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

Harry A Brown; Thomas H Topham; Brad Clark; Andreas D Flouris; Leonidas G Ioannou; Richard D Telford; James W Smallcombe; Ollie Jay; Julien D Périard

doi:10.1016/j.jsams.2023.10.006

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

J Sci Med Sport. 2023 Oct 16:S1440-2440(23)00427-9. doi: 10.1016/j.jsams.2023.10.006. Online ahead of print.

Authors

Harry A Brown¹, Thomas H Topham², Brad Clark³, Andreas D Flouris⁴, Leonidas G Ioannou⁵, Richard D Telford⁶, James W Smallcombe⁷, Ollie Jay⁸, Julien D Périard⁹

Affiliations

¹ Research Institute for Sport and Exercise Science (UCRISE), University of Canberra, Australia. Electronic address: https://twitter.com/_HarryBrown2.
² Research Institute for Sport and Exercise Science (UCRISE), University of Canberra, Australia. Electronic address: https://twitter.com/mt_topham.
³ Research Institute for Sport and Exercise Science (UCRISE), University of Canberra, Australia.
⁴ FAME Laboratory, Department of Physical Education and Sport Science, University of Thessaly, Greece. Electronic address: https://twitter.com/aflouris.
⁵ FAME Laboratory, Department of Physical Education and Sport Science, University of Thessaly, Greece. Electronic address: https://twitter.com/IoannouLeonidas.
⁶ Research Institute for Sport and Exercise Science (UCRISE), University of Canberra, Australia. Electronic address: https://twitter.com/DickTelford.
⁷ Heat and Health Research Incubator, Faculty of Medicine and Health, The University of Sydney, Australia. Electronic address: https://twitter.com/smallcombe2.
⁸ Heat and Health Research Incubator, Faculty of Medicine and Health, The University of Sydney, Australia. Electronic address: https://twitter.com/ollie_jay13.
⁹ Research Institute for Sport and Exercise Science (UCRISE), University of Canberra, Australia. Electronic address: Julien.Periard@canberra.edu.au.

PMID: 39492335
DOI: 10.1016/j.jsams.2023.10.006

Abstract

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.

Keywords: Core temperature; Heat adaptation; Sweating; Thermoregulation.