Study of whole-body thermotherapy (sauna) in human health and wellness
Creators: name: HUSSAIN, Juhaina
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 320599
subject: Medical biochemistry and metabolomics not elsewhere classified
classificationCode: 420899
subject: Traditional, complementary and integrative medicine not elsewhere classified

description: Worldwide many people sauna-bathe routinely for health reasons but the medical evidence to support these practices is still emerging.  Repeated sauna bathing, or whole-body thermotherapy, involving short-term exposures to elevated environmental air temperature, leads to mental and physical responses that are reportedly similar to exercise.  However, while the researched health benefits of exercise continue to abound, the research involving saunas is scant in comparison.  Therefore, the desire for a deeper understanding of how sauna bathing impacts systemic health, on both subjective and objective levels, potentially similar to exercise, was the overarching aim to undertaking these studies.  

A systematic review of the sauna medical literature concluded that those with cardiovascular-related disorders and rheumatological diseases, as well as athletes seeking improved exercise performance were the ones receiving the most beneficial health effects from sauna.  However, the mechanisms to explain the health effects were not clearly established.  As well, critically polled surveys of sauna bathing were sparse in the literature, and none were inclusive of women.  For this reason, a cross-sectional study was conceived to address a primary aim of this project, namely to better understand the health-related effects from a subjective viewpoint.  It involved conducting an online questionnaire polling frequent sauna bathers worldwide about their characteristics, motivations and experiences around sauna bathing.  

Analysis of this first-ever `Global Sauna Survey' conducted over 12 months (November 2016-2017) revealed all responders (n=482), involving nearly equal numbers of men (51.3%) and women (48.7%), were motivated to sauna bathe for relaxation (100%, n=460), as well as many (&gt;80%) reporting health benefits especially around mental well-being, pain relief (87.8%, n=404) and sleep (83.5%, n=423), with relatively few adverse effects.  More interestingly, these published study findings indicated many people sauna bathed for health reasons not yet established (e.g., `detox' or detoxification in 82.8%, n=381) and conversely, few people used sauna bathing for prevention or treatment of health conditions such as cardiovascular-related disorders where benefits have repeatedly been documented.  This mismatch between research focus and public behaviour highlighted important directions for future sauna research and influenced the choice of outcome measures when designing the next component of this project, a clinical interventional study.

Given the cardiovascular benefits reported across different types of sauna in the initial literature review, newer cardiovascular-related measurements of arterial stiffness and heart rate variability presented ideal choices of outcomes to further explore with sauna (and exercise).  Thus, in pursuing the additional aims of this project focused on attaining more objective health measures of sauna bathing, a novel randomised, controlled crossover trial of infrared sauna and exercise (indoor bicycling) in ten healthy premenopausal women was then conducted.  This `Sauna Exercise Clinical Crossover Study' included these two newer measurements and other relevant physiological outcomes collected from the women across both activities and an initial control resting intervention.  Data was gathered from August 2019 to March 2020.

The physiological results confirmed that elevations in core body temperature (i.e., tympanic) were detected with the infrared sauna as compared to both the exercise (mean difference = +0.79oC ± standard error of means - SEM -0.12 oC, 95% C.I.: 0.49 - 1.08, p&lt;0.0005) and control (+1.05 ± 0.12 oC, 95% C.I.: 0.73 - 1.36, p&lt;0.0005) at all the post-intervention time points (p&lt;.0005).  There was also an absence of hyperpnea (increased breathing rate) with the infrared sauna as compared to the moderate intensity bicycling (-6.66 ± 1.33 breaths/min, 95% C.I.: -10.11 to -3.20, p&lt;0.0005) and no differences to control (+1.01 ± 1.41 breaths/min, 95% C.I.: -2.65 to +4.66, p=1.000) at all the same time points.  However, there were curiously no differences in arterial stiffness or heart rate variability detected between any of the interventions.  These unexpected findings suggested thermoregulatory mechanisms, more so than exercise-mimetic hemodynamic, respiratory or cardiac autonomic nervous system responses to explain sauna's cardiovascular improvements.  

In the initial survey of sauna literature, several studies presented preliminary evidence that certain bio-accumulating environmental toxicants such as heavy metals, bisphenol-A (BPA), phthalates, perfluorinated compounds and organochloride pesticides, could be detected at greater concentrations in sweat produced with sauna (and exercise) interventions, as compared to respective concentrations in blood and urine.  Increased sweating was also hypothesised to contribute to positive health effects.  However, the methods particularly around sweat collection in these studies were heterogeneous and lacked sufficient rigour.  These findings spurred a second literature review, which examined the under-appreciated complexity of human sweating and sweat content.  This review highlighted several variables necessary to standardising sweat induction and collection for clinically meaningful analysis, which were then incorporated into the Sauna Exercise study.  

Consequently, another aim of this thesis was addressed by interrogating the toxicant content of sweat and urine collected with infrared sauna versus exercise activities.  Collaborations with the Queensland Academy of Sport (QAS) and the Queensland Alliance for Environmental Health Sciences (QAEHS) were organised to analyse the contents of the women's biofluid samples for environmental toxicants that had not (to our knowledge) been investigated in human sweat.  The choice of toxicants emerged to be four metabolites of organophosphate (OP) and pyrethroid (PYR) insecticides ubiquitously used worldwide.  

The results revealed OP and PYR metabolites were detected in 84.6% (22) and 26.9% (7), respectively, of the total 26 sweat samples (pooled per individual, per intervention).  Furthermore, median urinary levels of three of the four metabolites increased after infrared sauna bathing (z = 2.395, p=0.017; z = 2.599, p=0.009; z = 2.090, p=0.037) with the second of those three metabolites (3-phenoxybenzoic acid, a PYR metabolite) also increasing after exercise (z = 2.073, p=0.038), all compared to control.  Although definitive urine/sweat correlations were not detected with any of the metabolites, the presence of these metabolites in the two separate body compartments raises important questions about the human metabolism of synthetic insecticides that warrant further study.

The combined outcomes of this thesis provide new and novel insights into the multi-system effects of sauna bathing and sweating, especially in regard to female-inclusive findings and to deciphering its exercise-mimetic effects.  These multi-faceted findings support the use of sauna as a well-tolerated lifestyle tool promoting relaxation, mental wellness and beneficial thermoregulatory stress.  Furthermore, this thesis importantly adds to the growing research repository developing sauna as a therapeutic adjunct for prevention and treatment of several medical conditions, particularly relevant for CVD-related disorders.