Sauna

Bathing oneself in heat for the purposes of purification, cleansing, and healing is an ancient practice, dating back thousands of years and observed across many cultures. Variations of its use are seen today in the banyas of Russia, the sweat lodges of the American Indians, and, most famously, the saunas of Finland.

Sauna use, sometimes referred to as "sauna bathing," is characterized by short-term passive exposure to extreme heat. This exposure elicits mild hyperthermia – an increase in the body's core temperature – that induces a thermoregulatory response involving neuroendocrine, cardiovascular, and cytoprotective mechanisms that work together to restore homeostasis and condition the body for future heat stressors.[1]

In recent decades, sauna bathing has emerged as a means to increase lifespan and improve overall health, based on compelling data from observational, interventional, and mechanistic studies. Of particular interest are the findings from studies of participants in the Kuopio Ischemic Heart Disease Risk Factor (KIHD) Study, an ongoing prospective population-based cohort study of health outcomes in more than 2,300 middle-aged men from eastern Finland, which identified strong links between sauna use and reduced death and disease.

The KIHD findings showed that men who used the sauna two to three times per week were 27 percent less likely to die from cardiovascular-related causes than men who didn't use the sauna.[2] Furthermore, the benefits they experienced were found to be dose-dependent: Men who used the sauna roughly twice as often, about four to seven times per week, experienced roughly twice the benefits – and were 50 percent less likely to die from cardiovascular-related causes.[2] In addition, frequent sauna users were found to be 40 percent less likely to die from all causes of premature death. These findings held true even when considering age, activity levels, and lifestyle factors that might have influenced the men's health.[2]

The KIHD also revealed that frequent sauna use reduced the risk of developing dementia and Alzheimer's disease in a dose-dependent manner. Men who used the sauna two to three times per week had a 66 percent lower risk of developing dementia and a 65 percent lower risk of developing Alzheimer's disease, compared to men who used the sauna only one time per week.[3]

The health benefits associated with sauna use extended to other aspects of mental health, as well. Men participating in the KIHD study who used the sauna four to seven times per week were 77 percent less likely to develop psychotic disorders, regardless of the men's dietary habits, socioeconomic status, physical activity, and inflammatory status (as measured by C-reactive protein).[4]

Overview of sauna practices

The term “sauna” is a Finnish word, and it typically refers to an unpainted spruce- or pine-paneled room, with wooden benches made of aspen, spruce, or obeche.[5] The bulk of the research related to sauna bathing has been conducted in Finland or in regard to Finnish-style sauna practices. Not all saunas are Finnish-style, however, and saunas may differ according to their heat source, relative humidity, and duration of use.

Heat source

Historically, saunas were heated by wood fires – a practice still observed today in rural parts of Finland. Most modern saunas, however, are heated by conventional electric heaters or infrared heaters. Conventional heaters warm the air to a high temperature, ranging from 70°C to 100°C (158°F to 212°F), optimally at 80°C to 90°C (176°F to 194°F) at the level of the user's face.[5][6] The heat of the warmed air transfers to the body.

Infrared heaters emit thermal radiation, which heats the body directly while also warming the surrounding air. They operate at lower temperatures than traditional saunas, at 45°C to 60°C (113°F to 140°F).[7] Infrared heaters emit either near or far wavelengths. Near infrared heaters use incandescent bulbs to produce thermal radiation of varying wavelengths, ranging from near-infrared wavelengths (primarily) to middle-infrared wavelengths (to a lesser degree). Far infrared heaters use ceramic or metallic heating elements that emit energy in the far-infrared range, which is similar to energy produced by the sun.

Humidity

Saunas can be either dry or wet. In a dry sauna, the relative humidity is low (10 to 20 percent).[5] A common practice in Finland, called löyly, is to throw water on the heater rocks to increase the humidity slightly. The term “wet sauna” is a misnomer, however, referring to a steam sauna, where the humidity is extremely high (typically greater than 50 percent), which keeps sweat from evaporating.[8] A wet sauna may feel (subjectively) hotter than a dry sauna and is harder on the cardiovascular system, due to the reduced evaporative cooling.[8]

Duration and temperature

Finnish-style sauna bathing involves one to three sessions of heat exposure lasting five to 20 minutes each, interspersed with periods of cooling.[6] Some cooling methods can be rather extreme and involve rolling in snow or immersing in cold water, which further stresses the cardiovascular system.[9] Sauna poses little risk of cardiovascular complications in healthy people, however.[9]

The KIHD studies that found a dose-dependent reduction in cardiovascular-related mortality, all-cause mortality, and Alzheimer's disease incidence typically involved saunas that were heated to a temperature of at least 78.9°C (174°F) for at least 20 minutes. In fact, these studies found that the amount of time spent in the sauna also affected cardiovascular-related mortalities, with a longer duration of 19 minutes or more having a more robust effect than 11 to 18 minutes on lowering mortality rate.[2]

Another form of thermal treatment, called waon therapy, originated in Japan. Like traditional sauna, waon utilizes low humidity, but the temperatures are slightly lower, at approximately 60°C (140°F). Waon therapy is associated with improvements in multiple aspects of cardiovascular function.[10]

Physiological response to heat stress

Exposure to high temperature stresses the body, eliciting a rapid, robust response. The skin and core body temperatures increase markedly, and sweating ensues. The skin heats first, rising to 40°C (104°F), and then changes in core body temperature occur, rising slowly from 37°C (98.6°F, or normal) to 38°C (100.4°F) and then rapidly increasing to 39°C (102.2°F).[6]

Cardiac output, a measure of the amount of work the heart performs in response to the body's need for oxygen, increases by 60 to 70 percent, while the heart rate (the number of beats per minute) increases and the stroke volume (the amount of blood pumped) remains unchanged.[5] During this time, approximately 50 to 70 percent of the body's blood flow is redistributed from the core to the skin to facilitate sweating. The average person loses approximately 0.5 kg of sweat while sauna bathing.[11] Acute heat exposure also induces a transient increase in overall plasma volume to mitigate the decrease in core blood volume. This increase in plasma volume not only provides a reserve source of fluid for sweating, but it also acts like the water in a car's radiator, cooling the body to prevent rapid increases in core body temperature and promoting heat tolerance.

Repeated sauna use acclimates the body to heat and optimizes the body's response to future exposures, likely due to a biological phenomenon known as hormesis, a compensatory defense response following exposure to a mild stressor that is disproportionate to the magnitude of the stressor. Hormesis triggers a vast array of protective mechanisms that not only repair cell damage but also provide protection from subsequent exposures to more devastating stressors.[12]

The physiological responses to sauna use are remarkably similar to those experienced during moderate- to vigorous-intensity exercise. In fact, sauna use has been proposed as an alternative to exercise for people who are unable to engage in physical activity due to chronic disease, physical limitations, or injury.[13]

Molecular mechanisms involved in the heat stress response

The hormetic effects of heat stress are facilitated by molecular mechanisms that mitigate protein damage and aggregation and activate endogenous antioxidant, repair, and degradation processes. Many of these responses are also triggered in response to moderate- to vigorous-intensity exercise and include increased expression of heat shock proteins, transcriptional regulators, and pro- and anti-inflammatory factors.

Heat shock proteins

Heat-shock proteins (HSPs) comprise a large, highly conserved family of proteins that are present in all cells. They play prominent roles in many cellular processes, including immune function, cell signaling, and cell-cycle regulation.

Under normal conditions, cells maintain a constant, or "basal," level of HSPs to facilitate several aspects of the protein synthesis machinery, including assembly, folding, export, turn-over, and regulation. However, normal metabolic processes and immune function create reactive byproducts (such as reactive oxygen species and reactive nitrogen species) that can damage proteins and disrupt their structure.[14] Intrinsically disordered proteins are common features in cardiovascular diseases, and damaged, dysfunctional proteins, which can aggregate, or clump together, are strongly implicated in the pathogenesis of neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and Huntington's disease.[15][16] Increased expression of HSPs prevents protein disorder and aggregation by repairing proteins that have been damaged, and, in fact, animal evidence suggests that HSPs may offer protection against neurodegenerative diseases.[17]

When cells are stressed due to changes in their environment, cellular proteins can unfold or become damaged, impairing their normal function and further increasing their vulnerability to change. During exposure to stress from temperature extremes, reduced nutrient levels (as in fasting), or hypoxia (reduced oxygen), cells increase expression of HSPs to stabilize unfolded proteins and repair or re-synthesize damaged proteins.

Heat stress, in particular, robustly activates HSPs.[18] For example, after healthy men and women sat in a heat stress chamber for 30 minutes at 73°C (163°F), their levels of HSP72 increased by 49 percent.[19] In a different study, in which healthy men and women were exposed to deep tissue heat therapy for six days, participants' levels of HSP70 and HSP90 increased 45 percent and 38 percent, respectively.[20] In addition, their biomarkers of mitochondrial biogenesis improved, and their mitochondrial function increased by 28 percent compared to baseline levels. The activation of HSPs is sustained over time, suggesting that heat acclimation induces whole-body adaptations that increase heat tolerance, resulting in protective cellular adaptations.[18]

Nrf2

Nrf2 is a transcription factor commonly found in a cell's cytoplasm. Upon activation, Nrf2 travels to the nucleus, leading to the orchestrated regulation of a vast network of genes with cytoprotective, antioxidant, and anti-inflammatory functions and providing protection against oxidative stress, electrophilic stress, and chronic inflammation – the underlying causes of most chronic diseases.[21]

Heat exposure activates Nrf2, thereby upregulating a HSP called heme oxygenase-1, or HO-1, an enzyme that breaks down heme (a powerful pro-oxidant) to generate carbon monoxide (an anti-inflammatory gas) and bilirubin (an antioxidant).[22][23] The downstream effect of HO-1 upregulation includes inhibition of the expression of several pro-inflammatory molecules involved in the pathophysiology of cardiovascular disease, including E-selectin, vascular cell adhesion molecule-1, and intercellular adhesion molecule-1.[22]

FOXO3

FOXO3 proteins are members of the FOX family of highly conserved transcriptional regulators. They play important roles in human lifespan and healthy aging. FOXO3s regulate a vast number of genes that combat elements of cellular aging, such as damage to DNA, proteins, and lipids, and loss of stem cell function.[24] They also increase the production of genes that regulate DNA repair, tumor suppression, stem cell function, immune function, and protein aggregation to further mediate the deleterious effects of aging.[25][26][27][28] FOXO3s participate in autophagy, but when autophagic mechanisms are disturbed, FOXO3s confer cellular sensitization to apoptosis, a type of programmed cell death.[29]

Following heat stress, FOXO3 proteins form a complex with sirtuin 1, or SIRT1, an enzyme that influences aging and longevity via multiple molecular pathways. Sirtuins regulate a variety of metabolic processes, including release of insulin, mobilization of lipids, response to stress, and modulation of lifespan. SIRT1 enhances FOXO3's resistance to oxidative stress and its ability to induce cell cycle arrest, but it also inhibits FOXO3's ability to induce apoptosis, shifting FOXO3 activities away from cell death and toward stress resistance.[30]. Read more about FOXO proteins.

Interleukin-6 (IL-6) & Interleukin-10 (IL-10)

Inflammation is a fundamental cause of chronic disease processes. Maintaining the appropriate balance of pro- and anti-inflammatory factors is crucial for the development and subsequent resolution of an inflammatory response. IL-6 is a pro-inflammatory cytokine that plays an important role in the regulation of central homeostatic and immunological processes. However, IL-6 also dampens the inflammatory response through its activation of IL-10, a potent anti-inflammatory cytokine.[31] Hyperthermia induces a large increase in circulating IL-6 and, potentially, a reciprocal release of IL-10.[32] The role of IL-6, which can be robustly activated by muscle in the context of exercise, may be more complex than simple inflammatory mediator: as a "myokine," it is also necessary for the insulin-sensitizing effects of exercise.[33] Listen to discussion by scientist Dr. Charles Raison at 46:15 of this episode.

Health benefits associated with sauna use

Cardiovascular health

Heat exposure induces protective responses against the deleterious biological processes that drive cardiovascular disease and related disability. Some of these responses recapitulate those experienced during exercise. For example, heart rate may 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 vigorous-intensity physical exercise.[34][35] Furthermore, in a study involving 19 healthy adults, the effects of a single 25-minute sauna session were comparable to moderate physical exercise with respect to cardiovascular measures. During sauna use or moderate exercise, the participants' heart rate and blood pressure increased immediately, but after the sauna or exercise session, participants' blood pressure and heart rate began to drop below baseline levels measured pre-sauna or -exercise.[36] Like exercise, long-term sauna use generally improves blood pressure, endothelial function, and left ventricular function, and reduces inflammation.

Cardiovascular disease

The World Health Organization estimates that nearly 18 million people die each year from cardiovascular diseases, roughly one-third of all deaths worldwide. Cardiovascular disease is largely preventable with lifestyle behaviors such as sauna use.

As described above, large prospective studies conducted in eastern Finland have shown that compared to men who never use the sauna, moderate sauna users (two to three times per week) are 27 percent less likely to die from cardiovascular-related causes, and frequent users (four to seven times per week) are 50 percent less likely to die from cardiovascular-related causes.[2] In addition, the frequent sauna users were found to be 40 percent less likely to die from all causes of premature death, regardless of age, activity levels, and lifestyle factors.[2] Learn more about the role of sauna use in the prevention of cardiovascular disease in this podcast with author Dr. Jari Laukkanen.

Congestive heart failure

Congestive heart failure, or CHF, represents the culmination of all forms of cardiovascular disease. It leads to impaired blood flow to the heart and peripheral tissues with subsequent functional losses, shortness of breath, fluid retention, and left ventricular hypertrophy. Treatment is often limited to pharmaceutical, dietary, or palliative care.

Findings from a prospective, multicentered, randomized controlled trial involving 149 patients with advanced CHF demonstrated that two weeks of waon therapy improved the patients' endurance, heart size, and disease status compared to those who received standard medical care.[37] In a different randomized controlled trial involving 30 CHF patients with frequent premature ventricular contractions, or PVCs, a type of abnormal heartbeat, researchers found that two weeks of infrared dry sauna (waon therapy) reduced the number of PVCs the patients experienced in a 24-hour period (from a baseline of ~3097 to ~848). A control group that received conventional medical therapy showed no changes.[38]

Ischemic heart disease

Ischemic heart disease, also known as coronary artery disease, is characterized by reduced blood supply to the heart. It is the most common cause of death in most western countries. A randomized controlled trial examined the effects of sauna use in 24 patients with ischemic heart disease with chronic total coronary artery occlusion (full blockage of one or more of the arteries that supply the heart) who had not responded to non-surgical procedures and were not candidates for surgical interventions. The findings demonstrated that 15 waon sessions given over a three-week period improved the patients' vascular endothelial function. No improvements were observed in the control group that received standard medical care.[39]

Peripheral artery disease

Peripheral artery disease, or PAD, is a painful and debilitating condition in which the arteries that supply the head and extremities narrow. A pilot trial involving 20 patients with PAD who received 50 waon sessions over a period of 10 weeks demonstrated improvements in pain levels, walking endurance, and lower extremity blood flow.[40] A similar randomized controlled trial involving 21 patients with PAD showed comparable improvements.[41]

Dyslipidemia

Dyslipidemia, or abnormal blood lipid levels, is a strong predictor of cardiovascular disease risk. Two small studies have shown that regular sauna use modulates serum cholesterol and lipoproteins in healthy adults. Women who were exposed to seven 30-minute sauna baths over a period of two weeks exhibited reduced total cholesterol levels (from ~4.47mmol/L to ~4.25mmol/L) and reduced low-density lipoprotein, or LDL, levels (from ~2.83mmol/L to ~2.69mmol/L).[42] Similarly, men who were exposed to ten 45-minute sauna baths over a period of three weeks exhibited reduced total cholesterol levels (from ~4.50mmol/L to ~4.18mmol/L) and reduced LDL levels (from ~2.71mmol/L to ~2.43mmol/L).[43]

Hypertension

Hypertension, defined as a systolic pressure of 130 mm Hg or higher, or a diastolic pressure of 80 mm Hg or higher, is a chronic elevation of blood pressure.[44] It is a robust predictor of future incidence of stroke, coronary heart disease, heart attack, heart failure, and cardiovascular-related death. Central to the pathophysiology of hypertension is the loss of arterial compliance, or elasticity, which can have far-reaching effects on multiple organ systems, including the brain and kidneys. A common element among sauna users, however, is lower incidence of hypertension through improvements in arterial compliance. For example, men who used the sauna two to three sessions every week were found to have a 24 percent lower risk of developing hypertension, and men who used the sauna four to seven times per week had a 46 percent lower risk for hypertension, compared to men who used the sauna only once per week. [45] Just a single sauna session has been shown to lower blood pressure and improve arterial compliance. [46] As such, sauna use may serve as a non-pharmacological means to address, or even prevent, hypertension.

Endothelial dysfunction

The endothelium, the cell layer that lines the blood vessels, secretes substances that regulate blood vessel dilation (vasodilators) and constriction (vasoconstrictors). Endothelial dysfunction is characterized by decreased secretion of vasodilators and/or increased secretion of vasoconstrictors. This imbalance leads to impaired endothelium-dependent vasodilation, which is common among people who have CHF. Two weeks of sauna therapy, however, improved endothelial and cardiac function in patients with CHF.[47]

Left ventricular dysfunction

Dysfunction of the heart's left ventricle sets in motion a cascade of compensatory mechanisms that promote organ-level structural changes and elicit system-level hormonal adaptations. It is widely recognized as the end-stage of heart failure. Both single-session and long-term (five days per week for four weeks) sauna use improved ventricular function in men with CHF and may have therapeutic value for treating late-stage cardiovascular disease.[48][49]

Inflammation

Inflammation is a critical element of the body's immune response that involves immune cells, cell-signaling proteins, and pro-inflammatory factors. Acute inflammation occurs after minor injuries or infections and is characterized by local redness, swelling, or fever. Chronic inflammation occurs on the cellular level in response to toxins or other stressors and is often “invisible.” It plays a key role in the development of many chronic diseases, including cancer, cardiovascular disease, and diabetes.

C-reactive protein, or CRP, one of several blood proteins often referred to as acute phase reactants, participates in the body's inflammatory cascade. Elevated CRP is associated with the development of atherosclerosis, loss of arterial compliance, and incidence of cardiovascular events.[50] Sauna use, however, reduces blood levels of CRP. In a study of more than 2,000 men living in Finland, CRP levels were inversely related to the frequency of sauna bathing in a dose-response fashion, with lower levels linked to greater frequency.[51]

As described above, IL-10 is a potent endogenous anti-inflammatory protein. In a study involving 22 healthy male athletes and non-athletes who received two 15-minute sauna sessions at 98.2°C (208°F) separated by a 5-minute cool shower, the men's resting IL-10 levels increased, and this adaptation occurred faster in the athletes. A slight increase in some of the HSPs was also observed.[52]

Cognitive & mental health

Enhanced neurogenesis

Heat stress and exercise increase the expression of brain-derived neurotrophic factor, or BDNF,[53] a protein that acts on neurons in the central and peripheral nervous systems, to promote the growth of new neurons. BDNF modulates neuronal plasticity and ameliorates anxiety and depression from early-life stressful events.[54] It is active in the hippocampus, cortex, cerebellum, and basal forebrain – areas involved in learning, long term memory, and executive function. BDNF is also produced in exercising muscle tissue, where it plays a role in muscle repair and the growth of new muscle cells.[55]

Cognitive decline

Normal cognitive function relies on sufficient blood flow to the brain and peripheral nervous system, so cardiovascular diseases and cognitive decline often go hand-in-hand. For example, hypertension alters the structure of cerebral blood vessels and impairs blood flow to the brain. Poor cerebral blood flow is commonly observed in mice and humans and may contribute to impaired amyloid-beta clearance, thereby accelerating the progression of Alzheimer's disease.[56]

In addition, heat exposure increases the production of BDNF to promote neurogenesis – the growth of new neurons in the brain (as described above). Findings from a large observational study of middle-aged men living in Finland demonstrated that men who used the sauna four to seven times per week had a 65 percent reduced risk of developing Alzheimer's disease, compared to men who used the sauna only one time per week.[2]

Depression

Elevated biomarkers of inflammation are commonly observed in people who have depression. Chronic activation of the body's inflammatory response system promotes the development of depressive symptoms and induces changes in brain and neuroendocrine function. Sauna use has been shown to reduce symptoms of depression.

In a randomized controlled trial involving 28 people diagnosed with mild depression, participants who received four weeks of sauna sessions experienced reduced symptoms of depression – such as improved appetite and reduced body aches and anxiety – compared to the control group, which received bedrest instead of sauna therapy.[57]

In a randomized, double-blind study of 30 healthy adults diagnosed with depression, participants who were exposed to a single session of whole-body hyperthermia in which core body temperature was elevated to 38.5°C (101.3°F) experienced an acute antidepressant effect that was apparent within a week of treatment and persisted for six weeks after treatment.[58]

Similarly, in a double-blind randomized controlled trial in which core body temperature was elevated by 1.5°C (2.7°F), IL-6 levels in participants increased markedly, and participants with the highest IL-6 levels had the lowest levels of depression one week later (see discussion with Dr. Charles Raison at 44:18). Some of these benefits may be due to the effects of heat stress on circulating levels of pro-inflammatory IL-6 and the reciprocal effects of IL-10 (as described above).

Beta-endorphins and the opioid system. Another factor that may play a role in mental or cognitive effects from sauna use may depend on the opioid system. Beta-endorphins are endogenous opioids that are part of the body's natural painkilling system. They are produced and stored primarily in the anterior pituitary gland of the brain and play important roles in pain management and reward circuitry. Evidence suggests that beta-endorphins are responsible in part for the "feel-good" response to exercise.[59] The binding of beta-endorphins to mu-opioid receptors on nerve cells suppresses the release of pain-promoting substances in the brain. Sauna use promotes robust increases in beta-endorphins.[60] [61] [62]

The body also produces an opioid called dynorphin, which is generally responsible for dysphoria, a profound sense of unease or dissatisfaction. Dynorphin also mediates the body's response to heat, helping the body to cool.[63] Dynorphin is produced in many different parts of the brain, including the hypothalamus, the striatum, the hippocampus, and the spinal cord. The binding of dynorphin to kappa-opioid receptors triggers cellular events that promote pain and distress.

Sauna use (or intense exercise) promotes dynorphin release, which may be responsible for the general sense of discomfort experienced during heat exposure. Interestingly, in a biological feedback response that occurs after dynorphin binds to the kappa-opioid receptor, the brain produces more mu-opioid receptors, sensitizing them to endorphin and future endorphin exposure.[64]

Mental focus and attention span

Two key players in cognitive and mental function are norepinephrine, a hormone and neurotransmitter produced in the brain, and prolactin, a hormone released by the pituitary gland. Norepinephrine enhances focus and attention, while prolactin promotes myelin growth, which makes the brain function faster, a critical feature in repairing nerve cell damage.[65][66]

When young men stayed in a sauna that was heated to 80°C (176°F) until subjective exhaustion, their norepinephrine levels increased by 310 percent and their prolactin levels increased by 900 percent. Levels of cortisol, a hormone commonly associated with the stress response, were slightly decreased.[5][67] Similarly, in a study involving women who participated in 20-minute sessions in a dry sauna twice a week experienced a 86 percent increase in norepinephrine and a 510 percent increase in prolactin after the session.[68]

These findings suggest that since the norepinephrine response to exercise is blunted in children with attention deficit hyperactivity disorder, or ADHD, and norepinephrine reuptake inhibitors are frequently prescribed to treat ADHD, use of heat stress and its acclimation may one day be a useful alternative therapeutic approach employed by clinicians within some yet-to-be-established age-appropriate protocol.[69]

Hormonal and metabolic function

Growth hormone

Sauna use promotes growth hormone release, which varies according to time, temperature, and frequency. For example, two 20-minute sauna sessions at 80°C (176°F) separated by a 30-minute cooling period elevated growth hormone levels two-fold over baseline, but two 15-minute sauna sessions at 100°C (212°F) dry heat separated by a 30-minute cooling period resulted in a five-fold increase in growth hormone.[5][34]

Interestingly, repeated exposure to whole-body heat treatment through sauna use has an even more profound effect on boosting growth hormone immediately afterward: Seventeen men and women who were exposed to two one-hour sauna sessions at 80°C (176°F) dry heat (typical Finnish-style sauna) per day for seven days exhibited a 16-fold increase in growth hormone levels by the third day.[70] The growth hormone effects generally persisted for a few hours post-sauna.[5] It is noteworthy, however, that sauna use and exercise can synergize to significantly elevate growth hormone when used together.[71]

Insulin and glucose

Insulin regulates glucose homeostasis, primarily by promoting the uptake of glucose into muscle and adipose tissue. Repeated treatment with a far-infrared sauna has been shown to significantly lower fasting blood glucose levels.[72] When insulin resistant diabetic mice were subjected to 30 minutes of heat treatment three times a week for 12 weeks, they experienced a 31 percent decrease in plasma insulin levels and a significant reduction in blood glucose levels, suggesting re-sensitization to insulin.[73] The heat treatment specifically targeted the skeletal muscle by increasing the expression of a type of transporter known as GLUT 4, which is responsible for transporting glucose into skeletal muscle from the bloodstream.[73] Decreased glucose uptake by skeletal muscle is one of the mechanisms that leads to insulin resistance.

Physical fitness and athletic performance

Heat stress from using the sauna may modulate improvements in physical fitness and athletic performance by increasing endurance and maintaining or promoting gains in muscle mass.

Increased endurance

One small intervention study looked at the effects of repeat sauna use on athletic performance and other physiological effects in six male distance runners. The findings showed that one 30-minute sauna session twice a week for three weeks post-workout increased the time that it took for the study participants to run until exhaustion by 32 percent compared to their baseline.[74] These performance improvements were accompanied by a 7.1 percent increase in plasma volume and a 3.5 percent increase in red blood cells (RBCs).[74] During exercise, RBCs transport oxygen from the lungs to the body's tissues (especially working muscles) and deliver carbon dioxide to the lungs for expiration. Increases in RBC levels may facilitate these processes and improve performance.

Heat acclimation

During exercise, core body temperature increases, attenuating endurance and accelerating exhaustion. Heat acclimatization and acclimation, however, induce complex physiological adaptations that improve thermoregulation, attenuate physiological strain, and enhance athletic performance in hot environments. These adaptations are mediated via improved cardiovascular and thermoregulatory mechanisms that reduce the deleterious effects associated with elevated core body temperature, optimizing the body for subsequent increases in core body temperature during future exercise.

Whereas "acclimatization" refers to the body's response to heat exposure in natural environments, "acclimation" refers to the response in controlled environments, such as a sauna or heat stress chamber. The adaptations are the same in either scenario.[75]

In a small study involving nine female athletes who sat for 20 minutes a day for five days in a hot environment (50°C [122°F], in low humidity) wearing a sauna suit to replicate sauna conditions, the women experienced improvements in thermoregulatory, cardiovascular, and perceived strain compared to a control group.[76]

As described above, heat acclimation increases plasma volume and blood flow to the heart (stroke volume).[77][78] This results in reduced cardiovascular strain and lowered heart rate for the same given workload.[77] These cardiovascular improvements have been shown to enhance endurance in both highly trained and untrained athletes.[77][78][79]

Heat acclimation also increases blood flow to the skeletal muscles, fueling them with glucose, fatty acids, and oxygen, reducing their dependence on glycogen stores. Endurance athletes often “hit a wall” due to depletion of their liver and muscle glycogen stores. Heat acclimation has been shown to reduce muscle glycogen use by 40 to 50 percent compared to before heat acclimation, presumably due to increased blood flow to the muscles.[80][81][80]

Improvements in thermoregulatory function are also observed following heat acclimation. Heat exposure activates the sympathetic nervous system, increasing peripheral blood flow and the sweat rate to dissipate core body heat. After acclimation, sweating occurs at a lower core temperature and the sweat rate is maintained for a longer period.[77]

Muscle mass maintenance

Muscular atrophy, the shrinking or wasting away of muscle tissue, commonly occurs with muscle immobilization or disuse following sports injuries. Atrophy causes substantial strength losses, especially during the first week of immobilization or disuse, due to reduced protein synthesis and increased protein degradation.[82]

Maintaining muscle mass requires a balance of new protein synthesis and existing protein degradation. While new protein synthesis accompanies muscle use during exercise, protein degradation can occur during both muscle use and disuse. Of critical importance, therefore, is net protein synthesis.

Heat acclimation, which can be achieved through sauna use, may reduce the amount of protein degradation that occurs during disuse by increasing HSPs, reducing oxidative damage, promoting release of growth hormone, and improving insulin sensitivity.[83][84][85][73] Maintaining positive net synthesis has special relevance for recovery from injury since injury can tip the balance towards protein degradation and away from protein synthesis in the muscles, which can promote muscle atrophy.

A small intervention study in humans found that daily heat treatments applied locally to muscle during 10 days of immobilization prevented the loss of mitochondrial function, increased HSP levels, and attenuated skeletal muscle atrophy by 37 percent compared to a sham control group.[86] These results have been replicated in animal studies. For example, when rats received whole body hyperthermia at 41°C (105.8°F) for 30 minutes or 60 minutes, hindlimb muscle atrophy during disuse decreased by 20 percent or 32 percent, respectively.[84][87]

In another rodent study of the effects of heat stress, a 30-minute intermittent hyperthermic treatment at 41°C (105.8°F) for seven days induced a robust expression of HSPs (including HSP32, HSP25, and HSP72) in muscle, correlating with 30 percent more muscle regrowth than a control group subsequent to a week of immobilization.[83] This HSP induction can persist for up to 48 hours after heat shock.[83][84] Heat acclimation causes a higher basal expression of HSPs (even when not exercising) and a more robust induction upon elevation in core body temperature (such as during exercise).[18][88][89]

Heat shock proteins, described above, can prevent muscle protein damage by directly scavenging reactive oxygen species and by supporting cellular antioxidant capacity through their effects on maintaining the endogenous antioxidant glutathione.[83][84] In addition, HSPs can repair misfolded, damaged proteins, thereby ensuring proteins maintain their proper structure and function.[83][84]

Detoxification

Sweating facilitates the excretion of certain toxicants that bioaccumulate in the muscle, adipose tissues, and organs of humans. As described above, sauna use induces substantial sweat losses, with the average person losing approximately 0.5 kg of sweat while sauna bathing.[11]

Heavy metals

Heavy metals are naturally-occurring metallic elements that are toxic or poisonous at low concentrations. Exposures to heavy metals in everyday products such as arsenic in agricultural products, cadmium in cigarette smoke and automobile exhaust, lead in toys, and mercury in dental amalgam and certain types of fish, are common occurrences. Some heavy metals are excreted in sweat, and sauna use may facilitate their excretion. In a study in which the presence of various compounds (including heavy metals) in the blood, urine, and sweat of 20 adults was measured, markedly higher excretion of aluminum (3.75-fold), cadmium (25-fold), cobalt (7-fold), and lead (17-fold) was observed in sweat versus urine.[90]

Learn more about heavy metal excretion through sauna use in this episode featuring Dr. Dale Bredesen

Bisphenol A

Bisphenol A, or BPA, is a chemical used during the production of polycarbonate plastics and epoxy resins. It is ubiquitous in the environment and can be found in plastic beverage bottles and food packaging, metal food and beverage can linings, dental composites and resins, and other products. BPA is an endocrine disruptor that can mimic naturally occurring hormones in the body like estrogens, androgens, and thyroid hormones, potentially altering normal hormonal signals. High levels of urinary BPA are associated with increased risk for cardiovascular disease and diabetes.[91]

Bisphenol A bioaccumulates primarily in adipose tissue in humans, but some evidence indicates that it is excreted via sweat and, to a lesser degree, urine.[92] Sauna bathing may serve as means to facilitate BPA excretion via the skin to eliminate this toxicant from the human body.

Polychlorinated biphenyls

Polychlorinated biphenyls, or PCBs, are organochlorine compounds historically used in industrial and chemical applications such as coolants, transformer insulators, capacitors, motors, paints, and electrical wire coatings. Although PCBs were banned in the 1970s, they still persist in the environment, and many freshwater and farmed fish are contaminated with PCBs.[93] Adverse human health effects associated with PCB exposure are related primarily to endocrine disruption, particularly in a developing fetus, as well as increased risk for developing hypertension, cutaneous malignant melanoma, and non-Hodgkin's lymphoma. PCBs bioaccumulate in human muscle and adipose tissue, brain, liver, and lungs and have long elimination half-lives, ranging from 10 to 15 years.[94] Some, but not all, PCBs are excreted in sweat.[95]

Phthalate compounds

Phthalates are synthetic compounds that are used to create plastic products that are soft and malleable. They are used in other consumer goods including fragrances, paints, nail polish, and food and beverage packaging, and are a ubiquitous component of soft plastic toys as well as various other products, including vinyl floor tiles, shower curtains, synthetic leather, cosmetics, shopping bags, and pharmaceuticals.[96]

Urine samples analyzed from populations worldwide have found phthalates in up to 98 percent of participants, including pregnant women. Phthalates have a relatively short half-life of less than five hours, which means the widespread detection likely indicates chronic exposure rather than accumulation within the body.[97] Phthalates have been shown to lower testosterone levels and block the effects of testosterone on organs and tissues. Phthalates can pass from mother to fetus through the placenta and may result in abnormal sexual development. Some phthalates but not all are readily excreted through sweat.[90]

Sauna concerns and best practices

Male fertility

Heat exposure has notable, but reversible, effects on male sperm and fertility measures. In a study involving 10 healthy men who underwent two 15-minute sauna sessions at 80°C to 90°C (176°F to 194°F) every week for three months, the men experienced reduced sperm counts and motility. These measures returned to normal, however, within six months of ceasing sauna use.[98]

Special populations

Pregnant women

Some central nervous system birth defects, such as anencephaly and spina bifida, are linked with exposure to extreme heat during pregnancy. However, in Finland, where the majority of women practice sauna bathing at least once a week throughout their pregnancies, the incidence of anencephaly is the lowest in the world.[99] Similarly, observational studies conducted in Finland and the United States showed no links between sauna use and higher incidence of cardiovascular malformations, the most common form of birth defects.[6]

In an experimental study in which pregnant women were exposed to sauna-like temperatures, the women's skin and rectal temperatures increased during the exposure, but all the women gave birth to healthy babies.[6] Pregnant women with toxemia, however, exhibit increased resistance to blood flow in the uterine artery, which may compromise fetal health.[6] As such, they should avoid sauna use during pregnancy.

Sauna use during pregnancy may carry some health risks to a developing fetus.

Children

Children have less efficient thermoregulatory mechanisms than adults due to differences in their anatomy and physiology. In particular, they have lower sweat rates than adults, which can compromise their ability to dissipate body heat through evaporation.[100] Children with sinoatrial node disorders may be at greater risk of fainting during the cool-down phase after sauna bathing due to the sudden drop in blood pressure that can occur post-sauna use.[101] However, sauna bathing appears to be safe for healthy children over the age of 2 years under adult supervision.[102]

People who are ill or taking medications

People with acute illness accompanied by fever, or inflammatory skin conditions should avoid sauna use.[6] People taking any kind of medication, whether prescribed or over-the-counter, should consult a physician before sauna use.[6] Several studies have shown that people with certain types of cardiovascular disease may experience improvements in their symptoms and disease status with sauna use.[103] However, anyone with a diagnosed cardiovascular condition should consult a physician before using the sauna.

Hydration and electrolytes

Proper hydration and electrolyte balance are critical to maintain the body's fluid balance and to promote normal muscle contractility and nerve function. As described above, the average person loses approximately 0.5 kg of fluid as sweat during a single sauna session. Sweat rates vary between individuals and even between sessions, however, and some people may lose considerably more. Accompanying the fluid losses are losses of electrolytes, especially sodium, chloride, potassium, magnesium, and calcium.[104] Skeletal muscle cramps and fatigue are associated with dehydration and electrolyte deficits.

Sauna users should take care to drink sufficient fluids prior to and after sauna sessions and should consume electrolyte-rich foods post-sauna use, such as cooked spinach, avocado, tomatoes, fish, nuts, and seeds.[105] People who limit their caloric intake, eliminate one or more food groups from their diet, adhere to severe weight-loss practices, or eat unbalanced diets that are low in micronutrients may require supplements.[105]

Alcohol consumption before or during sauna use can cause severe dehydration, hypotension, arrhythmia, and possibly embolic stroke and should be avoided.[5]

Conclusion

Sauna bathing is associated with many health benefits, from cardiovascular and mental health to fertility and athletic endurance. It is generally considered safe for healthy adults and may be safe for special populations with appropriate medical supervision. Heat stress via sauna use elicits hormetic responses driven by molecular mechanisms that protect the body from damage, similar to those elicited by moderate- to vigorous-intensity exercise, and may offer a means to forestall the effects of aging.

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