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Brunt and colleagues at the University of Oregon randomised 20 young, sedentary adults, 10 per group, to eight weeks of passive heat therapy or a sham condition, published in The Journal of Physiology in 2016. The heat group sat in water at 40.5 degrees, four to five times a week, until rectal temperature had stayed at or above 38.5 degrees for 60 minutes. The sham group sat in thermoneutral water with the same number and length of sessions. The modality was hot water immersion in a tub, not a sauna.
In the heat group, flow-mediated dilation rose from 5.6 % to 10.9 %, while the sham group showed no such change. Also in the heat group, aortic pulse wave velocity fell from 7.1 to 6.1 metres per second, carotid intima media thickness from 0.43 to 0.37 millimetres and mean arterial pressure from 83 to 78 mmHg. Dynamic compliance of the femoral artery went from 0.06 to 0.09. None of these changes appeared under the sham condition. The authors rate these effects as equal to or larger than what exercise studies typically show in inactive people.
The sample is small, with ten people per group, and consisted of young, healthy, inactive participants rather than a population at risk. The protocol of 60 minutes at raised core temperature, four to five times a week, is hardly practical in everyday life. Because the heat came from a bath, the results do not carry over to a Finnish sauna.
The sham control is what sets this trial apart: several independent vascular markers moved in the same direction in the heat group and not in the control group. What it shows is a response under laboratory conditions in young adults, not a routine for anyone else.