Hot baths and blood vessels: a small controlled trial
A small randomized trial tested regular hot-water immersion, not sauna, against a sham treatment. Blood vessel function improved markedly in eight weeks: a promising signal from twenty people, not yet a broader answer.
The short version
In a small randomized trial, 20 sedentary young adults were assigned to eight weeks of hot-water immersion or a sham (thermoneutral) treatment, four to five sessions a week. In the heat group, blood vessel dilation nearly doubled, artery stiffness fell, and average blood pressure dropped by about five points; the sham group showed no change in these measures. The trial used hot baths, not sauna, and included just twenty young, healthy people. It is a well-controlled signal about passive heat and blood vessels, not evidence about sauna, and not tested against exercise directly.
Keep in mind
A well-controlled but very small trial (20 people), all young and healthy, testing hot-water immersion rather than sauna, over eight weeks. It is not a long-term or real-world test.
What they found
In the heat group, flow-mediated dilation (a measure of how well blood vessels widen) rose from 5.6% to 10.9%. Aortic pulse wave velocity, a measure of artery stiffness, fell from 7.1 to 6.1 meters per second. Carotid artery wall thickness fell from 0.43 to 0.37 millimeters, and average blood pressure fell from 83 to 78 mmHg.
None of these measures changed in the sham group.
The researchers describe these changes as similar in size to what exercise-training studies typically report in the published literature. This trial did not include an exercise group, so that comparison is drawn from outside studies, not from a head-to-head test here.
What this means for your practice
This is encouraging early evidence that passive heat exposure can change blood vessel measurements, from a small, well-designed trial. It used a monitored hot-water protocol, not a sauna, and did not test whether the same changes translate into fewer heart attacks or strokes.
This does not establish a sauna routine to follow, or show that a KUUME ritual has these effects.
Methods and limitations
What the researchers did
Researchers randomly assigned 20 sedentary young adults to eight weeks of either hot-water immersion (seated in 40.5°C water long enough to hold their core body temperature at 38.5°C or above for 60 minutes) or a sham treatment using thermoneutral water, four to five times a week.
This was a sham-controlled randomized trial, a stronger design than the observational Finnish cohort studies elsewhere in this library: participants were assigned to a group by chance, and the comparison group underwent a matched but inactive water treatment.
Before and after the eight weeks, the researchers measured blood vessel dilation (flow-mediated dilation), artery stiffness (aortic pulse wave velocity), carotid artery wall thickness, and blood pressure.
How much can this tell us?
The randomized, sham-controlled design is a real strength. It is the strongest study design in this library, and stronger evidence for cause and effect than the observational Finnish cohort studies.
The trial is very small (20 people) and used young, healthy, sedentary volunteers, so it does not show whether the same changes would occur in older adults or people with existing heart disease.
It tested hot-water immersion at a controlled, monitored temperature (core temperature held at 38.5°C or above for 60 minutes), not a typical sauna session. The findings should not be read as sauna research.
What makes it useful
- Randomized, with a genuine sham (inactive) control group
- Multiple validated vascular measurements, before and after
- Consistent improvement across several different measures
What limits it
- Very small trial: 20 people in total
- Young, healthy, sedentary volunteers only, not older or at-risk adults
- Hot-water immersion, not dry sauna: a different form of heat exposure
Where this study fits
Where this study fits
We score claims, not single studies. This one is part of the evidence for:
Regular heat keeps blood vessels healthy. Not supported
Low certaintyNo effect foundBody signals
Read the original
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. 2016;594(18):5329-5342.
DOI 10.1113/JP272453