Does Sauna and Heat Therapy Improve Heart Health? What a 2025 Review of 20 Trials Found

Saunas, hot water immersion and other forms of deliberate heat exposure have moved from the edges of wellness culture into the mainstream, and Ireland has been no exception, with infrared sauna studios and contrast therapy suites opening alongside the more familiar cold plunge trend. Much of the appeal rests on a genuinely interesting body of research from Finland, where large observational studies have linked frequent sauna bathing with lower rates of cardiovascular disease and death.

Those observational findings are real and worth taking seriously, but they come with a well-known catch. People who use a sauna several times a week tend to differ in many other ways from people who never do, from income and leisure time to baseline health and general activity levels, and observational studies struggle to fully separate the effect of the sauna itself from the effect of being the kind of person who regularly uses one.

That is exactly the gap a randomised controlled trial is designed to close, by taking similar people and assigning some to a heating intervention and some not, so that any difference that emerges is more plausibly due to the heat exposure. A 2025 review set out to gather all the trial evidence of this kind in one place, and its findings are a useful corrective to some of the bolder claims.

The review, published in the American Journal of Preventive Cardiology, was a systematic review and meta-analysis, meaning the authors searched the medical literature systematically and then statistically combined the eligible studies. They searched three major databases up to November 2024 and included only randomised controlled trials in adults that applied a passive heating intervention for at least one week and measured cardiometabolic or vascular outcomes.

Twenty trials met that bar. The interventions were varied, spanning hot water bathing, sauna use, hot yoga and local heating applied to part of the body, and the trials ran for between two and fifteen weeks. This breadth is a strength in that it captures the real-world variety of heat therapy, but it is also a limitation, because pooling quite different interventions together can blur genuine differences between them.

The outcomes examined were the ones that matter for heart and metabolic health, including blood vessel function measured as flow-mediated dilation, arterial stiffness measured as pulse wave velocity, resting heart rate and heart rate variability, fasting glucose and HbA1c, the full cholesterol panel and triglycerides, and C-reactive protein as a marker of inflammation. The review was pre-registered, which is a marker of good practice because it commits the authors to their plan before they see the results.

The central finding is straightforward and, for sauna enthusiasts, deflating. Across the pooled trials, passive heating produced no statistically significant benefit for the large majority of outcomes measured. Flow-mediated dilation, pulse wave velocity, resting heart rate, heart rate variability, fasting glucose, HbA1c, total, HDL and LDL cholesterol, triglycerides and C-reactive protein all showed no significant pooled effect.

That is a long list of null results covering precisely the outcomes that heat therapy is most often claimed to improve. It does not mean heat exposure does nothing for anyone under any circumstances, but it does mean that when you gather the controlled trials together and combine them properly, the averaged effect on these standard health markers is not distinguishable from no effect.

This is a good illustration of why a negative or null result deserves to be reported as clearly as a positive one. The observational research created a strong expectation that regular heating would improve these markers, and the trial evidence, which is better suited to testing cause and effect, has so far not confirmed it. Reporting that honestly is more useful than quietly emphasising the one positive finding and skipping past the rest.

There was a single exception to the pattern of null results. For systolic blood pressure, the top number in a blood pressure reading, the overall pooled estimate pointed towards a reduction but did not quite reach statistical significance across all the trials combined. When the authors looked closer, a significant reduction did appear in two specific situations.

The first was in whole-body heating interventions, as opposed to local heating of just one part of the body, where systolic blood pressure fell by around four millimetres of mercury. The second was in people who already had coronary risk or established cardiovascular disease, where a smaller but significant reduction of around two and a half millimetres of mercury was seen. In other words, any blood pressure benefit was concentrated in whole-body heat and in people who had more room to improve.

Even this finding came with an important caveat. The authors noted high heterogeneity, meaning the individual trials disagreed considerably with one another, and they explicitly advised interpreting the blood pressure result with caution. A modest average blood pressure reduction in higher-risk groups is a plausible and worthwhile effect, but it is not the same as the broad cardiovascular and metabolic overhaul heat therapy is sometimes sold as.

For an Irish audience, much of the current interest is in infrared saunas rather than traditional Finnish saunas or hot water immersion, so it is worth being precise about what the review does and does not tell us. The trials it pooled were dominated by conventional heating methods, and it did not establish that infrared saunas specifically deliver the outcomes measured. Evidence about one heating method cannot simply be assumed to apply to another.

The infrared-specific evidence that does exist is limited and not especially encouraging so far. A 2024 randomised crossover trial looked at whether a single infrared sauna session improved blood sugar handling in older adults with type 2 diabetes. It found that it did not, and in fact blood glucose rose slightly more after the heated session than after a thermoneutral control, with no improvement in insulin sensitivity.

That was a single session in a small group, so it is far from the last word, and the researchers themselves called for studies of longer, repeated infrared use. But it is a useful reminder that the specific, heavily marketed format of heat therapy that many Irish clinics now offer has less direct evidence behind it than the general enthusiasm would suggest, and that assuming infrared saunas match the observational Finnish sauna data is exactly the kind of leap the trial evidence does not support.

It would be a mistake to swing from overhyping saunas to dismissing them entirely, and this review does not justify the latter any more than it supports the former. A meta-analysis of relatively few, mostly small and short trials, with high variability between them, is a reason to lower confidence in strong claims, not proof that heat therapy is inert. Longer and larger trials could yet find effects these studies were too small or too brief to detect.

It is also true that these trials measured specific biological markers, and there are reasons people use saunas that sit outside that list entirely, including relaxation, sleep, muscle relaxation after exercise, and simple enjoyment. Those are legitimate benefits in their own right, and nothing in this review argues against valuing them. The point is narrower and important: the evidence that a sauna habit measurably improves your cholesterol, blood sugar or blood vessel function is currently weak.

For anyone making decisions with their health or their money, that distinction matters. Enjoying a sauna as a pleasant, relaxing part of a routine is entirely reasonable. Paying a premium for it, or relying on it, as a treatment for a cardiovascular or metabolic condition is a different proposition, and one this evidence does not support.

No single review is the final word, and this one has clear limitations that the authors were upfront about. The individual trials were small, typically enrolling only a few dozen people each, they were short, running for a matter of weeks rather than the years over which cardiovascular benefits would realistically accumulate, and they varied so much in method that combining them introduces real uncertainty. The high heterogeneity the authors reported is a genuine limit on how much weight the pooled numbers can bear.

On transparency, the paper disclosed the authors' potential conflicts of interest, which is good practice and worth reading alongside the findings. One author is listed as the owner of a health data company, and another reported investigator-initiated grants and honoraria from nutrition and consumer-health companies. None of the disclosed interests is a sauna or heat-therapy manufacturer, so there is no obvious commercial pull towards a positive sauna result, which if anything adds a little weight to the largely null findings.

Taken together, the picture is of a still-immature evidence base. The observational signal that got everyone interested is real, but the controlled trials needed to confirm it are so far too few, too small and too short to have done so, and the honest position is to hold the strong claims loosely until better trials arrive.

A 2025 meta-analysis of 20 randomised controlled trials found that passive heating, including saunas, produced no significant pooled improvement in most heart and metabolic health markers, with the only exception a possible reduction in systolic blood pressure that was largely confined to whole-body heating and to people already at cardiovascular risk. Evidence for infrared saunas specifically is thinner again, and a small trial found a single infrared session did not help blood sugar control.

If you use a sauna in Ireland and enjoy it, there is no reason in this evidence to stop, and there may be a modest blood pressure benefit for some people, alongside the relaxation and recovery benefits that drew you to it in the first place. What the current evidence does not support is treating a sauna, and an infrared one in particular, as an evidence-backed treatment for heart disease, diabetes or high cholesterol. For those conditions, the measures with strong trial evidence remain the first priority, and a sauna is at best a pleasant addition rather than a substitute. Anyone with a heart condition or high blood pressure should also check with their doctor before starting regular heat exposure.