Evidence of seasonal heat acclimatisation in recreationally active adults during a mild summer

Objectives: To assess the magnitude of seasonal heat acclimatisation in recreationally active adults and contextualise the process by documenting the factors that influence adaptations. Design: Longitudinal, repeated measures design. Methods: Seventeen (7 females) recreationally active adults (28 ± 8 yr, V̇O_2peak 54 ± 8 mL·kg⁻¹·min⁻¹) exercising outdoors a minimum of 5 h·wk⁻¹ completed a 45-min heat response test running at 60 % V̇O_2peak in 40 °C and 30 % relative humidity prior to, midway through, and following summer. Self-reported physical activity diaries were completed at the beginning and end of summer. Daytime wet-bulb globe temperature was calculated for each day of summer. Data were analysed using Bayesian ordinal regressions. Results: Daytime wet-bulb globe temperature was 22.0 ± 4.4 °C, with the most common hour for recreational exercise being 17:00 to 18:00. Following summer, the rise in oesophageal temperature and mean skin temperature during the heat response test was lower by 0.12 °C [90 % credible interval: − 0.30, 0.06; probability of direction = 87 %] and 0.43 °C [− 0.74, − 0.10; 98 %], respectively. Moreover, forearm local sweat rate increased by 0.26 mg·cm⁻²·min⁻¹ [0.15, 0.36; 100 %]. There was minimal evidence of a change in the increase in heart rate (1 beat·min⁻¹ [− 3, 5; 62 %]), or whole-body sweat rate (0.03 L·h⁻¹ [− 0.11, 0.15; 68 %]) during the heat response test. Conclusions: Although there was evidence of partial heat adaptation in recreationally active adults following summer, a combination of exercising later in the day and the prevailing environmental conditions (La Niña in South-Eastern Australia) may have blunted the development of further adaptations.