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.
Study Type
INTERVENTIONAL
Allocation
NA
Purpose
BASIC_SCIENCE
Masking
NONE
Enrollment
26
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.
Lithuanian Sports University
Kaunas, Lithuania
Dehydration (kg)
Dehydration (in kg) was assessed by calculating body mass loss (Tanita Body Composition Analyzer; USA) during heat exposure.
Time frame: 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).
Time frame: 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).
Time frame: 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.
Time frame: 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.
Time frame: 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.
Time frame: 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).
Time frame: 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).
Time frame: 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.
Time frame: 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).
Time frame: 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).
Time frame: 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).
Time frame: 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).
Time frame: 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.
Time frame: 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).
Time frame: 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).
Time frame: 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).
Time frame: From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).
Height (cm)
Height (in cm) was measured using a Harpenden anthropometer set (Holtain Ltd, UK).
Time frame: During enrollment
Body mass and body composition (kg)
Body mass and composition (in kg) was evaluated using Tanita Body Composition Analyzer (USA).
Time frame: From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).
Body mass index (kg/m^2)
Body mass index (in kg/m\^2) will be defined as the body mass divided by the square of the body height.
Time frame: From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).
Body composition (%)
Body composition (in %) was evaluated using Tanita Body Composition Analyzer (USA).
Time frame: From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).
Maximal oxygen consumption (in mL/min/kg)
Maximal oxygen consumption (VO2max) (in mL/min/kg) was defined through a maximal incremental exercise test conducted on an electromechanically braked cycling ergometer (Ergometrics-800S; Ergoline Medical Measurement Systems, Germany) and was measured on a breath-by-breath basis using a stationary MetaLyzer® 3B spiroergometry system (Cortex Biophysik GmbH, Germany).
Time frame: During enrollment
Prolactin (ng/mL)
Serum prolactin (in ng/mL) was measured using enzyme-linked immunosorbent assay kits and a Spark multimode microplate reader (Tecan, Austria).
Time frame: From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).
17beta-estradiol (pg/mL)
Serum 17beta-estradiol (in pg/mL) was measured using enzyme-linked immunosorbent assay kits and a Spark multimode microplate reader (Tecan, Austria).
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Time frame: From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).
Progesterone (ng/mL)
Serum progesterone (in ng/mL) was measured using enzyme-linked immunosorbent assay kits and a Spark multimode microplate reader (Tecan, Austria).
Time frame: From enrollment to the end of deacclimation (one month after the 14-day heat acclimation period).