Individual baseline differences outweigh personal traits in short-term heat acclimation adaptations

Area of Science:

• Environmental Physiology
• Human Adaptation
• Military Medicine

Background:

• Individual variability in long-term heat acclimation is known.
• Adaptations during short-term heat acclimation (STHA) and influencing factors remain less understood.

Purpose of the Study:

• To investigate the impact of individual characteristics on thermophysiological adaptations during STHA.
• To assess physiological and subjective changes in service members undergoing STHA.

Main Methods:

• Forty-six service members underwent an STHA protocol in controlled hot and humid conditions (35.1°C, 50% RH).
• Measurements included gastrointestinal temperature (Tgi), mean skin temperature (Tsk), heart rate (HR), sweat loss, and subjective scores.
• Mixed-effect modeling was used to analyze thermophysiological and subjective data.

Main Results:

• Significant declines in Tgi, Tsk, and HR were observed from day 1 to day 5 of STHA (p ≤ 0.011).
• Subjective heat tolerance scores improved significantly (p ≤ 0.005).
• Individual factors such as fat percentage, body mass, sex, and age were related to specific outcome measures (p ≤ 0.05).

Conclusions:

• STHA confers physiological and subjective benefits, leading to uniform heat adaptation in the military population.
• Despite identified relationships, individual characteristics explained only a minor portion of the variance in adaptations.
• No significant individual or group time interactions were found, suggesting consistent adaptation patterns.