Bring the Heat for Improved Cardiovascular Health

I have always loved to sweat. Nothing seems to get me in a better or more productive mood than a long run under the hot Florida sun — drenched by the end and in absolute, exhausted ecstasy. I feel pretty terrible if a day goes by when I don’t at least open up my pores and perspire for at least a good hour or so. Not so much due to the guilt of not exercising, but the fact that running myself dry just provides something euphoric. Runner’s high be it not, but something more primal.

I also happen to like saunas (for the same reason) which is why I’m pretty interested in the new fascination with them in the health and medical world. Some recent studies out of Finland (which we will call the “sauna capital of the world”) are providing some legit data that life-long use of the sauna leads to some beneficial long-term health outcomes.

, findings from the Finnish Kuopio Ischemic Heart Disease Risk Factor Study showed that more frequent and longer-duration sauna users had lower risk of sudden coronary death (SCD), coronary heart disease (CHD), cardiovascular disease (CVD), and all-cause mortality.

The men who reported using the sauna 4–7 times per week and >19 minutes per session had the lowest risk for all of the above health outcomes. Compared to just 1 session per week, even those who were 2–3 times per week users of the sauna experienced a benefit.

While this wasn’t an experimental study (they just asked the men how many times and for how long they used the sauna each week), the observational data are pretty pervasive. Sauna bathing has a rich history in the Finish culture, among others and, in my opinion, something so steeped in tradition probably has a good reason for being there.

These associations along with other data on sauna use have led to a research interest in the effects of heat on cardiovascular health — something referred to as “heat science.”

Why the interest in heat? Sweltering temperature is now known to be far more than an environmental stress — it might have beneficial health outcomes when applied “therapeutically.” For instance, many studies have shown that experimental heat therapy — things like heating pads or hot water immersion — can improve the function of blood vessels, lower blood pressure, and reduce the stiffness of arteries — which all increase with age and contribute to CVD development. compiles the data on some traditional and experimental heat therapy strategies to give an idea of if and how they influence vascular health

This is a potentially beneficial area of research because, as the authors of the paper note (but don’t need to), many populations and individuals fail to get enough of another thing known to improve blood vessel health — exercise. In this way, heat might be an “exercise mimetic” (though I despise this word).

Some people, due to aging or other disease-related factors, just can’t exercise. Others just choose not to.

Either way, if heat therapy can simulate some of exercise’s beneficial cardiovascular properties, it might be an avenue to help stimulate and improve health aging individuals. Forget the treadmill, hop in the dry sauna.

The paper delves into some various forms of “traditional” heat therapy used in cultures, which I won’t go into detail on. These include things like Waon therapy (basically a dry sauna followed by lying down wrapped in a warm blanket), Japanese Onsen Bathing (hot springs), Finnish sauna bathing, and Bikram (hot) yoga. In sum; most of these practices are shown to result in outcomes like improved symptoms of heart failure (in CHF patients), prevention of hypertension and cardiovascular disease, and lower levels of markers of arterial stiffness — just by getting in the heat a few times per week.

The question then becomes why these particular practices reduce cardiovascular disease risk. Knowing why is important because sure, it’s interesting but also for the reason that if the mechanisms for the benefits are known, we can develop therapies that “mimic” the pathways stimulated by heat and gain the health benefits in a clinical or experimental setting.

One of the primary mechanisms discussed at large in the paper is that heat stress causes the blood vessels to dilate, or relax. This is a necessary reaction, since it allows blood flow to the skin to increase, helping us to dissipate heat and cool down. As heat increases in intensity, so does the amount of blood flow to the skin. Increased blood flow leads to something called shear stress — essentially the friction of blood against the blood vessel wall, leading to the release of molecules like nitric oxide (NO) which cause the vessels to relax even more.

Along with increase the amount of available nitric oxide, shear stress is the critical stimulus responsible for the improvements in endothelial function and other vascular adaptations. Our arteries are highly “adaptive ”(although the capacity is somewhat lost with aging). When the body signals that there is an increased demand it responds by increasing capacity to handle the greater blood flow and eventually improves its overall function — think of it as resistance training for the arteries.

With one bout of heat exposure (i.e. a sauna session), . Thinking more long term (for those 4–7 times a week for 20+ years sauna using Finns), with each little bit of stress, the adaptations progress a little more. Over time, the improvement in health can be vast. This is why chronic heat exposure as a therapy may prove to be promising and has already shown health-protective benefits in some populations.

Let’s not forget about the heat shock proteins (HSPs). These little heat-responsive molecules are known to play a variety roles in the protective benefits of heat (and exercise) stress. In response to heat, HSPs lead to increased nitric oxide production (remember, the key relaxing molecule for blood vessels) and also protect against inflammation and oxidative stress in our body.

Heat shock proteins and their function relate a bit to the concept of hormesis, which I’ve written about . A little bit of stress (in this case, heat) triggers just enough damage to lead to an adaptive response, making us stronger (and healthier) in the long term.

A heat shock protein (HSP). From:

How to apply all of this interesting information around heat. Heat is, well, not a new concept. However, using heat to treat disease and improve health is. If heat stress can improve vascular function and other cardiovascular parameters, perhaps it can reduce the burden of CVD around the world — it’s currently the leading cause of death in the U.S and other countries. Impaired vascular function is a precursor to atherosclerosis and cardiovascular disease. Keep the arteries healthy and we can avoid many long-term adverse consequences.

The research world has now developed heating models to study mechanisms and efficacy of”lower key” treatments. Things like using heating pads to warm the upper and lower limbs and full or partial body immersion in warm water (basically, a hot bath). These can also be applied chronically (i.e. “training”) for periods such as 6–8 weeks in order to see if larger adaptations take place in the vasculature.

So far, improvements have been seen in some, but not all studies. In one neat and comprehensive — participants underwent a protocol of 4–5 sessions of hot water immersion per week involving 90 minutes sessions where core temperature was increased about 1.5 degrees Celsius. A pretty drastic protocol, but it worked. Even in young, healthy participants, several measures including flow-mediated dilation, femoral artery compliance, measures of arterial stiffness like pulse-wave velocity, thickening of the arterial intimal wall, and blood pressure, improved.

Do heat therapy, sauna use, and the like, represent an “exercise alternative”? Again, I don’t think anything can perfectly mimic exercise. With heat therapy, sure, you get some of the cardiovascular-stress benefits of exercise. What you don’t get is any muscular contraction when you’re sitting passively — therefore losing many of the benefits that the act of moving provides.

Nevertheless, chronic sauna use or even the persistent applications of some of the techniques used in experiments like lower-body hot water immersion might be “tools” to achieve benefits for the vasculature in people with disease, the inability to exercise. Additionally, these can be used in addition to exercise in healthy humans to further optimize function.

I love the idea of combining many stresses, and think optimal health involves playing with many strategies. For instance, I wrote about how, in stark contrast to our current topic, . Ice baths and cryotherapy are also “hot” right now. Many are using alternating hot-cold therapy as a longevity enhancer, health promoter, or just to wake up in the morning (nothing like a good polar plunge to start the day.)

I love following a good run with a hot sauna session.In the summer, perhaps I’ll try and ice bath. Double the pleasure (or pain, if you prefer).

Time to break a sweat.

References

Cheng JL, Macdonald MJ. Effect of Heat Stress on Vascular Outcomes in Humans. J Appl Physiol. 2019

Lee E, Laukkanen T, Kunutsor SK, et al. Sauna exposure leads to improved arterial compliance: Findings from a non-randomised experimental study. Eur J Prev Cardiol. 2018;25(2):130–138.

Laukkanen T, Khan H, Zaccardi F, Laukkanen JA. Association between sauna bathing and fatal cardiovascular and all-cause mortality events. JAMA Intern Med. 2015;175(4):542–8.

Brunt VE, Howard MJ, Francisco MA, Ely BR, Minson CT. Passive heat therapy improves endothelial function, arterial stiffness and blood pressure in sedentary humans. J Physiol (Lond). 2016;594(18):5329–42.

PhD candidate at the University of Florida — Science writing with a particular focus on exercise and nutrition interventions, aging, health, and disease.

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