Hormonal intrauterine devices and heat exchange during exercise

Regulation of body temperature and blood pressure in women: Mechanisms and implications for heat illness risk

Increasing global temperatures due to ongoing climate change phenomena have resulted in increased risk of exertional heat illness in otherwise healthy, young individuals who work or play in the heat. With increasing participation of women in athletic, military and industrial activities that involve exertion in the heat, there is a growing need to study female physiology in this context. Mechanisms controlling blood pressure and body temperature have substantial overlap in humans, largely due to autonomic mechanisms which contribute to both. Similarly, illnesses that result from excessive heat exposure can often be traced back to imbalances in one or more of these autonomic mechanisms. In recent years, there has been increased recognition of the importance of sex as a biological variable for basic and applied research in these areas. The goal of this paper is to present an update on the integrative physiology and pathophysiology of responses to heat stress in women (thermoregulation and blood pressure regulation). In this context, it is often the case that differences between sexes are presented as 'advantages' and 'disadvantages' of one sex over the other. In our opinion, this is an over-simplification of the physiology which ignores the nuances and complexities of the integrative physiology of responses to heat exposure and exercise, and their relevance for practical outcomes.

Time to reach equilibrium deep body temperatures in young and older adults resting in the heat: a descriptive secondary analysis

It is commonly thought that steady-state thermoregulatory responses are achieved within 30-90 min of compensable heat stress. However, this assumption is based on measurements of whole body heat exchange during exercise, which stabilize (equilibrate) more rapidly than deep body temperatures, especially under resting conditions. To support the design of ecologically relevant heat exposure studies, we quantified equilibrium times for deep body temperature, as indexed by rectal temperature, in young and older adults resting in the heat. We also evaluated the lag in rectal temperature equilibrium relative to whole body heat storage (direct calorimetry). Equilibrium times were estimated with data from two laboratory-based trials (NCT04353076 and NCT04348630) in which 83 adults aged 19-80 yr (34 female) were exposed to simulated heat-wave conditions for 8-9 h. When assessed at the group level, it took rectal temperature 3.3 [bootstrap 95% confidence interval: 2.9-3.9] h to reach thermal equilibrium (<0.05°C/h rate of change) in young adults exposed to 40°C, 9% relative humidity (RH). In older adults, who were exposed to a greater range of conditions (31°C-40°C, 9-45% RH), equilibrium times were longer, ranging from 4.4 [3.8-5.3] to 5.2 [4.9-5.4] h. Furthermore, rectal temperature equilibrium was delayed 0.9 [0.5-1.4] and 1.8 [0.9-2.7] h compared with whole body heat storage in young and older adults, respectively (only assessed in 40°C, 9% RH). Individual-level equilibrium times ranged from 1 to 8 h. These findings highlight the importance of ecologically relevant exposure durations in translational research assessing the physiological impacts of hot weather.NEW & NOTEWORTHY Deep body (rectal) temperature took 3-5 h on average and up to 6-8 h at the individual level to reach thermal equilibrium in young and older adults resting in the heat. Furthermore, stable rectal temperatures were delayed by up to 2 h relative to the achievement of heat balance (0 kJ/min rate of heat storage). We provide the first quantification of the temporal profiles of thermal strain during extended rest in conditions simulating hot weather.