Slayer's Den

[01dragonslayer]

The Science of the Sauna​

Joe Rogan loves his sauna. But do saunas really extend your life, sweat out toxins, and burn fat? Here's the actual science.​

The gym’s sauna used to be packed with nothing but sweaty old fat guys. I used to think, “What’s the point? Are they trying to feel like they’ve actually exercised?” But these days, the sauna is filled with bodybuilders, athletes, mixed martial artists, and fitness competitors.

Thanks in part to Joe Rogan, the sauna is hot again. But is the sauna really beneficial to our health? Let’s take a deep dive into the science and find out.

Interventional studies found that the sauna can reduce blood pressure, at least in the short-term (1, 2). A long-term observational study (adjusted for confounders like cardiorespiratory fitness) in men who used a sauna 4-7 times per week found the risk of developing high blood pressure was nearly 50% lower over a 25-year period (3).

Ideally, though, establishing to what extent a sauna can help reduce blood pressure would require randomized, controlled studies performed for at least several months with repeated sauna use as an intervention.

One observational study found the risk of cardiovascular disease, sudden cardiac death, fatal coronary heart disease, and all-cause mortality were inversely associated with the duration and frequency of sauna use (3). This was after adjusting for potential confounders like physical activity, BMI, socioeconomic status, and lifestyle. Another study found stroke risk was also decreased with sauna use (4-7 times per week versus once a week).

One observational study found that men who reported frequent sauna use (5-7 times a week) had a 65% reduction in the risk of Alzheimer’s and dementia compared to those who reported using it once a week. This included adjustment for confounders like physical activity, BMI, alcohol consumption, and smoking. (4).

The risk for experiencing psychosis was also reduced by 78% for men who reported 4-7 sauna sessions per week compared to those who had only one sauna session per week. This included adjustment for confounders like physical activity and energy intake (5).

There’s some evidence that the sauna may help with delayed onset muscle soreness (DOMS) (6). Additionally, it might help reduce headache intensity in those suffering from chronic tension headaches (7). There’s also some limited evidence that infrared sauna might help with certain inflammatory conditions like rheumatoid arthritis (8).


Exercise1240×698 148 KB

Since some of the studies are observational in nature, there’s always the possibility of one or more confounding variables influencing the associations between sauna use and a given effect. This can result in incorrect observations or conclusions. Accounting for these variables helps us identify whether the association is being influenced. These studies have largely done so.

However, the only way to know with great certainty is through a randomized controlled trial, and, in this case, it would need to be large and long-term. Whenever a single variable has a fairly significant impact on the risk of various diseases in observational studies, it can mean one of three things:

1. The effect is real, and the pathways involved in various disease states are modified in such a way that a positive effect is seen.
2. The variable is an excellent marker for health, but not the cause of it.
3. There’s a mixed effect: some disease states are truly impacted while others are merely due to other confounding variables.

Sauna modifies variables like blood pressure (1-3) and cholesterol (albeit in uncontrolled trials) which are risk factors for cardiovascular disease, while various other effects typically seen with exercise are also seen with sauna (9).

The greatest benefits from the sauna are probably attributable to the taxing effect on the body and cardiovascular system that, in effect, mimics a moderate form of aerobic exercise. However, other mechanisms may be at play.

For example, when you combine sauna with exercise (not at the same time!) the benefit may be greater than either exercise or sauna alone. In long-term observational studies, the risk of cardiovascular death, sudden cardiac death, and all-cause mortality were lower in those who exercised AND hit the sauna than in those with high cardiorespiratory fitness or sauna-bathing alone (10, 11).

The additional benefit of sauna beyond exercise alone isn’t enormous, but it suggests there may be mechanisms at play that are unique to sauna. But, this is an observational study. There may be may some unique confounders that result in a lower risk of death and all-cause mortality.

After all, it’s not difficult to imagine that those able to exercise and use the sauna frequently may enjoy a lifestyle that an average person may not. They may be elite athletes, wealthy, experience little to no stress, or have better diets. The best way to answer all remaining questions is through well-designed interventional studies.

When you jump into the sauna, your heart has to work harder to pump blood away from your internal organs toward the surface of your body. This helps keep your core temperature cooler. This cardiovascular stress could lead to some of the benefits (9).

The heat stress itself also likely plays a role. Sauna can cause neuromuscular fatigue, especially after lifting. This demonstrates it produces a stressful effect on the body which might stimulate a beneficial adaptive response similar to exercise (12).

Interestingly, one study concluded that the cardiovascular stress of sauna is similar to moderate-intensity aerobic exercise. This may explain at least part of the potential benefits (13).

However, some other potential mechanisms (some of which may be a response to the heat stress itself) include alterations in natriuretic peptides, improved vascular function, immunologic effects, up-regulated heat shock protein (HSP) expression, increased brain-derived neurotrophic factor (BDNF), and decreased oxidative stress (1-3, 9, 14-16).

Of course, some of these also overlap (or are interconnected). Many of these benefits are seen with exercise as well (9, 17, 18), calling into question whether sauna has many unique benefits beyond what you’d experience with exercise designed to improve cardiorespiratory fitness alone. In fact, for HSP, which seems to be a popular molecular target, the increase from exercise is greater than that seen from heat stress (15).


Respiratory Disease1240×698 325 KB

Frequent sauna seems to decrease the risk for respiratory diseases in men based on a long-term prospective observational study (19).

In a smaller interventional study, sauna users only had half the colds of those who didn’t sweat with the oldies (20). The severity and duration of the colds weren’t different, however (21). Interestingly, while a hot, dry sauna doesn’t seem to help with cold symptoms, a steam bath may help with allergic rhinitis symptoms (22). Smaller studies in children and adults also found a reduction in respiratory infections like the common cold and flu (23).

How does the sauna do this? It might affect white blood cell and HSP activity. A lot of doctors now see fever as a positive mechanism – it helps the body fight off infection, directly and indirectly. This slight hyperthermic state may be how the sauna provides such benefits (24).

A note of caution: While there’s evidence that sauna may reduce the likelihood of respiratory infections, it shouldn’t be used to treat viral infections. Never visit a sauna while you have a fever. Even when you’re not sick, consult your physician before using a sauna.

Some say the sauna helps with fat loss. Is that true?

Sauna probably does increase lipolysis, and it does increase energy expenditure, but it may be that sauna increases lipolysis without affecting lipid oxidation to a significant extent (25-27), much like caffeine. The increased energy expenditure may instead be coming from increased carbohydrate utilization, but more research is needed.

For the time being, it’s best not to count on the sauna to make up for those extra cookies.


Finnish Sauna1240×698 215 KB

I’ve always grouped this into the same new-age hooey that includes “detoxing” with various drinks, fasts, and other methods that aren’t supported with much evidence. Here’s why:

• Most chemicals we encounter are metabolized and excreted very quickly, although there are exceptions (some organic compounds and heavy metals) that are stored in tissues like fat and bone as well as organs like the brain, liver, or kidneys.
• Most things we consider toxins aren’t eliminated by sweating. The major routes are urine and feces. Minor routes aside from sweat include tears, saliva, semen, hair, skin, and breast milk (28,29). Exhalation can also be a major route of elimination for gaseous or sometimes highly volatile toxicants. Cerebrospinal fluid can also be a source of elimination from the brain to the blood.
• Even if the rate of excretion is higher in sweat than in urine, in some cases, the total volume is generally lower with sweat.

However, I decided to see if any studies might show exceptions. I found some indicating that induced sweating via sauna or other methods may allow for significant elimination, even exceeding the excretion rate in urine. This was true, at least for some heavy metals and certain lipophilic compounds that bioaccumulate in humans.

Remember, this doesn’t amount to much ordinarily, but you can sweat up to 2 liters per hour in a sauna. That’s pretty significant considering that we pee around 1.5 liters a day (25, 30). Even if you only sweated out half that amount in a 20-minute session, it’s still significant. This may be especially useful for those highly lipophilic compounds. The kidneys don’t easily excrete them even after metabolism by the liver.

One author was critical of the methods used in these studies, arguing that it may have been sebum rather than sweat responsible for increased excretion (31). (Sebum is an oily substance the body produces to keep the skin moisturized.) However, the critical point is that these substances were still eliminated from the body after induced sweating at an accelerated rate. That’s significant!

More info:

These are flame retardants found in household plastics and textiles, and commonly detected in humans. They may alter hormone levels (particularly thyroid) and be carcinogenic. While the excretion of individual congeners in sweat was fairly low, nothing was detected in urine (30).

DDT and methoxychlor are the most widely known, but they were eventually banned in the United States and Canada due to potentially adverse effects. Due to their persistence in the environment, they’re still detected in humans. For some, excretion in sweat was generally greater than urine (32).

Also banned now, but also still detected in humans. At least some PCB congeners were excreted in sweat to a greater degree than urine (33). A single case study also found enhanced excretion in sebum in response to a protocol that included induced sweating (34). A larger study found reductions in PCB congeners in adipose tissue (35). Considering that heat exposure may increase sebum secretion from the sebaceous glands, this seems to be a reasonable route for the excretion of lipophilic compounds (36-38).

While not lipophilic, some heavy metals can be excreted in sweat to a greater degree than urine (39,40). Cadmium has shown consistently greater excretion in sweat than in urine, while lead and mercury have shown mixed results. Arsenic is excreted in sweat but appears to favor urine.

Note: We’re all exposed to organic pollutants and heavy metals throughout our lives. The background levels of these compounds in our bodies probably have little, if any, effect on our well-being, but we can’t be 100 percent certain. That said, it’s difficult to argue that you’d be worse off by decreasing the total body burden of the compounds.

Now, someone could make this argument: The potential lipolysis experienced during induced sweating could result in compounds stored in fat being freed and redistributed elsewhere where they may cause harm, like the brain (41).

But exercise already causes lipolysis. And it appears that with induced sweating, these compounds excrete through the skin rather than favoring redistribution.


Rocks1240×698 226 KB

• Most studies are observational or uncontrolled, making the data behind them kinda weak and vulnerable to confounders.
• Many studies utilize a Finnish sauna (80-100°C or 176-212°F) or something similar, along with rather low humidity (10-20%). Infrared saunas are a much newer iteration and may not be comparable to a traditional sauna.
• While an exception to the rule, there’s evidence that some compounds are preferentially excreted either in sweat or sebum. What sort of health benefits this produces, if any, remains to be determined. Case reports and one pilot randomized controlled trial have evaluated some aspects in extreme cases, but any potential benefits are confounded by a regimen consisting of other interventions. Also, it’s extremely difficult to control for potential placebo effects (34, 35, 38, 42).
• Sauna probably provides the greatest benefit to those who don’t already have high cardiorespiratory fitness. A newer randomized controlled trial did find that supplementing regular exercise with sauna caused additional beneficial changes in cardiorespiratory fitness, blood pressure, and cholesterol (43). However, these individuals were sedentary and had at least one risk factor for cardiovascular disease.