Sex Differences in Human Thermoregulation: Relevance for 2020 and Beyond

Effects of sex and menstrual cycle on sweating during isometric handgrip exercise and postexercise forearm occlusion

NEW FINDINGS

What is the central question of this study? Do sex and menstrual cycle modulate sweating during isometric handgrip exercise and muscle metaboreceptor stimulation? What is the main finding and its importance? Sex modulates sweating during isometric handgrip exercise, as indicated by the lower sweat output per gland in women than in men, but not during muscle metaboreceptor stimulation. Sweat output per gland during isometric handgrip exercise and muscle metaboreceptor stimulation were lower in the mid-luteal phase than in the early follicular phase in women. Cholinergic sweat gland sensitivity might explain, in part, the individual variation of the response. Our results provide new insights regarding sex- and menstrual cycle-related modulation of the sweating response.

ABSTRACT

We investigated whether sex and menstrual cycle could modulate sweating during isometric handgrip (IH) exercise and muscle metaboreceptor stimulation. Twelve young, healthy women in the early follicular (EF) and mid-luteal (ML) phases and 14 men underwent two experimental sessions consisting of a 1.5 min IH exercise at 25 and 50% of maximal voluntary contraction (MVC) in a hot environment (35°C, relative humidity 50%) followed by 2 min forearm occlusion to stimulate muscle metaboreceptors. Sweat rates, the number of activated sweat glands and the sweat output per gland (SGO) on the forearm and chest were assessed. Pilocarpine-induced sweating was also assessed via transdermal iontophoresis to compare the responses with those of IH exercise and muscle metaboreceptor stimulation, based on correlation analysis. Sweat rates on the forearm and chest during IH exercise and muscle metaboreceptor stimulation did not differ between men and women in either menstrual cycle phase (all P ≥ 0.144). However, women in both phases showed lower SGO on the forearm and/or chest compared with men during IH exercise at 50% of MVC, with no differences in muscle metaboreceptor stimulation. Women in the ML phase had a lower forearm sweat rate during IH exercise at 50% of MVC (P = 0.015) and SGO during exercise and muscle metaboreceptor stimulation (main effect, both P ≤ 0.003) compared with those in the EF phase. Overall, sweat rate and SGO during IH exercise and muscle metaboreceptor stimulation were correlated with pilocarpine-induced responses (all P ≤ 0.064, r ≥ 0.303). We showed that sex and menstrual cycle modulate sudomotor activity during IH exercise and/or muscle metaboreceptor stimulation. Cholinergic sweat gland sensitivity might explain, in part, the individual variation of the response.

Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies

A rise in body core temperature and loss of body water via sweating are natural consequences of prolonged exercise in the heat. This review provides a comprehensive and integrative overview of how the human body responds to exercise under heat stress and the countermeasures that can be adopted to enhance aerobic performance under such environmental conditions. The fundamental concepts and physiological processes associated with thermoregulation and fluid balance are initially described, followed by a summary of methods to determine thermal strain and hydration status. An outline is provided on how exercise-heat stress disrupts these homeostatic processes, leading to hyperthermia, hypohydration, sodium disturbances, and in some cases exertional heat illness. The impact of heat stress on human performance is also examined, including the underlying physiological mechanisms that mediate the impairment of exercise performance. Similarly, the influence of hydration status on performance in the heat and how systemic and peripheral hemodynamic adjustments contribute to fatigue development is elucidated. This review also discusses strategies to mitigate the effects of hyperthermia and hypohydration on exercise performance in the heat by examining the benefits of heat acclimation, cooling strategies, and hyperhydration. Finally, contemporary controversies are summarized and future research directions are provided.

Integrative cardiovascular control in women: Regulation of blood pressure, body temperature, and cerebrovascular responsiveness

Over the past several decades, it has become increasingly clear that women have distinct cardiovascular profiles compared to men. In this review, our goal is to provide an overview of the literature regarding the influences of female sex and reproductive hormones (primarily estradiol) on mechanisms of cardiovascular control relevant to regulation of blood pressure, body temperature, and cerebral blood flow. Young women tend to have lower resting blood pressure compared with men. This sex difference is reversed at menopause, when women develop higher sympathetic nerve activity and the risk of systemic hypertension increases sharply as postmenopausal women age. Vascular responses to thermal stress, including cutaneous vasodilation and vasoconstriction, are also affected by reproductive hormones in women, where estradiol appears to promote vasodilation and heat dissipation. The influence of reproductive hormones on cerebral blood flow and sex differences in the ability of the cerebral vasculature to increase its blood flow (cerebrovascular reactivity) are relatively new areas of investigation. Sex and hormonal influences on integrative blood flow regulation have further implications during challenges to physiological homeostasis, including exercise. We propose that increasing awareness of these sex-specific mechanisms is important for optimizing health care and promotion of wellness in women across the life span.

Fundamental Concepts of Human Thermoregulation and Adaptation to Heat: A Review in the Context of Global Warming

The international community has recognized global warming as an impending catastrophe that poses significant threat to life on earth. In response, the signatories of the Paris Agreement (2015) have committed to limit the increase in global mean temperature to <1.5 °C from pre-industry period, which is defined as 1850–1890. Considering that the protection of human life is a central focus in the Paris Agreement, the naturally endowed properties of the human body to protect itself from environmental extremes should form the core of an integrated and multifaceted solution against global warming. Scholars believe that heat and thermoregulation played important roles in the evolution of life and continue to be a central mechanism that allows humans to explore, labor and live in extreme conditions. However, the international effort against global warming has focused primarily on protecting the environment and on the reduction of greenhouse gases by changing human behavior, industrial practices and government policies, with limited consideration given to the nature and design of the human thermoregulatory system. Global warming is projected to challenge the limits of human thermoregulation, which can be enhanced by complementing innate human thermo-plasticity with the appropriate behavioral changes and technological innovations. Therefore, the primary aim of this review is to discuss the fundamental concepts and physiology of human thermoregulation as the underlying bases for human adaptation to global warming. Potential strategies to extend human tolerance against environmental heat through behavioral adaptations and technological innovations will also be discussed. An important behavioral adaptation postulated by this review is that sleep/wake cycles would gravitate towards a sub-nocturnal pattern, especially for outdoor activities, to avoid the heat in the day. Technologically, the current concept of air conditioning the space in the room would likely steer towards the concept of targeted body surface cooling. The current review was conducted using materials that were derived from PubMed search engine and the personal library of the author. The PubMed search was conducted using combinations of keywords that are related to the theme and topics in the respective sections of the review. The final set of articles selected were considered “state of the art,” based on their contributions to the strength of scientific evidence and novelty in the domain knowledge on human thermoregulation and global warming.