Humans: A homeothermic animal that needs perturbation?

I have just returned from an excellent Physiology & Pharmacology of Temperature Regulation meeting in Split, where Experimental Physiology sponsored the student prizes for the best oral presentations. The meeting included several papers on the benefits of thermal therapies for physiological function. This begs the question, why do these work? Perhaps humans have become too comfortable? Our technical ability to control our environment has meant that we can be sedentary, physiologically unresponsive/unchallenged and, as a result, increasingly unhealthy. This is well recognized in terms of the impact of physical inactivity, but it also seems to apply to our ability to control our thermal environment. Perhaps this, plus the decline in the amount of physical activity-related heat production, has made us ‘thermostatic’. Humans are homeostatic animals, seeking a relatively constant internal environment in terms of temperature, biochemistry, blood gases etc. However, it is becoming increasingly apparent that, for thermal balance at least, it is beneficial if the dynamic equilibrium that produces this homeostasis (homeothermy) is challenged and stress-related responses are evoked. Cultures, such as those of the Scandinavians and Japanese, have known this for many years. Ofuro bathing is a component of Japan's national culture and identity, and it is believed that healthy persons may benefit from the physiological effects of hot water immersion on the body's homeostatic systems (Becker, 2009; Choukroun & Varene, 1990; Hayasaka, Shibata, Goto, Noda, & Ojima, 2010; Kataoka & Yoshida, 2005). Others adopt more idiosyncratic approaches, Thomas Jefferson used a cold foot bath every morning for six decades to ‘maintain his good health’. Many claim health benefits from regular whole-body cold water immersion (Tipton, Collier, Massey, Corbett, & Harper, 2017). More recently, claims for the benefits of challenging the thermal status of the body have been extended to include less healthy individuals. In general, we have arrived at a situation where many humans are so thermostatic that the perturbations that were once caused by exercise and exposure to hot and cold environments must now be provided by bespoke thermal therapeutic interventions. It seems that many have fallen below the low levels of exercise/thermal stress necessary for a beneficial hormetic response (Mattson, 2008; Peake et al., 2015). So it is that in the last few years we have seen numerous scientific papers reporting that exposure of otherwise sedentary healthy and unhealthy individuals to static, local or whole-body heating, cooling or a combination of both is beneficial for a wide range of physiological responses, including the following: resistance to cardiovascular disease and mortality (Kunutsor, Khan, Laukkanen, & Laukkanen, 2018; Laukkanen, Khan, Zaccardi, & Laukkanen, 2015); endothelial function and arterial stiffness (Brunt, Howard, Francisco, Ely, & Minson, 2016); walking ability and lower limb perfusion; shear pattern, blood pressure and circulating endothelin-1 concentrations (Chiesa, Trangmar, & Gonzalez-Alonso, 2016; Neff et al., 2016; Thomas, van Rii, Lucas, & Cotter, 2016); glucose metabolism (Kimball, McCue, Petrie, & Shields, 2018); autonomic nervous activity (Kuwahata et al., 2011); cerebral protection (Coombs & Tremblay, 2018); and stress resistance (Brunt, Wiedenfeld-Needham, Comrada, & Minson, 2018). Furthermore, both hot (Naumann et al., 2017) and cold (Van Tulleken, 2018; Van Tulleken, Tipton, Massey, & Harper, 2018) water exposures have been reported to improve mental health. Although the efficacy of these thermal interventions is, sadly, a possible consequence of the capability and inclination of modern humans to be thermostatic, this is an interesting area that requires more work. The various doses of heat and cold, local and general exposure, skin and deep body temperature changes necessary to promote optimal responses need to be established. Although it is not easy to separate exercise and heat as factors in these beneficial adaptations, the fact that passive heat and cold exposure have beneficial effects supports the hypothesis that although humans are homeotherms, this homeothermy needs to be challenged if we are to remain healthy. That is, the dynamic equilibrium that underpins homeostasis needs to be perturbed. This hypothesis is unlikely to be limited to thermal balance; one also wonders about the possibility of cross- or combined stressor therapies. Watch this space.