Tom Cullen, Neil D. Clarke, Mathew Hill, Campbell Menzies, Christopher J. A. Pugh, Charles J. Steward, and C. Douglas Thake
Centre for Sport Exercise and Life Sciences, Coventry University, Coventry, United Kingdom; and Cardiff School of Sport & Health Sciences, Cardiff Metropolitan University, Cardiff, United Kingdom. Submitted 14 July 2020; accepted in final form 28 September 2020
Exercise can induce numerous health benefits that can reduce the risk of chronic diseases and all-cause mortality, yet a significant percentage of the population do not meet minimal physical activity guidelines. Several recent studies have shown that passive heating can induce numerous health benefits, many of which are comparable with exercise, such as improvements to cardiorespiratory fitness, vascular health, glycemic control, and chronic low-grade inflammation. As such, passive heating is emerging as a promising therapy for populations who cannot perform sustained exercise or display poor exercise adherence. There appears to be some overlap between the cellular signaling responses that are regulated by temperature and the mechanisms that underpin beneficial adaptations to exercise, but detailed comparisons have not yet been made. Therefore, the purpose of this mini review is to assess the similarities and distinctions between adaptations to passive heating and exercise. Understanding the potential shared mechanisms of action between passive heating and exercise may help to direct future studies to implement passive heating more effectively and identify differences between passive heating and exercise-induced adaptations.
Physical inactivity increases the risk of several chronic diseases, such as cardiovascular disease, type 2 diabetes, and obesity. In contrast, regular exercise elicits a variety of health benefits and attenuates traditional cardiovascular disease risk factors, including blood pressure and body weight/adiposity, as well as improved blood lipid profiles, insulin sensitivity, and cardiorespiratory fitness. More recently, this understanding has extended to include nontraditional factors such as anti-atherogenic effects propagated by functional and structural adaptations within the vasculature and the anti-inflammatory effects of exercise. Despite the overwhelming evidence for its efficacy, exercise is typically not well adhered to, with common self-reported barriers including lack of motivation, time, poor physical fitness, and low self-esteem. However, older populations also avoid exercise due to an increased fear of injury and pain. As such, alternative or adjunct therapies capable of eliciting similar systemic health benefits have considerable clinical implications and warrant further investigation. In recent years, there has been a resurgence of interest in the potential health benefits of passive heating or “’thermal therapy,” with some authors promoting heat therapy as a potential alternative to exercise for populations with physical disabilities and those who find adherence to exercise difficult. Thus far, a range of different methods of passive heating have been used such as sauna bathing, hot water immersion, water-perfused suits, or microwave diathermy. Epidemiological studies from Finland, where sauna bathing is common, have demonstrated that regular sauna bathing and a high level of cardiorespiratory fitness (argued to be a surrogate of regular physical activity) independently reduce the risk of death by cardiovascular disease, but this risk is further reduced by a combination of high cardiorespiratory fitness and regular sauna bathing. If the protective effects of these therapies are complementary, it raises the question as to “how” these protective effects are conferred and to what extent these mechanisms overlap. A growing number of studies have begun to elucidate the mechanisms by which the protective effects of thermal therapy may be conferred, and the reader is directed to reviews on these topics (e.g., 9, 25, 28). Given that studies of passive heating are still in their relative infancy, understanding the potential shared mechanisms of action with exercise may help to direct future studies and the implementation of passive heating more effectively. To focus on the potential overlap of mechanisms, this review will primarily draw upon literature from nondi-seased populations and will only make a reference to other populations where useful to do so. The purpose of this mini review is to assess the similarity and distinctions between the cardiovascular and metabolic health benefits induced by passive heating and exercise, to highlight any areas by which passive heating may lack some of the benefits of exercise, and to discuss important areas of future study.