Thermophysiological adaptations to passive mild heat acclimation

Abstract

Passive mild heat acclimation (PMHA) reflects realistic temperature challenges encountered in everyday life. Active heat acclimation, combining heat exposure and exercise, influences several important thermophysiological parameters; for example, it decreases core temperature and enhances heat exchange via the skin. However, it is unclear whether PMHA elicits comparable adaptations. Therefore, this study investigated the effect of PMHA on thermophysiological parameters. Participants were exposed to slightly increased temperatures (∼33°C/22% RH) for 6 h/d over 7 consecutive days. To study physiologic responses before and after PMHA, participants underwent a temperature ramp (UP), where ambient temperature increased from a thermoneutral value (28.8 ± 0.3°C) to 37.5 ± 0.6°C. During UP, core and skin temperature, water loss, cardiovascular parameters, skin blood flow and energy expenditure were measured. Three intervals were selected to compare data before and after PMHA: baseline (minutes 30–55: 28.44 ± 0.21°C), T1 (minutes 105–115: 33.29 ± 0.4°C) and T2 (minutes 130–140: 35.68 ± 0.61°C). After 7 d of PMHA, core (T1: −0.13 ± 0.13°C, P = 0.011; T2: −0.14 ± 0.15°C, P = 0.026) and proximal skin temperature (T1: −0.22 ± 0.29°C, P = 0.029) were lower during UP, whereas distal skin temperature was higher in a thermoneutral state (baseline: +0.74 ± 0.77°C, P = 0.009) and during UP (T1: +0.49 ± 0.76°C, P = .057 (not significant), T2:+0.51 ± 0.63°C, P = .022). Moreover, water loss was reduced (−30.5 ± 33.3 ml, P = 0.012) and both systolic (−7.7 ± 7.7 mmHg, P = 0.015) and diastolic (−4.4 ± 4.8 mmHg, P = 0.001) blood pressures were lowered in a thermoneutral state. During UP, only systolic blood pressure was decreased (T2: −6.1 ± 4.4 mmHg, P = 0.003). Skin blood flow was significantly decreased at T1 (−28.35 ± 38.96%, P = 0.037), yet energy expenditure remained unchanged. In conclusion, despite the mild heat stimulus, we show that PMHA induces distinct thermophysiological adaptations leading to increased resilience to heat.

Drivers of diversity in human thermal perception – A review for holistic comfort models

“…Especially for metabolic rate, further studies need to make a clear distinction between behavioral effects on metabolic rate and physiological ones, given the ambiguous findings of field and laboratory studies, especially regarding heat adaptation. In addition, it has been shown previously that the process of physiological adaptation to both heat and cold is highly individual, even in relatively homogenous study populations [88,89,149]. It is likely that physiological adaptation differs even more, for example, between groups with different age and/or body composition.…”

“…Based on the results of a comprehensive literature review on the physiological aspects of heat adaptation by Taylor et al [90] and other physiological studies [89,139], it can be assumed that a decrease of metabolic rate in hot environments, which has earlier been suggested to be a physiological consequence of heat adaptation [140], is not due to physiological changes but is rather attributable to behavioral adjustments [90]. However, an enhanced metabolic response to cold exposure has consistently been reported as a result of cold adaptation [141].…”

“…Adaptation processes occur within three broad stages: 1) acute adjustments and physiological accommodation (within seconds to approximately two days), 2) short-term acclimation/acclimatization processes (approximately two days up to two weeks), and 3) full long-term acclimation/acclimatization to the specific thermal environment (as off approximately two weeks, long-term habituation). The reader is referred to a number of publications describing and reviewing acute and longer-term physiological responses to heat and cold exposure, as this is not the focus of this review [86][87][88][89][90].…”

Physiological and perceptual responses to exercising in restrictive heat loss attire with use of an upper-body sauna suit in temperate and hot conditions

“…resting in hot-dry or hot-humid conditions) (Armstrong and Kenney 1993; Racinais et al 2016;Pallubinsky et al 2017), or by implementing thermal exposures, such as hot water immersion (HWI) (Zurawlew et al 2015;Ruddock et al 2016), or sauna post-exercise (Scoon et al 2007), the most common and potentially most potent HA methods require exercise-heat stress (Racinais et al 2015). A variety of HA protocols have been published ranging in durations of 4-20 days, utilising prolonged exposures (30-120-mins) typically in hot-dry or hot-humid conditions (~40°C, 40% relative humidity [RH]) (Tyler et al 2016).…”

Adaptive changes in physiological and perceptual responses during 10-day heat acclimation training using a water-perfused suit

“…Typically, exercising in heat (active HA strategy) is the most commonly accepted method to induce the following HA responses [10], all of which contribute to improved heat tolerance: (1) decrease in body core temperature; (2) reduction in heart rate; (3) lessened physiological strain index (PSI); (4) increase in sweat rate; (5) alleviated perceptual strain. Other studies advocated passive HA strategies, subjecting individuals to heat stress without any exercise mainly by immersion in hot water [11], entering a heat chamber [12], or sauna [13]. These studies suggested that active strategies are not necessary to induce HA as the main stimulus for heat adaptation is simply a repeated rise in body core temperature [14].…”