Thirty minutes in 40C water lowered blood pressure more than moderate cycling in a 25-person trial

Hot water immersion, like exercise, increases blood flow and shear stress along the inside walls of blood vessels. That stress triggers adaptations that improve vascular function, according to ScienceAlert's summary of the research ^[6]. Earlier studies found that repeated immersion improved vessel function, reduced arterial stiffness and helped vessels widen through nitric oxide. Those studies recruited young, healthy adults ^[7].

The new trial, published in the European Journal of Applied Physiology ^[1], went after a different group. "These findings suggest that hot water immersion may act as an 'exercise-like' vascular stimulus, but evidence in older individuals with risk factors for cardiovascular disease who are unable to engage in conventional exercise is lacking," the researchers wrote ^[8]. They cite global estimates that "approximately 31 percent of adults worldwide do not meet physical activity guidelines" ^[5].

The crossover is the right design for 25 people. Each participant is compared with themselves across both sessions ^[10]. Between-person differences in fitness, weight and resting pressure drop out of the comparison. In a two-group study this small, those differences could swamp the effect. Randomising the order means neither session gains from always coming second.

The authors are candid about where the larger vessel response comes from. "Collectively, these findings suggest that the larger microvascular responses observed after hot water immersion likely reflect the combined effects of direct local skin heating and greater systemic thermal strain imposed by passive heating," they wrote ^[16]. They tie the widening of vessels after immersion to the body shedding heat by moving blood to the skin ^[17]. By their own account, part of the margin over cycling comes from warming the skin that the scanner was imaging ^[16].

ScienceAlert's report does not give the size of the blood pressure falls in millimetres of mercury, or how long they lasted once participants were out of the water. Each arm was one 30-minute bout ^[11][12], so the result describes an acute response to a single session.

Every participant also completed the cycling arm at 60 to 70 percent of maximum heart rate ^[12]. The sample, in other words, was made up of people able to do moderate exercise ^[18].

I think the evidence supports the modest version of the claim. ScienceAlert's write-up frames passive heat as a possible way for people unable to exercise to get some of the same benefits ^[4]. The same write-up notes that this was a small experiment and that nobody is suggesting people give up exercise for hot baths ^[3].