Mechanisms of Action: Cardiovascular and Heat Stress

Mechanisms of Action: Cardiovascular and Heat Stress
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).

Does Sauna Help With Respiratory Diseases?
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.

Fat Loss and Energy Expenditure
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.

Can You Really “Sweat Out the Toxins?”
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:

Polybrominated diphenyl ethers (PBDES)
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).

Organochlorinated pesticides (OCPS)
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).

Polychlorinated Biphenyls (PCBS)
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).

Heavy Metals
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.

The Take-Home Points
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.