HEAT EXPOSURE THERAPY

Heat stress increases core body temperature, promotes blood vessel dilation (vasodilation) and blood redistribution to the skin and extremities to facilitate cooling. Increase in 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.
Cardiac output (the amount of blood pumped from the heart per minute) is increased and driven primarily by an elevation in heart rate. This is comparable to the increase seen with moderate intensity exercise.
During the cool down period, HRV (heart rate variability) increases as parasympathetic tone is enhanced and sympathetic output dampens.
Heat positively affects blood vessels by increasing shear stress, mitigating protein damage and aggregation, activating endogenous antioxidant and anti-inflammatory pathways, and increasing nitric oxide.
Acute heat stress also activates and upregulates heat shock proteins , which protect cells from the damaging effects of heat and other stressors and facilitate normal cell function.
Heat exposure can also change hormone levels. Preliminary data demonstrate increases in growth hormone, testosterone, prolactin and decreases in cortisol during sauna bathing. Changes in hormones appear to be short lived and data on longitudinal alterations is lacking.
In the brain, heat stress promotes a robust increase in beta-endorphins and BDNF which suppress pain, modulate reward circuitry and regulate functions that are critical to cognition and memory.

MODALITIES OF PASSIVE HEATING

Hot Water Immersion
Waon Therapy (Far Infrared Sauna)
Finnish (Dry) Sauna

Exercise can induce numerous health benefits that can reduce the risk of chronic diseases and all-cause mortality, yet a significant percentage of the population do not meet minimal physical activity guidelines. Several recent studies have shown that passive heating can induce numerous health benefits, many of which are comparable with exercise, such as improvements to cardiorespiratory fitness, vascular health, glycemic control, and chronic low-grade inflammation. As such, passive heating is emerging as a promising therapy for populations who cannot perform sustained exercise or display poor exercise adherence. There appears to be some overlap between the cellular signaling responses that are regulated by temperature and the mechanisms that underpin beneficial adaptations to exercise, but detailed comparisons have not yet been made. Therefore, the purpose of this mini review is to assess the similarities and distinctions between adaptations to passive heating and exercise. Understanding the potential shared mechanisms of action between passive heating and exercise may help to direct future studies to implement passive heating more effectively and identify differences between passive heating and exercise-induced adaptations.
There is considerable overlap between the protective effect of passive heating and exercise, with beneficial adaptations reported in cardiorespiratory fitness, vascular function, and metabolism. Yet, heating does not appear to confer all the important benefits of exercise and potentially not to the same degree in all cases (see Fig. 2 for a summary). It is important to consider that our understanding of the health benefits of exercise has developed over several decades, providing considerable detail and nuance to how different populations respond to different forms of exercise, while the study of passive heating is still in its relative infancy. In this regard, exercise should be considered as the primary route for maintaining and improving health. Having said this, the health benefits of passive heating have been observed in healthy sedentary and some diseased populations, supporting the supposition that passive heating may be a promising therapy in those who display poor exercise adherence. In this regard, research should continue to focus on those specific populations who may benefit most, and a wide range of populations remain unstudied. It will be hugely important to investigate the risks and potential adverse outcomes associated with passive heating. These remain relatively unexplored but include potential heat illness, orthostatic intolerance, and an increased risk of falling, especially in older individuals (28, 45). Similarly, the physiological basis of these events and any subsequent mitigating strategies should be developed as they have been with exercise.
Beyond pleasure and relaxation, emerging evidence suggests that sauna bathing has several health benefits, which include reduction in the risk of vascular diseases such as high blood pressure, cardiovascular disease (CVD), stroke, and neurocognitive diseases; nonvascular conditions such as pulmonary diseases including common flu; mortality; treatment of specific skin conditions; as well as pain in conditions such as rheumatic diseases and headache.

The physiological responses produced by an ordinary sauna bath correspond to those produced by moderate- or high-intensity physical activity such as walking.
The beneficial effects of sauna baths on CVD and mortality may be mediated via reduction in blood pressure, improvement in endothelial function, reduction in oxidative stress and inflammation, beneficial modulation of the autonomic nervous system, improved lipid profile and arterial compliance, and improvement in the cardiorespiratory system.
Sauna bathing is a safe activity and can even be used in patients with stable CVD, provided it is used sensibly for an appropriate period of time.
Many people sit in saunas for relaxation and general wellness. But experts believe sauna bathing may offer benefits for:

Arthritis
Chronic fatigue
Cold symptoms
Muscle soreness
Psoriasis
Stress

Finnish sauna bathing

The Finnish tradition of sauna bathing dates back thousands of years. On average, Finnish people take saunas two or three times a week, staying in the wood-lined rooms of hot, dry air for up to 20 minutes. In 2018, a team of Finnish researchers published a review of health benefits of sauna bathing in Mayo Clinic Proceedings. Several studies link frequent sauna use (four to seven times a week) to lower blood pressure and a decreased risk of cardiovascular disease, including sudden cardiac death and stroke. According to the authors, these benefits may stem from better blood vessel function, improved cholesterol levels, and less inflammation observed in frequent sauna users. But it's also possible that the relaxation from taking a sauna — not to mention a lifestyle that allows for frequent saunas — may explain the heart-related outcomes.

Sauna and heart health

Sitting in a sauna may be particularly good for your heart. The heat can raise your skin temperature and cause heavy sweating — just a short time in the sauna can produce a pint of sweat. As your body attempts to keep cool, your heart rate increases and may reach 100-150 beats per minute.

Sauna bathing (and an increased heart rate) causes your blood vessels to open, increases circulation and reduces stress levels — like the effects of low or moderate exercise. As a result, sitting in a sauna also benefits:

Blood pressure

Evidence over the past few decades shows that sauna bathing has an immediate positive effect on blood pressure. But for a bigger benefit, combine your sauna bathing with exercise. Recent research finds that using the sauna for 15 minutes after a workout, three times a week, results in a more significant improvement in blood pressure than exercise alone.

Total cholesterol

High blood cholesterol — a waxy substance in your cells — is a major risk factor for heart disease. Lowering your total blood cholesterol by 10% can decrease your risk of heart disease by 30%. The good news is that sweating can raise your good cholesterol levels (HDL) to improve your total cholesterol.

Exercise will certainly help you work up a sweat, and sauna bathing on its own can also reduce total cholesterol levels. But combining the two — sitting in a sauna after your workout — has a more significant effect on total cholesterol.

Cardiovascular respiratory fitness level

Cardiovascular respiratory fitness (CRF) is your body’s ability to deliver oxygen to your muscles and organs during physical activity. A higher CRF level can decrease your risk of heart disease and death.

Regular exercise is one way to improve CRF levels. But whether your CRF levels are high or low, research shows that sauna bathing following exercise is beneficial. If your CRF level is low, exercise followed by 15 minutes in the sauna improves CRF more than exercise alone. If your CRF level is high, adding sauna bathing to your workout routine can reduce your risk of heart-related death, including sudden cardiac death.

Risk of death from heart-related disease

Heart disease causes one out of every five deaths in the United States. And sudden cardiac death (SCD) often has no warning signs. Research shows that sauna bathing may be able to help lower your risk of heart disease. One studyfollowed 2,300 sauna bathers for 20 years and found that the participants who visited the sauna more frequently (four to seven times a week) had lower death rates from heart disease and stroke.

Mounting evidence shows that hitting the heated chambers can produce some of the same cardiovascular benefits as aerobic exercise. While a post-workout sauna can compound the benefits of exercise, the hormetic effects of heat therapy alone can produce significant gains for microvascular and endothelial function, no workout required.

Performance Benefits of Heat Exposure

Now the health benefits of heat exposure are all well and good, but I’m sure you’d like to know how jumping in the sauna can help you get in shape, right? Well thankfully there is good research to suggest that heat exposure has some major performance benefits as well.

Aerobic System

If you are one of those people who believes in conditioning as a part of overall fitness (like myself), you will be happy to know that sauna use can help. Through the activation of heat shock proteins and their effect on inflammation, mitochondrial health is improved substantially. Healthy mitochondria enable us to make more ATP per unit of time. This has huge implications for aerobic exercise with respect to delivering energy to the exercising muscle. Not only will you be able to sustain a higher rate of exercise, but you’ll be able to recover from that exercise more efficiently.

On top of that obvious benefit, heat exposure has been shown to have a profound impact on the thermoregulatory and cardiovascular systems. Through heat acclimation, the body is better able to control body temperature, which helps to keep you cooler during exercise [12]. Additionally, blood volume and fluid balance are improved from heat acclimation. All these adaptations allow you to stave off any ill effects which result from dehydration or increased body temperature during exercise. Going even further, we see that cardiac stability and blood perfusion to the heart are improved, which allows the heart to work more efficiently during exercise [12]. While it is not clear whether these adaptations result in improved time trial/race performance, they do seem to improve time to exhaustion during endurance exercise [14].

Skeletal Muscle

Of course, most of you didn’t come here to read about the endurance benefits of sauna use. You want to know how heat exposure can benefit you in the weight room. Interestingly, heat exposure seems to have an even greater effect on skeletal muscle than it does on the aerobic system.

The HSP’s that are activated from heat exposure actually have a robust effect on net protein balance in the body. While they do not necessarily trigger muscle protein synthesis (MPS), they do inhibit protein breakdown [15]. This effectively shifts the balance into positive net protein balance which, when training is applied, results in enhanced muscle hypertrophy.

Additionally, heat exposure causes a huge increase in growth hormone (GH) release and may improve insulin sensitivity [7][8]. While GH may not be anabolic per se, it does offer some nice anti-catabolic effects as well as connective tissue benefits. However, GH also stimulates the release of insulin like growth factor 1. IGF-1 can then activate the mTOR pathway leading to MPS as well as activate FOXO, which further inhibits protein degradation.

Looking at the effect of improved insulin sensitivity, we also see benefits in these same areas. Insulin stimulates MPS directly after a meal and also inhibits protein breakdown. Being more sensitive to insulin allows our body to perform these tasks more efficiently.

Over time, these robust benefits to MPS and inhibition of protein breakdown will definitely lead to greater muscular adaptations. Heat exposure may also serve as a useful tool in dieting individuals who are more prone to experiencing muscle breakdown.

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References

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