The elderly have a higher risk of adverse health due to extremely hot weather compared to other age groups in terms of heat-related morbidity and mortality (D'Ippoliti et al., 2010; Hajat and Kosatky, 2010; Hajat et al., 2007; Li et al., 2015). In East Asia, health registry data from China indicates that older individuals face an elevated risk of heat-related health issues when compared to younger age groups. (Gu et al., 2016). Another health registry study conducted in Korea observed that for each 1 °C increase in maximum temperature, the relative risk of heat-related illnesses among those aged 65 or older was 1.86, while it was 1.63 for individuals between the ages of 20 and 64 (Na et al., 2013). The causes for such vulnerability are multifactorial, including physiologic impairment (e.g., impaired cardiopulmonary reserve, obesity, sweat gland dysfunction, blunted heat and thirst sensation, chronic diseases, and thermoregulation-interfering medication), social conditions (e.g., poor living conditions, social isolation, and lack of air conditioning, and correct knowledge to heat prevention), and personal adaptation behaviors (Epstein and Yanovich, 2019; Worfolk, 2000). Although an experimental study suggests that the elderly might have diminished perceptual awareness of increasingly challenging thermal environments (Waldock et al., 2018), perceived heat-related symptoms, which can signal conditions such as heatstroke or heat-related death, are notably prevalent among the elderly. For instance, a Dutch study reported thirst (42.7%), sleep disturbances (40.6%), and excessive sweating (39.6%) as common symptoms among the elderly (van Loenhout et al., 2016). Research from Germany reported 74.6% of senior participants experiencing moderate to severe heat strain during summer months (Kemen et al., 2021). In India, a study during the summer found elderly respondents frequently reporting discomfort due to heat (82.3%), excessive sweating (81.3%), excessive thirst (81.9%), fatigue or weakness (76.5%), disturbed sleep (62.3%), heat rash (62.3%), muscle cramps (54.2%), and other related symptoms (Mukhopadhyay and Weitz, 2022). Assessing heat-related symptoms offers warning signs of individual heat responses, insights into underlying pathophysiological processes, and benchmarks for tracking heat adaptation at the population level.

Thermoregulatory function during exercise in hot conditions is notably affected by aging (Anderson and Kenney, 1987; D'Souza et al., 2020; Kenney and Anderson, 1988; McGinn et al., 2017; Stapleton et al., 2015). This decline can be partly attributed to age-related decreases in cutaneous vasodilation and sweating, which hinder the overall body's heat loss mechanisms—both dry and evaporative heat exchange (Anderson and Kenney, 1987; D'Souza et al., 2020; Kenney and Anderson, 1988; Stapleton et al., 2015). Such constraints intensify body heat storage, particularly during moderate to vigorous exercise intensities. There is an approximate 4% decrease in whole-body heat loss per decade with aging, observed consistently in both genders (D'Souza et al., 2020; McGinn et al., 2017). One specific study focusing on women revealed that older females (aged 51 ± 7 years) stored body heat at rates 29% and 16% higher than their younger counterparts (aged 25 ± 4 years) under dry and humid conditions, respectively (Notley et al., 2017b). The pronounced influence from dry heat suggests that the age-related decrease in total body heat loss primarily arises from a reduced evaporative heat loss. This reduction is mainly due to a decrease in sweat gland output (Inoue and Shibasaki, 1996), potentially correlated with reductions in cholinergic sensitivity (Inoue et al., 1999).

Obesity, characterized by excessive fat accumulation, has been identified as a risk factor for heat-related illnesses (Kenney, 1985). Several mechanisms have been proposed in the literature to explain this association (Kenney, 1985). Primarily, obesity increases the cardiopulmonary load and reduces thermal response reserves (Alexander, 1964; Buskirk et al., 1965; Meyers et al., 1991). Additionally, subcutaneous fat tissue reduces the body surface area-to-mass ratio and limits the rate of heat exchange with the environment (Kenney, 1985). Moreover, adipose tissue with low water content has a specific heat of about 0.4 kcal/g/°C, compared to the higher specific heat of roughly 0.8 kcal/g/°C for fat-free body mass, which has a higher water content. Consequently, for the same heat load per unit of body weight, the increase in tissue temperature will be greater in an obese person than in a lean one (Kenney, 1985). Furthermore, the density of sweat glands is lower on skin overlying areas with higher adipose content, which weakens heat dissipation through sweat evaporation (Bar-Or et al., 1968). During weight-bearing exercises like walking, individuals with obesity inherently carry more weight compared to their leaner peers, which triggers increased responses in core temperature, heart rate, and sweating (Bar-Or et al., 1969). Despite this knowledge, the literature on obesity or body fat's influence on thermoregulation offers varying conclusions. In controlled experiments with fixed, non-weight-bearing workloads, obese participants have shown either similar (Adams et al., 2015; Limbaugh et al., 2013) or heightened (Dervis et al., 2016) core temperatures when compared to non-obese subjects. The direct impact of obesity or body fat on heat-related symptoms in the elderly remains ambiguous. Notably, body fat, which often increases with age as muscle mass diminishes (Zamboni et al., 2003), is a modifiable factor. Consequently, we aim to discern any potential links between body fat in the elderly and the prevalence of heat-related symptoms.

People can use heat adaptation methods to reduce exposure to high temperatures and minimize health risks. Some studies have surveyed heat adaptation behaviors among the elderly in different populations, yielding varied findings regarding their correlation with heat-related health outcomes. A study in Guangdong, China shows that drinking more water and wearing light clothes reduce the risk of heatstroke (Liu et al., 2013). In contrast, a study from the United States shows a positive association between prevention behaviors and heat illness, implying a motivation for changing behaviors due to experience with health problems (Hass and Ellis, 2019). Moreover, research from Germany demonstrated that the range of coping strategies employed is directly proportional to the perceived intensity of heat strain (Kemen et al., 2021). Because there may be complex interactions between symptoms related to heat exposure and heat adaptation behaviors, this study aimed to assess the protective effects of specific adaptive behaviors on the elderly by exploring the effect modifications of adaptive behaviors on risk association between body fat and heat-related symptoms.