The effect of heat acclimation or acclimatisation on physiological markers of heat adaptation: preliminary meta-analysis data

Introduction

Exercise in the heat places a greater physiological strain upon the body than exercising in temperate conditions, so a number of strategies have been adopted to attenuate this strain. Heat acclimation (or acclimatisation) (HA) has regularly been reported to induce beneficial cardiovascular and thermoregulatory adaptations. However, the magnitudes of benefit reported range from none to substantial, and the differences reported may be due to a wide range of HA protocols being used. The aim of this meta-analysis was to quantify the magnitude of effect that HA has on key physiological markers of adaptation, and to see whether the magnitude of effect is related to the volume or intensity of heat stress experienced.

Methods

The PubMed database was searched (09/01/15) using the first-order search terms acclimation, acclimatization, acclimatisation and adaptation and second-order search terms heat, exercise, performance, capacity and training. Using the four-stage process identified in the PRISMA statement the initial number of results (9,369) was reduced to 92. Data (N, mean, SD) were extracted from these articles in duplicate or triplicate. A subset of the data (n = 46 manuscripts) is presented here; manuscripts were included if resting core temperature (T_core), resting heart rate (HR), resting plasma volume (PV) and/or core temperature at sweat onset (T_{sweat onset}) data were reported. All HA protocols regardless of duration, frequency, ambient conditions or exercise modality were used. Hedge's g ( ± 95% CI) were calculated and Spearman's correlation analyses were performed between the effect size and total HA time (HA_time), and HA temperature (HA_temp).

Results

The 46 manuscripts reviewed used a mean (SD) of 9 (0) [range: 4 - 16] HA sessions separated by 0 (0) [0 - 1.5] days. Total HA_time was 868 (558) min [150 - 2,880], and the HA_temp and HA_humidity were 39 (5) °C [28 - 50] and 36 (16) % [14 - 86], respectively.

Conclusion

HA is an effective way to reduce resting T_core and HR; increase resting PV, and lower the T_{sweat onset}. The magnitude of effect appears to be independent of HA_time or HA_temp for each of the 4 variables with the exception of T_{sweat onset}, which may be inversely related to HA_time; however, these latter data are derived from only 6 investigations.

Table 1 The effect of HA on resting T_core, resting HR, resting PV and T_{sweat onset}.