A crossover control study of three methods of heat acclimation on the magnitude and kinetics of adaptation

Abstract

New Findings What is the central question to the study?Are primary indices of heat adaptation (e.g., expansion of plasma volume and reduction in resting core temperature) differentially affected by the three major modes of short‐term heat acclimation, that is, exercise in the heat, hot water immersion and sauna? What it the main finding and its importance?The three modes elicited typical adaptations expected with short‐term heat acclimation, but these were not significantly different between modes. This comparison has not previously been made and highlights that individuals can expect similar adaptation to heat regardless of the mode used. Abstract Heat acclimation (HA) can improve heat tolerance and cardiovascular health. The mode of HA potentially impacts the magnitude and time course of adaptations, but almost no comparative data exist. We therefore investigated adaptive responses to three common modes of HA, particularly with respect to plasma volume. Within a crossover repeated‐measures design, 13 physically active participants (five female) undertook four, 5‐day HA regimes (60 min/day) in randomised order, separated by ≥4 weeks. Rectal temperature (Tre) was clamped at neutrality via 36.6°C (thermoneutral) water immersion (TWI; i.e., control condition), or raised by 1.5°C via heat stress in 40°C water, sauna (55°C, 52% relative humidity), or exercise in humid heat (40°C, 52% relative humidity; ExH). Adaptation magnitude was assessed as the pooled response across days 4–6, while kinetics was assessed via the 6‐day time series. Plasma volume expansion was similar in all heated conditions but only higher than TWI in exercise in the heat (ExH) (by 4%, P = 0.036). Approximately two‐thirds of the expansion was attained within the initial 24 h and was moderately related to that present on day 6, regardless of HA mode (r = 0.560–0.887). Expansion was mediated by conservation of both sodium and albumin content, with little evidence for these having differential roles between modes (P = 0.706 and 0.320, respectively). Resting Tre decreased by 0.1–0.3°C in all heated conditions, and systolic blood pressure decreased by 4 mmHg, but not differentially between conditions (P ≥ 0.137). In conclusion, HA mode did not substantially affect the magnitude or rate of adaptation in key resting markers of short‐term HA.

Acute physiological and psychophysical responses to different modes of heat stress

“…Short-term adaptation effects of the different modes of heating are addressed in the companion paper (Kissling et al, 2022), and the haemodynamic stimuli for health-related effects were recently reported by Francisco et al (2021). Relevant to both of these contexts, the present study demonstrates that thermal AUC of core temperature, as a primary stimulus for adaptation, was not dampened with multiple exposures -perhaps unsurprising given their thermal uncompensability.…”

“…Short‐term adaptation effects of the different modes of heating are addressed in the companion paper (Kissling et al., 2022), and the haemodynamic stimuli for health‐related effects were recently reported by Francisco et al. (2021).…”

“…The 60‐min duration was chosen as a balance between the volume and intensity of exposure that would be tolerable yet sufficient to allow comparisons across modes of heating whilst also driving adaptation within the accompanying heat acclimation studies (see below). A 5‐day period was used for studying habituation effects because the companion paper (Kissling et al., 2022) investigates the kinetics of short‐term heat acclimation, and much of the psychophysical and physiological cardiovascular effect becomes evident within this timeframe (Sawka et al., 2000).…”

Myths and methodologies: Considerations for evaluating the time course of thermoregulatory adaptation during heat acclimation

“…Unfortunately, individual heat adaptation data from constant stress and isothermal heat acclimation are rarely reported and so the key variables that influence heat acclimation success, and their relative contributions, are currently unknown. Differences in variables such as age (Wagner et al., 1972 ), sex (Mee et al., 2015 ), hormonal profile (Wickham et al., 2021 ), ethnicity (Taylor, 2006 ), aerobic fitness (Pandolf et al., 1977 ), initial heat adaptation status (Taylor, 2006 ), previous acclimation history (Weller et al., 2007 ), the magnitude of the thermal impulse provided (Kissling et al., 2022 ) and the environment (Griefahn, 1997 ) may explain some of the variation and merit further study. Interrogating individual responses to any intervention, including heat acclimation, is associated with numerous statistical pitfalls so we encourage reading reviews on the topic before engaging in individual responder analysis (Atkinson et al., 2019 ).…”