Endocrine effects of sauna bath

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC-ND 4.0 https://creativecommons.org/licenses/by-nc-nd/4.0/ Endocrine effects of sauna bath © 2019 Elsevier Ltd. Accepted version (Final draft) Huhtaniemi, Ilpo T.; Laukkanen, Jari A. Huhtaniemi, I. T., & Laukkanen, J. A. (2020). Endocrine effects of sauna bath. Current Opinion in Endocrine and Metabolic Research, 11, 15-20. https://doi.org/10.1016/j.coemr.2019.12.004 2020 Journal Pre-proof Endocrine effects of sauna bath Ilpo T. Huhtaniemi, Jari A. Laukkanen PII: S2451-9650(19)30104-8 DOI: https://doi.org/10.1016/j.coemr.2019.12.004 Reference: COEMR 127 To appear in: Current Opinion in Endocrine and Metabolic Research Received Date: 24 October 2019 Revised Date: 7 December 2019 Accepted Date: 14 December 2019 Please cite this article as: Huhtaniemi IT, Laukkanen JA, Endocrine effects of sauna bath, Current Opinion in Endocrine and Metabolic Research, https://doi.org/10.1016/j.coemr.2019.12.004. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved. 1 Endocrine effects of sauna bath Ilpo T. Huhtaniemi 1 & Jari A. Laukkanen 2 1 Institute of Reproductive and Developmental Biology, Department of Metabolism, Digestion and Reproduction, Imperial College London, Hammersmith Campus, Du Cane Road, London W12 0NN, UK; E-mail: ilpo.huhtanie[email protected] 2 Institute of Public Health and Clinical Nutrition, University of Eastern Finland, Kuopio, Finland; Faculty of Sport and Health Sciences, University of Jyvaskyla, Jyvaskyla, Finland; Central Finland Health Care District, Department of Internal Medicine, Jyväskylä, Finland: E-mail: [email protected] Corresponding author: Prof. Ilpo Huhtaniemi Declaration of competing interests: NONE 2 Abstract Sauna bath brings about numerous acute changes in hormone levels, partly akin to other stressful situations, partly specific for sauna. Norepinephrine increases in those accustomed to sauna bath. Sweating increases the production of antidiuretic hormone, and the renin-angiotensin system becomes activated. Of the anterior pituitary hormones, growth hormone (GH) and prolactin (PRL) secretion is increased. Also β-endorphin has been frequently reported to increase, whereas the responses of ACTH and cortisol are variable, probably depending on the type of sauna exposure. Sperm production decreases in particular in sauna-naïve men, but reduced fertility has not been associated with regular sauna habits. Minor sex differences exist, the hormonal responses being somewhat greater in women. Sauna-naïve women may experience mild disturbances in menstrual cycle, but no effects of fertility have been reported. The hormone responses are short-lived, normalizing soon after sauna exposure during the recovery. Adaptation to regular sauna use plays an important role in the responses, which attenuate upon frequent exposure. Key words: heat stress, hot bath, hyperthermia, norepinephrine, growth hormone (GH), prolactin (PRL), cortisol, β-endorphin, renin-angiotensin, spermatogenesis, adaptation 3 1. Introduction Heat stress in the passive (hot baths and sauna) or active (exercise) form evokes multiple physiological responses, some of them involving the endocrine system. Sauna bathing, a common life-style habit used mainly for relaxation, cleaning, pleasure, and to release bodily stress, has modulating effects on the hormonal and autonomic nervous system activity (1). The purpose of this review is to summarize the current knowledge about the endocrine effects of sauna. Besides the Finnish sauna-type exposure to dry heat, rather similar endocrine responses have been observed upon immersion in hot water. A wealth of information is available on effects of sauna (and hot bath) on hormones from older studies carried out 20-30 years ago, but additional novel information about sauna and hormones is rather scarce. Some older reviews exist on the topic (e.g. 2,3), but in recent years the endocrine effects of sauna have not been addressed comprehensively. We therefore summarize the key information available from older literature with the more recent findings. A summary of the hormone responses detected upon sauna bath is presented in Table 1. 2. Neuroendocrine and pituitary hormones The neuroendocrine responses to sauna have concentrated on stress hormones such as β-endorphin and ACTH, which respond variably, ranging from decrease to no change to increase (4-8). When the heat exposure reaches level of subjective discomfort the ACTH/β-endorphin increase is more consistent (9). The increase in β-endorphin may play a role in the feeling of wellbeing after sauna. The thermal stress response of these hormones is abolished in alcohol, 4 cocaine and heroin addicts, which is explained by the disruption of hypothalamic opioid neurotransmission upon chronic addiction (7,10). No effect of sauna on gonadotropins has been found in either sex (4,6). Findings on TSH are conflicting with reports ranging from increase (11) to no change (4,12) to decrease (both TSH and T4)(13,14). The two anterior pituitary hormones with the most consistent responses during exposure to sauna are GH and PRL. GH increase has been documented in numerous studies (4,9,15). Leppäluoto et al. (16) documented that the GH response was under control of the hypothlalamic GH-releasing hormone (GHRH). Furthermore, an intriguing age difference was found: while GHRH and GH responded with significant increase in younger men (31-46 yrs), no increase of either was found in older men (49-66 yrs). The same investigators detected attenuation of the GH responses following repeated sauna exposure in men (7 days twice daily) (4). The attenuation of GH response was not reproduced in young women (13). Stimulation of peripheral thermoregulators (heating of arm) was found sufficient to increase the GH levels (17). The other consistent endocrine response to sauna is the increase in PRL levels (4,5,9). The increase is brisk (up to > 10-fold), remained similar in men upon repeated sauna exposure (4), but attenuated in women (13). As an attempt to study the mechanism of PRL increase upon hyperthermia, Low et al. (18) subjected men to exhaustive bicycle exercise at 33 °C temperature, and to passive heating in a 41.5 °C water bath. Similar increase in body core 5 temperature (to 38.8 °C) and a 2-fold PRL response was observed. It was concluded that the thermoregulatory afferents, rather than cardiovascular, provide the stimulus for PRL increase, probably through changes in serotoninergic and dopaminergic activation, in agreement with the response to sauna. However, it remains controversial whether the body core (17) or skin (19) temperature rise is more important in the PRL rise. Another study showed that facial cooling prevented the PRL increase and increased thermal comfort during sauna bath, indicating that a small part of total skin area (10%) can have a disproportionate role in the PRL response (20). 3. Cortisol Findings on the acute effects of sauna bathing on the distal response of the hypothalamic-pituitary-adrenal axis, i.e. serum cortisol, are variable. In some studies cortisol levels have increased (21), in some an initial decrease was followed by an increase (8), and also unchanged levels have been reported (9). Similar variable findings have been made during the post-sauna cooling period (5,8,9,22). The differing responses are best explained by differences in the duration and temperature of sauna bath. A general trend is that higher humidity and temperature evoke higher increase in cortisol (9,22). Excessive sauna exposure of men (twice a day for 7 days) suppressed the GH, ACTH and cortisol responses at the end of the experiment (4). A very recent study examined the effects of various types of physical exercise (endurance, strength, combined), followed by sauna, on serum hormone levels (GH, testosterone, cortisol) in men (23). The expected increases in the levels of 6 the three hormones were found when studied in the afternoon, but not in the morning - a confounding factor not taken into account in all sauna studies. Sauna after the exercise enhanced neuromuscular fatigue, but did not change the hormonal responses evoked by exercise. 4. Effects on stress and cardiovascular hormones Responses to sauna of the cardiovascular system, with its essential regulatory hormones related to hemodynamic consequences of heat exposure, have been studied in recent years. Regular sauna bathing has been suggested to exert a potential therapeutic effect on elevated blood pressure, which may be partly explained via favorable effects on the cardiac autonomic nervous system balance, improved vascular function and increased excessive fluid loss (24). Sauna baths have shown to increase the demands of cardiovascular function (25). Sauna bathing leads to an increase in heart rate (HR), thereby imitating effects of moderate physical activity on cardiovascular system without active skeletal muscle work. Sauna bathing causes an increase in HR and heart rate variability (HRV), which is a marker of autonomic nervous system balance. HRV analysis provides an insight into cardiovascular responses to sauna baths. One recent study showed the effects of a typical Finnish sauna on HR and HRV in a population with cardiovascular risk factors among regular sauna bathers (26). The results indicate that sauna bathing improves cardiac autonomic nervous system balance, leading to increase in vagal tone and decrease in sympathetic tone, with favorable modulations in blood pressure during the recovery from sauna which may also be related to a lowered cardiovascular event risk (27). 7 Increased sweating during sauna bathing is accompanied by reduction in blood pressure and a higher HR, whilst cardiac stroke volume is largely maintained; although a part of blood volume is diverted from the internal organs to peripheral body parts with decreasing venous return which is not facilitated by active skeletal muscle work (1). Indeed, comparable with exercise-induced adaptations, heat stress could increase HR up to 150 per minute (26). During the hot sauna bath HR increases gradually from the beginning of sauna bath until the end, corresponding with an increase in body temperature and heat load. An earlier study conducted in young healthy men showed that the plasma norepinephrine concentrations increased approximately 2-fold during bath sessions with a mean temperature of 88 °C whereas there were no substantial changes in plasma epinephrine and serum thromboxane concentrations (28). The effectiveness and safety of sauna bath as an additional diaphoretic or diuretic therapy requires more studies. Although intensive sweating, induced by sauna with dry or wet heat usually increases losses of water, urea, sodium, potassium and chloride, a single sauna bath session does not cause significant longer-term changes in serum electrolyte or creatinine levels (25). Ohori et al. (29) demonstrated that 3 weeks of repeated thermal treatment (Waon therapy) in patients with chronic heart failure was associated with decreases in levels of brain natriuretic peptides (BNPs) and plasma norepinephrine. However, there is only moderate evidence that sauna bath improves left ventricular ejection fraction and decreases BNP levels (30). Some other studies in which patients with heart failure were treated with infrared-ray sauna therapy for several 14 transient hormonal responses observed cannot be considered harmful for individuals in good health, but they may represent a burden for those in suboptimal health status – an issue deserving more detailed study in the future. 15 References *1. Laukkanen JA, Laukkanen T, Kunutsor SK. Cardiovascular and other health benefits of sauna bathing: A review of evidence. Mayo Clinic Proceedings. 2018;93:1111-1121. This is the latest review summarizing comprehensively the various health effects of sauna bath. 2. Hannuksela ML, Ellahham S. Benefits and risks of sauna bathing. Am J Med 2001;110:118-126. 3. Leppäluoto J, Huttunen P, Hirvonen J, Väänänen A, Tuominen M, Vuori J. Endocrine effects of repeated sauna bathing. 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Appl Biochem Biotechnol 2013;171:1253-1261. 21 Table 1. Documented changes in hormone levels during sauna bath, compiled from ref. 2 and papers published thereafter (as listed in the references). Hormone Change Adiponectin ↔ ↔↔ ↔ Adrenocorticotropic hormone ↔ ↔↔ ↔,↑ ↑↑ ↑* Aldosterone ↑ ↑↑ ↑ Angiotensin II ↑ ↑↑ ↑ Arginine vasopressin ↑ ↑↑ ↑ Atrial natriuretic peptide ↑ ↑↑ ↑ Beta-endorphin ↑ ↑↑ ↑ Brain natriuretic peptide ↑ ↑↑ ↑,↓ ↓↓ ↓ Cholecystokinin ↔ ↔↔ ↔ Costisol ↔ ↔↔ ↔,↑ ↑↑ ↑,↓ ↓↓ ↓ Epinephrine ↔ ↔↔ ↔,↑ ↑↑ ↑ Follicle-stimulating hormone ↔ ↔↔ ↔ Ghrelin ↑ ↑↑ ↑ Glucagon ↑ ↑↑ ↑ Growth hormone ↑ ↑↑ ↑ Growth hormone-releasing hormone ↑ ↑↑ ↑ Insulin ↔ ↔↔ ↔ Leptin ↑ ↑↑ ↑ Luteinizing hormone ↔ ↔↔ ↔ Melatonin ↔ ↔↔ ↔ Motilin ↑ ↑↑ ↑ Norepinephrine ↑ ↑↑ ↑ Prolactin ↑ ↑↑ ↑ Renin ↑ ↑↑ ↑ Somatostatin ↔ ↔↔ ↔ Testosterone ↔ ↔↔ ↔, ↑ ↑↑ ↑ Thyroxine ↔ ↔↔ ↔, ↑ ↑↑ ↑ Thyroid-stimulating hormone ↔ ↔↔ ↔, ↑ ↑↑ ↑ Vasoactive intestinal polypeptide ↑ ↑↑ ↑ _____________________________________________________________________________________________ * ↔, no change: ↑, increase; ↓, decrease