Harnessing Heat Therapy for Better Health: A review
Heat therapy has been an integral part of many cultures since the dawn of human civilization. It seems reasonable to conclude that given its wide acceptance and staying power amongst various peoples, it must be doing something beneficial for our bodies. A growing body of research is now substantiating this ancient intuition with new modalities for heat therapy—such as infrared saunas—and places to purposefully sweat are popping up everywhere from gyms to spas.
The majority of the research on heat therapy—specifically whole body thermotherapy— comes from sauna bathing, specifically the use of Finnish dry saunas. These wooden saunas contain Conventional heaters that 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). Humidity in dry saunas is typically low at low (10–20%). Finnish dry sauna typically involves short exposures (5−20 minutes) at a time with 10- to 30-min rest periods outside the sauna in between. Other (but less studied) forms of direct contact thermotherapy include wet saunas (or steam rooms) which have high humidity, as well as hot baths.
Some of the first studies of dry heat therapy for human clinical populations were done by a group of cardiologists in Japan conducted some of the first studies of dry heat therapy in humans using Waon therapy, which consists of 15 min in a 60°C far infrared sauna followed by 30 min of recovery. Waon therapy reportedly improved the hemodynamics, cardiac function, ventricular arrhythmias, vascular endothelial function, neurohormonal factors, sympathetic nervous system function, and symptoms in patients with chronic heart failure (CHF). These early investigations prompted widespread investigation into the therapeutic uses of heat therapy for both the prevention and treatment of disease.
Infrared saunas are a modern technology that utilize light at a particular wavelength to make heat. The infrared waves generated by these saunas are able to penetrate the skin, effectively heating the body from the inside out. Infrared heat penetrates more deeply than warmed air, allowing users to reach an elevated core body temperature at a lower ambient temperature than in traditional saunas. There are three levels of infrared light - near, mid, and far. Near-infrared light measures 700-5,000 nanometers (nm), mid-infrared is between 5,000- 40,000nm, and far-infrared is 25,000-350,000nm.
Acute Physiological Response to a Heat Stimulus
The goal of passive heat therapy is to increase the body's core temperature. The rate at which body core temperature increases is dependent on the rate of transfer of heat from the environment to the body and the effectiveness of the body’s heat loss mechanisms, both of which vary across different modalities.
The acute cardiovascular effects of passive heat stress have been well characterized. Heat stress increases core body temperature, promotes blood vessel dilation (vasodilation) and blood redistribution to the skin and extremities to facilitate cooling. Increased core temperature also prompts increased sweat production. Active vasodilation and sweating are initiated once core body temperature reaches a temperature threshold, usually ∼0.4°C above resting core temperature. Sweat evaporates from the skin surface and produces cooling that facilitates temperature homeostasis.
Cardiac output (the amount of blood pumped from the heart per minute) is increased and driven primarily by an elevation in heart rate. The elevation of cardiac output primarily supports the redistribution of blood to the skin surface, compensating for fluid loss with sweating. The magnitude of increase correlates with the rise in core body temperature. The increase in heart rate during heat therapy can be substantial, to levels comparable to moderate-intensity exercise. While blood pressure may stay fixed or even increase during sauna use due to the increase in cardiac output, it tends to be lower for up to 60 min following passive heat stress, which is comparable to the duration of lowered blood pressure observed following aerobic exercise. Heat stress also stimulates the body to produce more plasma which contributes to increased cardiac output.
A higher heart rate variability (HRV), a marker of autonomic nervous system balance, is indicative of a greater capacity of the cardiovascular system to respond to changes in stressful conditions. Higher HRV typically indicates improved cardiovascular health overall. During the actual experience of the stress of heat heart rate and sympathetic tone increase. However, during the cool down period, HRV increases as parasympathetic tone is enhanced and sympathetic output dampens.
In addition to measures directly related to heart function, heat therapy has also been implicated in positive vascular changes. In a comprehensive review of the molecular mechanisms underlying the benefits of heat therapy, Brunt and Minson summarize how whole-body thermotherapy (both wet and dry forms) induce discrete cellular and biochemical changes that beneficially change shear stress within blood vessels, mitigate protein damage and aggregation, activate endogenous antioxidant and anti-inflammatory pathways, increased NO (nitric oxide) bioavailability, and increase insulin sensitivity. Indeed, in the original Waon therapy studies, it was noted that heat specifically induced endothelial nitric oxide synthase (eNOS), a powerful vasodilator, and that eNOS was a critical regulator of the angiogenesis induced by this therapy.
Anterograde shear stress, the mechanical frictional force exerted by the blood on the arterial walls as it flows away from the heart, improves endothelial function and is antiatherogenic. And passive heat therapy has been shown to increase anterograde and reduce retrograde shear stress. Studies in humans show that beneficial adaptations in the vasculature are mediated by increased anterograde shear stress, which seem to be due to the combined effect of increased blood flow and increased temperature.
Acute heat stress also activates and upregulates heat shock proteins (HSPs), which protect cells from the damaging effects of heat and other stressors and facilitate normal cell function. Under stressful environmental conditions, cellular proteins can unfold or become damaged, impairing their normal functions. HSPs process stress-denatured proteins and prevent disruption of structural proteins. The magnitude of expression and activation of heat shock proteins is directly correlated with the amount of heat exposure—a factor of both duration and temperature. It appears that HSP expression occurs relatively immediately following heat exposure and persists above baseline levels on the order of hours to days depending on the tissue type. In addition to (and likely mediated directly by) HSPs, heat stress increases levels of antioxidative enzymes, increases nitric oxide production, reduces proinflammatory molecules, and confers protective mechanisms against cell death. This has direct implications on diseases outside of CVD as the activation of HSPs, specifically HSP72, has been elegantly demonstrated to protect against diet or obesity induced insulin resistance.
Heat exposure can also change hormone levels. Preliminary data demonstrate increases in growth hormone, testosterone, prolactin and decreases in cortisol during sauna bathing. However, the time course and amplitude of these neuroendocrine changes remains unclear and seems to vary according to duration of exposure, temperature, comfort with or prior adaptation to exposure, and sex of the study subject. In general, the hormone responses are short-lived, normalizing soon after sauna exposure during the recovery and long term adaptations are yet to be elucidated.
In the brain, heat stress promotes a robust increase in beta-endorphins which are endogenous neuropeptides that bind to mu opioid receptors to suppress pain and modulate our reward circuitry. While endorphins are responsible for feelings of ease and happiness, dynorphin—endogenous neuropeptides that activate the kappa opioid receptor—signal sensations of discomfort such as those experienced in intense heat. Interestingly, repeated heat therapy may sensitize mu-opioid receptors to endorphins via dynorphin upregulation, thereby increasing the feelings of euphoria and calm after heat exposure. Heat stress also induces the expression of brain-derived neurotrophic factor (BDNF), a protein that acts on neurons in the central and peripheral nervous systems, to promote the growth of new neurons and regulate functions that are critical to cognition and memory.
Chronic Physiologic Adaptation of Repeat Heat Stimulus
The immediate molecular and cellular responses to heat therapy strongly imply that regular heat exposure, similar to regular exercise, triggers a cascade of stress-response-and-adaptation functions that lead to what is known as hormetic adaptation and an increased level of “heat fitness” needed to support this intermittent stressor. Hormesis is generally defined as a process in which exposure to a low dose of a chemical agent or environmental factor that is damaging at higher doses induces an adaptive beneficial effect on the cell or organism. As basal HSP concentrations are higher in heat-acclimated individuals, it can be suggested that heat acclimation results in protective cellular and system-wide adaptations.
Cardiovascular adaptations include an increase in total body water, plasma volume expansion, better sustainment and/or elevation of stroke volume, reduction in heart rate, improvement in ventricular filling and myocardial efficiency, and enhanced skin blood flow and sweating responses. A comprehensive review article by Brunt and Minson summarizes how heat stress can lead to short-term, long-term, and lifelong cardiovascular adaptations.
Health Impact of Heat Therapy
Data on deliberate heat therapy demonstrate robust and widespread beneficial effects on cardiovascular health, in particular.
Heat therapy provides similar benefits as cardiovascular exercise including increasing left ventricular ejection fraction and reducing left ventricular ejection time, enhancing arterial compliance, and improving vascular endothelial function. Based on a comprehensive review of the data, regular heat therapy facilitates long-term reductions in resting blood pressure. Autonomic function may also be improved, as measured by reduced low-frequency (sympathetic) and increased high-frequency (parasympathetic) components of heart rate variability
The first studies to investigate CV benefits of passive heat therapy in humans were conducted by a group of cardiologists in Japan using infrared, Waon therapy in patients with heart disease. In Japanese, Waon means “soothing warmth or comfortable warmth that refreshes the patients’ mind and body.” In Waon therapy, the body is warmed for 15 minutes at 60 degrees Celsius. After the core body temperature has increased 1-1.2 degrees Celsius, the warmth is retained by covering the patient for an additional 30 minutes. Finally, the patient drinks cold water corresponding to the amount of perspiration. Numerous studies using Waon therapy demonstrate improved biomarkers and functional status in patients with CHF, ischemic heart disease, and peripheral artery disease. Cardiac event rate, ventricular arrythmias, heart failure symptoms and quality of life, and exercise tolerance can all be improved with Waon therapy. Based on this growing evidence base, Waon therapy is indicated as a Class I treatment for patients with chronic heart failure in Japanese medical guidelines.
The largest and most solid evidence base for the health impacts of deliberate heat exposure comes from the Kuopio Ischemic Heart Disease (KIHD) Risk Factor Study, a population-based longitudinal follow-up study designed to investigate risk factors of cardiovascular diseases in a population in Eastern Finland with one of the highest recorded rates of coronary heart disease. Beginning in 1984, the KIHD involved nearly 3,000 middle-aged (42–60 years) men from the Kuopio region. Ten years later, women of same age (N > 1,000) were recruited to the study. A major part of the original cohorts have been reexamined 4, 11, and 20 years after the baseline, including frequent medical examinations and questionnaires on their sauna bathing habits. In the initial cohort of men, both the frequency (sessions per week) and duration per session of sauna bathing predicted incidence of sudden cardiac death, fatal cardiovascular disease and all-cause mortality after adjusting for confounding factors such as blood pressure, resting heart rate, smoking status, Type 2 diabetes, previous myocardial infarction, LDL levels, and alcohol consumption. Their findings included a 63% risk reduction [HR 0.37 (0.18–0.75), p = 0.005] of sudden cardiac death, and a 40% risk reduction [HR 0.60 (0.46–0.80), p < 0.001] of all-cause mortality. Notably, the cardiovascular benefit appears to be dose dependent. The risk of CVD-related mortality was 48–50% lower in men who used sauna four to seven times per week compared with those who used it only once per week.
In this same KIHD cohort, the researchers also found a 66% risk reduction [HR 0.34 (0.16–0.71), p = 0.004] of dementia a 65% risk reduction [HR 0.35 (0.14–0.90), p = 0.03] of Alzheimer's disease in a similarly dose dependent fashion. This may be in part explained by the fact that heat shock proteins, which increase following sauna use, demonstrate critical roles in preventing Alzheimer's disease. In addition, 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. And there is some initial evidence that sauna can reduce symptoms of depression. In a randomized controlled trial involving 28 individuals diagnosed with mild depression, participants who received 4 weeks of sauna sessions experienced reduced symptoms of depression, such as improved appetite and reduced somatic complaints and anxiety, compared to the control group, which received bedrest instead of sauna therapy. More recently, a team of researchers experimented with an infrared sauna whole-body hyperthermia protocol in twenty five adults with depression and reported reductions in self-reported depressive symptoms after only one session.
Back in 1999, Philip Hooper published his results in the New England Journal of Medicine after taking five of his patients with type 2 diabetes and demonstrating that hot tub therapy improved their glycemic control as indicated by lower fasting glucoses and hemoglobin A1c. Since then, the majority of research investigating the metabolic effects of deliberate heat therapy has involved animal models. Heat treatment has been shown to improve glucose tolerance, restore insulin-stimulated glucose transport, and increase insulin signaling via upregulation of heat shock proteins in the muscles of rats fed a high-fat diet. Similar results were seen in select white adipose tissue in rats, with improved insulin signaling following a single bout of heat treatment. In women with polycystic ovarian syndrome, which is tightly linked with metabolic dysfunction and insulin resistance, 30 one-hour hot tub sessions over 8-10 weeks improved fasting glucose levels and insulin signaling compared to controls. Overall, however, the data to date on the impact of heat therapy on metabolic diseases remains limited.
Heat has been a longstanding and welcome therapy for musculoskeletal pain due to its reassuring, and hence analgesic effects. Heat helps take the edge off for various types of pain including the acute soreness from over-exertion, stiffness and pain in specific areas related to osteoarthritis, muscle “knots”, and muscle cramps and spasms. With regard to inflammatory musculoskeletal conditions, a study of 34 patients diagnosed with either rheumatoid arthritis (RA) or ankylosing spondylitis (AS) reported decreased pain and stiffness in the RA (p < 0.05) and AS (p < 0.001) groups during 4 weeks of sauna therapy that was not sustained after the 4 weeks. A single-group study of 44 patients diagnosed with fibromyalgia with or without another rheumatological disorder (i.e., systemic lupus erythematosus, systemic sclerosis, rheumatoid arthritis, Sjogren's syndrome, Behcet's disease, or aortitis syndrome) reported subjective improvements in pain, symptoms improved quality of life and tenderness on exam after 12 weeks of combined far-infrared sauna and underwater exercise therapy.
There are limited studies that indicate the benefit of heat therapy on several other clinical outcomes. Although clinical trials are generally lacking, there is suggestive evidence of the effectiveness of heat therapy for primary dysmenorrhea, the most common gynecologic condition in women which entails menstrual pain without pelvic pathology. And there is some preliminary data for the effectiveness of heat therapy in chronic pain, chronic fatigue syndrome, COPD and respiratory illness, including pneumonia. Lastly, heat therapy may have implications for healthy aging overall by modulating HSPs. By repairing proteins that have been damaged, HSPs prevent protein misfolding and aggregation, which is a hallmark of the aging process and many chronic conditions—neurodegenerative disorders in particular. Although an association with heat therapy specifically has not been elucidated, HSPs have been associated with human longevity in longitudinal cohort studies.
Finally, with the increasing attention to toxic chemical exposures and evidence that persistent toxicants are retained within the human body long after the primary exposure, there is interest in sweating as a mechanism for clinical detoxification. Sweating with heat and/or exercise has been viewed throughout the ages, by groups worldwide, as “cleansing.” To date, evidence on sweating as a mechanism for toxic element detoxification is limited, but studies do suggest that this may be a way to excrete physiologically unnecessary, and even harmful chemicals.
Performance Effects of Heat Therapy
Heat therapy has been used for centuries by many cultures, with widespread reports of improved overall well-being and quality of life. Its staying power is anecdotal testament to its perceived effectiveness as a performance enhancing modality.
With regard to physical performance, heat therapy has been called an “exercise mimetic” due to its ability to enhance cardiorespiratory fitness and endurance and preserve muscle mass. For patients with chronic heart failure, 3 weeks of Waon therapy improves VO2 max, and exercise tolerance, (measured using both 6-min walk distance), time-to-fatigue on a modified Bruce test, and overall quality of life. In runners, post-training sauna sessions increase run time to exhaustion and plasma and red-cell volumes and induce heat acclimatization to aid performance. By aiding in thermoregulation, regular heat therapy reduces cardiovascular strain and lowered heart rate for the same given workload, which translates into improved cardiovascular stability and exercise performance.
Heat therapy may also have a direct effect on muscle physiology. A recent study in humans shows that heat stress can provide protection against atrophy in skeletal muscle, likely via increased heat shock protein expression and improved mitochondrial function. This could be an important means of maintaining lean muscle mass if the setting of injury or other physical limitations, or a potent complement to an existing training routine. Heating also augments the benefits in range of motion achieved through stretching.
Maintaining the appropriate balance of pro- and anti-inflammatory factors is crucial for the development and subsequent resolution of an inflammatory response. The pathways that maintain this balance often become dysregulated and contribute to an inflammatory bias that elicits a state of chronic inflammation, which is increasingly recognized as a root cause for a range of chronic diseases. Like exercise, passive elevation of body temperature can induce acute increases and chronic reductions in inflammatory markers and improve glycemic control. There is correlative evidence that increased sauna use is associated with reduced circulating inflammatory markers, namely CRP, in humans.
To date, the data on heat therapy use as a recovery intervention from physical training (both endurance and strength exercise) is too limited to draw any meaningful conclusions or evidence-based protocols. However, there are limited data to suggest possible associations.
Heat therapy may directly impact both performance and recovery by improvements in sleep. Indeed, some research suggests that it may not just be the physical exhaustion induced by exercise that promotes deeper, slow wave sleep, but actually the concomitant increase in body temperature. Research on deliberate heat exposure alone, such as with sauna bathing, and the impact on sleep is largely nonexistent to date. Yet, mechanistically it makes sense that purposefully elevating one’s core body temperature would promote more restfulness and relaxation. Body temperatures rise rapidly for about 30 minutes of sauna time then the body expends energy to cool down, and this subsequent lowering of core body temperature is sleep promoting.
Safety, Efficacy, and Effectiveness Considerations of Heat Therapy
To date, the most robust evidence for heat therapy derives from the modality of a dry (aka “Finnish”) sauna. Frequent Finnish sauna bathing improves a variety of subjective and objective health parameters and is associated with improved outcomes such as reduced overall mortality and reduced incidence of cardiovascular events and dementia, at least in men. At this time, there is not yet enough evidence to distinguish any particular health differences between Finnish-style and infrared sauna bathing.
The strongest data to date on the health benefits of heat therapy pertain to cardiovascular health. Despite differences in sauna types, temperature, frequency, and duration of interventions, the far-infrared sauna (“Waon therapy”) studies involving cardiovascular disease and congestive heart failure patients suggest favorable outcomes that reinforce earlier findings of interventional Finnish sauna studies and cardiovascular disease. The mechanisms for these effects may include increased bioavailability of nitric oxide to vascular endothelium, heat shock protein-mediated metabolic activation, immune and hormonal pathway alterations, enhanced excretions of toxicants through increased sweating, and other hormetic stress responses. At this time, much of the work linking these biochemical/molecular pathways to improved CV function is correlative, not causal.
As is also true for cold therapy, we are lacking a robust number of long-term and interventional studies utilizing the various modalities of heat therapy. Most direct intervention studies (as opposed to correlative cohort studies like KIHD), are highly variable with small sample sizes, poorly described methodology, absent control groups, differing types of sauna and sauna protocols, variable duration and frequency of sauna interventions, and inconsistent mention of cooling therapies or rehydration protocols along with divergent outcome measures. This heterogeneity across studies makes meaningful comparisons across studies difficult and provides insufficient evidence to recommend specific temperature, frequency, or duration of sauna bathing for any specific health outcome.
Despite the variable quality of evidence to date on the effect of deliberate heat exposure as a regular lifestyle intervention for health, the practice itself is largely safe for the majority of participants, particularly older adults, patients with untreated hypertension, and even though stable and relatively asymptomatic heart failure and coronary artery disease.
That said, there are some clear contraindications for heat therapy. Primarily, these include unstable conditions for which exercise would also be contraindicated—severe aortic stenosis, unstable angina pectoris, recent myocardial infarction, recent stroke or transient ischemic attack, and elderly individuals prone to orthostatic hypotension. For those of reproductive age, there have been small studies describing notable, but reversible, effects on male sperm count and motility measures for men who are sauna naive. For women who are pregnant, the suggested teratogenic threshold for core body temperature in pregnant women is 39.0 °C (102.2 °F). Some studies have shown an increased risk of birth defects in babies of women who had an increased body temperature during the first trimester of pregnancy. For this reason, the American College of Gynecology discourages deliberate heat exposure for long periods of time, especially during the first trimester of pregnancy. And heat therapy should be avoided for those who have a high risk pregnancy.
One final caution is to rehydrate efficiently after heat therapy. Dehydration of 2% of body weight or more can have significant negative effects on both mental and physical performance. The best way to rehydrate after sweating in the heat is to weigh yourself before and after and replenish at least 100% that amount, ideally 125%. If weighing is not an option, a good estimate is for every 15 minutes of exposure, drink (body weight in lbs)/30 in oz of water. So, for a 120lb person that is exposed for 30 minutes, (120/30)x2 = 8oz of water replenishment (i.e. approximately ½ a standard athletic water bottle).
The Bottom Line: A practical application of Heat Therapy
To date, the scientific literature on the beneficial effects of heat therapy is more robust, strong, and convincing than that for cold therapy. Despite the heterogeneity of studies, there seem to be some clear take-aways and applications of the literature to date:
The health benefits of heat therapy appear to be dose dependent. So barring safety concerns and taking care of hydration needs, the more time you spend in the heat, the better. With increasing duration and amplitude of heat exposure, the greater “heat fitness” you will develop.
Combine the stimulus of exercise with the stimulus of heat for an additive, synergistic effect. In a recent randomized controlled trial that looked at the combined effect of an exercise program followed by sauna use over 8 weeks, the researchers found that when combined with exercise, sauna bathing demonstrated a substantial synergistic effect on cardiorespiratory fitness, systolic blood pressure, and total cholesterol levels.
If for some reason an individual is unable to exercise or has physical impairments, consider employing heat therapy as an “exercise mimetic” that provides cardiovascular benefits and preserves lean muscle mass. To be clear, this is not a suggestion to replace exercise with heat therapy, but instead to use heat therapy if the ability to exercise is impaired.
Heat therapy is probably less useful as a recovery tool for physical performance. But to the extent that it provides a soothing sensation for tired muscles and painful joints, stress relief, and a space to pause, use it to reboot and recharge.
As a rule of thumb, if a person can walk into a sauna, he or she can walk out of it.
Another rule of thumb: if you are contemplating trying any new health intervention and you have pre existing medical conditions, it is always prudent to check with your healthcare provider before starting a regular thermotherapy routine.