Seasonal Heat Acclimatisation in Healthy Adults: A Systematic Review.
Level 2 - randomized trial
Systematic review of non-randomized longitudinal cohort studies
PubMed 35460514 · doi:10.1007/s40279-022-01677-0
What was done
Systematic review of literature across MEDLINE, SPORTDiscus, CINAHL Plus, Scopus, and Cochrane up to July 15, 2021 (PROSPERO CRD42020201883). Studies examining seasonal heat acclimatisation in the same sample of healthy adults tested at least 3 months apart were included. Two authors independently evaluated risk of bias using a modified McMaster critical review form. Extracted variables included participant demographics, outdoor physical activity metrics, heat response test parameters, environmental conditions (wet-bulb globe temperature, WBGT), and physiological metrics (core and skin temperatures, heart rate, sweat rates, sweat sodium, skin blood flow, and plasma volume).
What was found
Twenty-nine studies including 561 participants across eight countries were reviewed (mean summer daytime WBGT of 24.9 °C, range 19.5–29.8 °C). Significant reported adaptations following summer (associated with mean summer WBGT of 25.2 °C, range 19.6–28.7 °C) were: two studies reported reduced resting core temperature (0.16 °C; p < 0.05); 11 reported increased sweat rate (range: 0.03–0.53 L·h⁻¹; p < 0.05); two observed reduced heart rate during heat response tests (range: 3–8 beats·min⁻¹; p < 0.05); and six noted reduced sweat sodium concentration (range: -22% to -59%; p < 0.05).
Why it matters
This review synthesizes evidence showing that natural seasonal summer exposure induces meaningful physiological heat acclimatisation in healthy adults. The extent of adaptation is influenced by a combination of outdoor environmental heat levels and physical activity duration and intensity.
Limits
Included studies primarily examined young healthy male adults, limiting generalizability to females and broader populations. There were substantial inconsistencies and poor reporting across studies regarding outdoor exposure duration, exercise tasks, and specific environmental conditions.
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