Science Of Sauna Blog

- Paul Taylor, BSc, MSc, MSc (Med Sci)

A former British Royal Navy Aircrew Officer, Paul is an Exercise Physiologist, Nutritionist and Neuroscientist who is currently completing a PhD in Applied Psychology. 

Paul is the host of The MindBodyBrain Project Podcast and his latest venture is The Resilient Mind, an online program and App that has proven benefits for resilience, mental wellbeing and reducing levels of burnout of participants.

Executive Summary

Sauna use, which sometimes referred to as “sauna bathing,” has been popular in a range of cultures for hundreds of years and is characterised by short-term exposure to high temperatures, typically ranging from 45 0C to 100 0C (113 0F to 212 0F), depending on the modality chosen. This heat exposure elicits mild hyperthermia and stimulates a wide range of co-ordinated bodily responses including cardiovascular, neuroendocrine, and cytoprotective mechanisms that position sauna bathing as a viable means to extend both lifespan and healthspan.

Sauna Basics

Heat and sauna therapy for the purposes of healing, cleansing and purification is an ancient practice that can be found across cultures throughout hundreds, if not thousands, of years. Variations of heat therapy include ancient Romans baths, Native American sweat lodges, Japanese hot soaking tubs, Russian banyas and Finnish saunas. Modern saunas come in 3 main varieties: dry, steam and infrared, and all utilise short-term exposure to high temperatures, typically ranging from 45 0C to 100 0C (113 0F to 212 0F), depending on the modality chosen (1). 

A dry sauna is based on the traditional Finnish sauna, with low humidity and a high temperature, typically from 80 to 100° C (176 to 212° F) (2). A steam sauna has higher humidity, which makes sweating and cooling the body more challenging and therefore cannot be as hot as a dry sauna and are reported to be more uncomfortable and stressful than dry sauna (3). Infrared saunas use infrared radiation lamps that emit both visible and infrared light, with the infrared light being either near- or far-infrared spectrums. Far-infrared saunas emit longer wavelengths of infrared light that penetrate tissue to 0.1 mm deep, whilst near-infrared saunas emit shorter wavelengths that can penetrate the body up to 5 mm (4). Some modern saunas capable of emitting both wavelengths and these are known as full-spectrum infrared saunas. Because of the deep tissue penetration, infrared saunas operate at cooler temperatures than dry saunas while still heating up the body, typically 45 0C to 60 0C (113 0F to 140 0F) (1, 4).

Physiological & endocrine response to heat stress

All varieties heat the body to the point where the usual means of cooling through sweating is not enough to compensate for the extreme heat, so the body elicits a rapid, robust response that affects primarily the skin and cardiovascular systems. The skin heats first, rising to approximately 40 0C (104 0F), followed by changes in core body temperature, rising slowly from 37 0C to approximately 38 0C (98.6 0F to 100.4 0F) and then increasing rapidly to approximately 39 0C (102.2 0F) if exposure lasts long enough (5, 6, 7). Cardiac output may increase by as much as 60–70%, while the heart rate increases and the stroke volume remains stable (8). Additionally, approximately 50–70% of the body’s circulation redistributes from the core to the skin to facilitate sweating, driving fluid losses at a rate of approximately 0.6 to 1.0 kg per hour (9).

The endocrine system responds to the heat by increasing the production of several important hormones (2). There are increases in Betaendorphins, which are responsible for the “pleasure” and “analgesic” effects of a sauna. Norepinephrine (sometimes called Noradrenaline), which is associated with improved mood, focused attention and raised heart-rate, also increases (5). Growth hormone, whose secretion progressively declines with age and may contribute to sarcopenic obesity and frailty, increases in a manner that is dependent on time, temperature and frequency of exposure. For example, two 20-minute sauna sessions separated by a 30-minute cooling period has been shown to result in a 2 to 5-fold elevation in Growth Hormone secretion and sauna use and exercise work synergistically to significantly elevate growth hormone when used together (10, 11).

Immune Effects

Sauna bathing stimulates the immune system by increasing white blood cell, lymphocyte, neutrophil, and basophil counts, which may translate to fewer illnesses (12). For example, a six-month study reported that participants who engaged in regular sauna baths had significantly fewer colds than the control group over the same time period (13). The positive impacts on the immune system are at least partly derived by the increases in molecular proteins known as Heat Shock Proteins (HSPs), which are activated when the body is exposed to stressors like exercise, heat and cold. Evidence suggests that certain HSPs play roles in both innate and adaptive immunity (14) and it has also been suggested that HSPs may offer protection against neurodegenerative diseases (15).

In one study, healthy men and women who were exposed to heat for 30 min at 73 0C (163.4 0F) increased HSP72 levels by 49% (16). In another study, healthy men and women had their HSP70 and HSP90 levels increased by 45% and 38%, after undergoing deep tissue heat therapy for six days (17).

Cardiovascular Effects

Heat exposure from sauna bathing induces protective mechanisms that promote cardiovascular health, some of which are the same as those experienced during exercise. For example, heart rate has been shown to increase up to 100 beats per minute during moderate-temperature sauna bathing sessions and up to 150 beats per minute during hotter sessions, similar to the increases observed during moderate- to vigorousintensity physical exercise (18).

Heart disease was once contraindicated for saunas, but recent research is proving that saunas can be not only safe, but beneficial for people with cardiovascular disease. A range of findings on cardiovascular health have been reported from the ongoing landmark Kuopio Ischemic Heart Disease (KIHD) Risk Factor Study, which has been following 2,315 middle-aged Finnish men for more than 20 years. The study has found that those men who frequented saunas the most (four to seven times per week) had a significantly lower risk of sudden cardiac death, fatal coronary heart disease, and fatal cardiovascular disease compared to those who visited saunas two to three times per week or one time per week (3). In addition to these findings, the study found that increased sauna use was also associated with a 40% reduction in all-cause mortality (19).

Brain & Mental Health

Sauna bathing is associated with reduced risk of developing agerelated neurodegenerative diseases such as dementia and Alzheimer’s disease, in a dose-dependent manner (more frequent bathing has a bigger effect). In the KIHD study mentioned earlier, men who reported using the sauna 4–7 times per week had a 66% lower risk of developing dementia and a 65% lower risk of developing Alzheimer’s disease, compared to men who reported using the sauna only once weekly (20).

There may be multiple mechanisms by which frequent sauna use may protect against neurodegenerative diseases, such as improved blood flow to the brain, improved cardiovascular health and the positive health effects of Heat Shock Proteins (1). The health benefits of sauna use also extended to other aspects of mental health as well. Men participating in the KIHD study who reported using the sauna 4–7 times per week had a 77% reduced risk of developing psychotic disorders (21). In a randomised controlled trial of depressed individuals, those who received 4 weeks of sauna sessions experienced reduced symptoms of depression and participants in another randomised controlled trial who received a single session of heat therapy experienced an acute antidepressant effect that was apparent within 1 week of treatment and persisted for 6 weeks after treatment (22).

Detoxification

Many websites promote sauna bathing as a way of increasing detoxification, but the evidence for this is not yet as robust as the evidence for other health impacts, such as cardiovascular benefits mentioned above. However, several small studies have demonstrated a benefit. In one study, Police officers were treated successfully for methamphetamine exposure using a combination of exercise, nutritional support, and sauna therapy (23) and women with occupational exposure to solvents improved after therapy that included sauna use (24). Interestingly, the high rate of sweating to assist with cooling the body during sauna bathing has been reported to facilitate higher excretion of some heavy metals including aluminum (3.75- fold), cadmium (25-fold), cobalt (7-fold), and lead (17-fold), compared to elimination via urine (10). Sauna use to aid in detoxification is a promising additional health benefit, but further research is required to understand the mechanisms.

Other benefits

A number of studies highlight many other benefits of regular sauna use, including increased left ventricular ejection fraction, cardiac output, endothelial function, lower oxidative stress markers and improved exercise tolerance with infrared sauna use (25, 26, 27, 28, 29). Sauna use has also been reported to reduce pain in Fibromyalgia patients (30) and to reduced fatigue, anxiety, and depression in individuals with chronic fatigue syndrome (31). Sauna use may also help prevent or treat diabetes Sauna use may also help prevent or treat diabetes by improving insulin sensitivity (32) and improvements in respiratory symptoms, including vital capacity, minute ventilation, and forced expiratory volume of lungs have also been reported (33,34).

Contraindications

Although it has been recommended in the past that pregnant women avoid saunas are often contraindicated, several studies have concluded that saunas use does not disturb the development of the fetus in healthy women, especially if the heat is not extreme (35, 36, 37).

People with hypotension (low blood pressure) should obtain medical advice before using the sauna and those with heat sensitivities (such as those with multiple sclerosis), a recent myocardial infarction, unstable angina and people who are experiencing an illness accompanied by a fever should avoid the sauna (38). A sauna should also be avoided if you are drinking alcohol, as most sauna accidents and death involve alcohol consumption (39).

Conclusion

Sauna bathing is clearly associated with many health benefits, from cardiovascular and cognitive health to physical fitness and immune system support. It is generally considered safe for healthy adults and may be safe for special populations with appropriate medical supervision. Heat stress via sauna use upregulates positive molecular mechanisms that protect the body and brain from damage, similar to the responses elicited by moderate- to vigorous-intensity exercise, and may offer a means to slow aging and extend healthspan.

References

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