Hot baths: 30,000 people, 20 years, and one lab trial

The sauna literature gets the attention. The bath literature is in some ways better, because it has both a very large cohort and a controlled trial pointing the same direction.

The cohort

The Japan Public Health Center-based Study followed about 30,000 adults for around 20 years, grouping them by how often they took a tub bath.

Almost-daily tub bathing, against zero to two times a week, was associated with a 28% lower risk of cardiovascular disease.

The baths in question were Japanese domestic baths, typically around 40 to 42 °C, and the practice is a daily habit for much of the population rather than an intervention. That is part of why the cohort is large and part of why the comparison group is unusual — in Finland the low-exposure group is “once a week”, and in Japan it is “hardly ever”, which in that population may itself be a marker for something.

The same caveats apply as to any cohort. Observational, habits reported at baseline, and people who bathe daily may differ from people who do not in ways nobody measured.

The trial, which is the more interesting half

Brunt and colleagues took sedentary participants and gave them eight weeks of hot-water immersion at 40.5 °C, then measured vascular function directly.

Flow-mediated dilation is a measure of how well an artery widens when blood flow increases — a standard index of endothelial function, and one that predicts cardiovascular events. Roughly doubling it over eight weeks is a large change.

This is a small controlled trial rather than a cohort, and it measures an intermediate outcome rather than heart attacks. But it does what the cohort cannot: it assigned the exposure, so the direction of causation is not in question, and it shows a plausible mechanism by which the cohort association could be real.

The proposed mechanism is shear stress. Heating raises skin blood flow substantially, which increases the frictional force blood exerts on the vessel wall, which is one of the known stimuli for endothelial adaptation. It is the same signal exercise provides, arrived at without the exercise.

How Contrast counts it

Over the last four weeks, a bath day is a day with at least five minutes of hot bath in total. The weekly average is placed in one of the study’s three groups: 0–2 times a week, 3–4 times, or almost daily.

The five-minute floor matters because a two-minute rinse is not what the cohort was describing.

The app shows which group you are in. It does not multiply that by 28%, and it does not tell you to take more baths.

Why baths and saunas are kept apart

Contrast compares your sauna habit against the Finnish cohorts and your bath habit against this one, and never mixes them.

The reason is that nobody has established an exchange rate. An 80 °C sauna and a 40 °C bath both raise core temperature and skin blood flow, by different amounts, over different timescales, with different cardiovascular loads — water immersion adds hydrostatic pressure that dry heat does not. Whether twenty minutes of one equals twenty minutes of the other is an open question with no data behind it.

The weekly heat total in the app does add them together, and the metric page says outright that this is a simplification with no study behind it. Keeping the cohort comparisons separate is how the app avoids compounding that.

The practical reading

The bath evidence is, if anything, more accessible than the sauna evidence. Most people have a bath. Almost nobody has a 79 °C Finnish sauna.

Two details from the trial are worth carrying over. The temperature was 40.5 °C, which is hotter than a comfortable bath and requires deliberate effort to maintain. And the participants were sedentary — the effect in already-fit people may be smaller, since the shear-stress stimulus is one they already get.

The Contrast band chart puts 40.5 °C in the Hot band, between 38 and 41. If you are trying to reproduce the trial’s conditions rather than approximate them, that band is what to aim at, and the app will tell you how many minutes you actually spent there rather than how hot you ran the tap.