to induce adaptation in participants through 14 consecutive days of controlled 5-minute whole-body immersion in water at 45°C.

Heat acclimation

Eligibility Criteria

• Age: 18 Years - 35 Years
• Sex: all
• Healthy Volunteers: Yes
• Age Groups: Adult (18-64)

You may qualify if:

• Healthy, non-obese males and females aged 18 to 35 years with a body mass index between 18.5 and 29.9 kg/m².
• No participation in temperature manipulation programs (e.g., cold or heat acclimation) within the past three months.
• No engagement in excessive formal exercise.
• Female participants must have a regular menstrual cycle.

You may not qualify if:

• Current smokers.
• Fear of needles or blood.
• Use of medications or supplements that could affect study outcomes.
• History of neurological, cardiovascular, metabolic, or inflammatory diseases that could be aggravated by hot water exposure.

Sponsors & Collaborators

• Lithuanian Sports University: lead

Study Sites (1)

Lithuanian Sports University

Kaunas, Lithuania

Location

Study Design

• Study Type: interventional
• Phase: not applicable
• Allocation: NA
• Masking: NONE
• Purpose: BASIC SCIENCE
• Intervention Model: SINGLE GROUP
• Sponsor Type: OTHER
• Responsible Party: SPONSOR

Study Record Dates

• First Submitted: September 25, 2025
• First Posted: September 30, 2025
• Study Start: October 1, 2020
• Primary Completion: May 31, 2024
• Study Completion: May 31, 2024
• Last Updated: December 24, 2025

Record last verified: 2025-12

Locations

LI (1)