The field of health, fitness, nutrition and supplements is bewildering. It can be near impossible to know what to believe with the sheer volume of research being reported in the media and the almost daily rebuttals, controversies and debunkings. Are eggs good or bad for you? Will red meat give you cancer? Is saturated fat bad? Are mushrooms more effective than chemotherapy? Unfortunately, journalists often present misleading (and sometimes plain false) versions of the science.
My idea with this new blog series is to provide an insight into what the science of health really says, paper by paper. Most of us will never have the time to survey the whole landscape of research on any particular subject. But you can learn a lot by paying careful attention to particular, high quality studies. I will only look at papers that are well regarded by people that I trust as adjudicators on these fields. I’ll be casting a critical (if non-professional) eye over the research to give you my take on what conclusions we should draw, what potential actions we should take and the degree to which we should trust the results. Almost everything will be in shades of grey and forms of ‘it depends’. That’s the nature of health science and you should get used to it.
I am by no means a medical professional or academic scientist, so you should certainly not take anything I say as advice and you should probably add a pinch of salt too. I’m just a nerd who loves to dig in to the science and has the inclination to read papers start to finish with a critical eye. If you think I’ve got anything wrong, I’d love to hear your thoughts. I’m more than happy to pointed in the right direction. Just email me at email@example.com
Today’s topic will be the sauna. Saunas have become very popular and all sorts of health benefits are touted. But what does the science say? In particular, what does the science say about saunas and heart disease?
In this paper, the authors followed a group of 2,315 Finnish men for 20 years to investigate the association between sauna use and cardiovascular disease and death. The headline: they found that men who used the sauna 2-3 times per week were 27% less likely to die of cardiovascular disease and 24% less likely to die of any cause than were men who used the sauna only once a week, after controlling for other major risk factors. These numbers improved more for men that used the sauna 4-7 times per week. They had reduced risk of CVD and all-cause mortality of 50% and 40% respectively. There were also similar effects seen for sudden cardiac death and coronary heart disease (see below).
One of the things that can be misleading when it comes to research in this field is the difference between relative risk and absolute risk. The truth is, if an intervention reduces your risk of a disease from 0.0001% to 0.00005%, you don’t really care, even though that represents a 50% reduction in risk. The difference in relative risk is 50% but the difference in absolute risk is 0.00005%. It’s clearly the latter that we care about. So what are the absolute risks we are talking about in this study?
The good news is that since we are considering prevalent diseases, the absolute risk effects are fairly large. Men who used the sauna once a week had a 22% risk of dying of CVD over the 20 year period. Men who used the sauna 2-3 times per week had a 16.5% risk and men who used the sauna 4-7 times per week had an 11.9% risk. These numbers don’t take into account other risk factors so there may be some confounding involved, but the absolute risk differentials of 5.5% and 10.1% respectively tell a significant story. You definitely care if you can reduce your risk of dying of CVD from 22% to 11.9%!
That being said, there are a few important limitations to the study. It’s always difficult to tease out causality and mechanisms from associative studies. Did sauna users have better outcomes because of the sauna or because those who sauna more are typically less busy and less stressed? Is it the sauna itself that created the benefit or the relaxation time that it implies or the social effect of having a sauna with friends?
The researchers controlled for physical activity and certain other lifestyle factors, but it’s possible there is still a hidden ‘healthy user bias’. In other words, it’s possible that people who care more about their health use the sauna more and it could therefore be their generally healthier lifestyles that account for better cardiovascular outcomes rather than the sauna itself. The concept of the healthy user bias is a powerful tool to keep in mind when reading research and can radically change the way you interpret results about, for example, associations between red meat and cancer.
The other major issue is that the population being studied were not, in my opinion, particularly healthy. The mean LDL cholesterol level was 4 mmol/L (numbers shown in mg/dL below). Typically, being over 3 mmol/L is considered a risk factor for CVD. 30% of the participants were smokers, the average blood pressure was 134 / 88 and the mean VO2 max was 30. I wonder what effect the sauna would have on healthier individuals. What benefits does the sauna have over and above an active lifestyle? The authors themselves note that some of the benefits of the sauna seem to be mediated by mechanisms similar to exercise. The sauna raises your heart rate for example, sometimes to over 150 bpm, suggesting that using the sauna is similar to doing cardio. The researchers point out that the sauna seemed to confer greater benefits on those who had lower levels of cardiovascular fitness. I would love to see a study done on healthy, active individuals to tease out the sauna-specific effects.
So it might be that sauna use is less protective of heart health for active, healthy individuals than it is for the sedentary. The truth is that most interventions will benefit the unhealthy more than the healthy. It’s a bit like newbie gains. In the beginning, most things will work. But I think the case for sauna use being protective of heart health is still fairly strong, just based on the fact that the effect sizes are so large. There is a lot of margin for error in the results. Although some of the sauna’s mechanisms are similar to exercise, some are distinct. The level of heat-stress caused by saunas is much greater than with exercise, for example. For my part, I’m going to continue to use the sauna 3-4 times per week, for 15 minutes at around 75-80 degrees C.