Scientists launch push to organize sauna health research

Frequent sauna use tied to lower cardiovascular risk in studies

The two-day gathering took place at The Well, a resort outside Oslo where nude bathing is the norm and essential-oil-infused rituals based on traditions such as the German Aufguss are held throughout the day. It convened on the eve of the International Sauna Congress, an event that draws hundreds of sauna lovers and experts every four years. The symposium was funded by the Norwegian Sauna Society, the resort where it took place, and the Sauna Research Foundation, a nonprofit backed by a Minnesota-based longevity entrepreneur and several other private funders.

Organizers described the meeting as an attempt to turn what has been a niche and disparate area of research into a credible scientific field — and to dispel what some acknowledged is a whiff of wellness hype. “We want to introduce this as an actual field of research, not something fringy, or woo-woo,” said Ashley Mason, a clinical psychologist at the University of California, San Francisco, and an associate professor of psychiatry and behavioral science who is running trials on heat therapy for depression, sleep and other aspects of mental health.

Although sauna has been studied on and off for decades, researchers said very little of that work comes from well-controlled human trials. Existing studies are small, methods vary significantly, and key measurements are sometimes missing, making firm conclusions difficult. The nuances of measuring environmental humidity and body temperature — or how to best define heat therapy — may be afterthoughts to the average sauna-goer, but all of this matters enormously when pulling together data from different countries and institutions. “We’re all over the place,” Mason said. “If we want to be taken seriously, we need to have consistent terms, so that people understand how to refer to and talk about our work.”

Earric Lee, a postdoctoral researcher at the Montreal Heart Institute who is spearheading the initiative, said scientists first need to agree on standards — a painstaking process involving dozens of other experts that will set the stage for larger, multisite trials. Researchers want answers to basic questions: what the actual benefits are, how often a person needs to go, what the risks are, and just how hot it needs to be. “People want information. I know ‘listen to your body’ doesn’t cut it,” Lee said, “but I really can’t give you recommendations, and that’s the honest truth of it.”

The scientific interest in heat’s therapeutic potential has deep roots. In the late 1700s, British scientists ran bold experiments on thermoregulation, subjecting themselves — and a dog — to a piping hot room and observing that, unlike the steak and eggs accompanying them, their own bodies did not cook. The acute physiological response to extreme heat is now well understood: blood vessels dilate, heart rate speeds up, and blood is pushed to the skin, where it is cooled as the body sweats; the body releases hormones and activates heat shock proteins that protect cells. That understanding has shaped medical care for heat-related illness. But the potential therapeutic benefits of heat exposure have received considerably less attention from modern science. Meanwhile, age-old practices built around stressing the body with heat, such as sauna, are enjoying newfound popularity. From social sauna meetups to the broader wellness and optimization movement, the level of public interest has surprised even long-time devotees such as Dr. Hans Hägglund, a hematologist and stem-cell researcher at the Karolinska University Hospital in Stockholm, dubbed the “sauna doctor” in his home country thanks to a popular book he authored on the subject. “It’s really booming,” Hägglund said. “Unfortunately, there is still not very much evidence out there.”

Known as “passive heat therapy” in the scientific literature, heat delivered as a strong stimulus in a time-limited manner works the cardiovascular system in ways similar to light physical activity, delivering some of the same benefits, including improvements in blood pressure, arterial stiffness and vascular health. Some experiments also indicate improvement in markers of inflammation, though Lee said the findings, as in many areas, are not conclusive. Ant Shepherd, a professor of clinical environmental physiology at the University of Portsmouth in the U.K., said there is particular interest in how heat can help people with cardiovascular and metabolic conditions, as well as the elderly. His lab has shown that spending time in a hot tub improved resting metabolic rate and some measures of insulin sensitivity for people with Type 2 diabetes. “Exercise is king, exercise is medicine, but for those who can’t do that easily, can we get some of the same effects?” Shepherd asked. “It looks like we can.”

In one of the only randomized controlled trials on sauna bathing, Lee found that adding sauna to an exercise regime conferred added benefits for people at risk of cardiovascular disease. He is now running a trial on patients undergoing cardiac rehabilitation. Separately, several research groups are exploring whether heat can help treat depression, building on a landmark study that showed meaningful decreases in symptoms after one session in a special infrared machine. “We’ve got lots and lots of lovely little studies,” Shepherd said. “The governments aren’t going to listen until we put the hardcore, randomized-control trials in front of them.”

The recent uptick in scientific interest largely stems from a series of observational studies done in Finland. The first, published just over a decade ago, showed that using the sauna four to seven times per week was associated with a 40% to 60% lower risk of cardiovascular disease and death, compared with going once a week. “It was a strong signal and the impetus for a lot of labs to stand up and say, ‘Hey, there might be something here,’” said Lee, who earned his Ph.D. working with Dr. Jari Laukkanen, the research leader of the group at the University of Eastern Finland. Subsequent studies suggested other benefits, including lower rates of Alzheimer’s disease and respiratory disease and lower markers of chronic inflammation; long-running studies from Japan have also linked regular hot tub use with lower risk of cardiovascular disease. Experts stress that observational data relies on self-reports and can be skewed by other factors. “The effects are quite large, so we’re fairly optimistic, but that’s about as far we can go as scientists,” Shepherd said.

In countries where heat practices are deeply interwoven into society, such as Finland and Japan, public funding has supported some of this research. In the U.S., the National Institutes of Health is funding Mason’s mental-health work. Lee said the newly formed group hopes to secure funding from public agencies in Europe, but the kind of expensive, large-scale trials scientists envision may largely rely on foundations and private funders. Laukkanen, a professor of internal medicine at the University of Eastern Finland who has led much of the influential research on sauna in the country and was not at the gathering, welcomed the effort to organize the field and to look for ways to replicate the Finnish findings elsewhere. He noted that the challenge of conducting such studies mirrors the difficulty of studying other lifestyle habits like exercise or diet. “They are really demanding,” Laukkanen said. “Pharmaceutical companies are doing this kind of study, but they have the resources. We don’t have those yet.”