… The idea that passive heating can improve cardiovascular function received further support when the University of Oregon published a study the following year showing that regular hot baths can lower blood pressure.
In a second study, the same group looked at the mechanism responsible for these improvements. They found that passive heating raised levels of nitric oxide, a molecule that dilates blood vessels and reduces blood pressure. This has implications for treating high blood pressure and improving peripheral circulation in people with type 2 diabetes. As type 2 diabetes is associated with reductions in nitric oxide availability, passive heating may help re-establish a healthier nitric oxide level and reduce blood pressure.
In order to establish the effect of increasing body temperature passively, as opposed to through exertion, another study matched the intensity of heating from water immersion to that of running on a treadmill. Water immersion resulted in a greater increase in body temperature compared with exercise, as well as a greater reduction in average arterial blood pressure. This is important as a reduction in blood pressure is closely associated with a reduced risk of developing heart disease. This study points to the promising effect that may result from passive heating. It also suggests some of the cardiovascular effects of passive heating may be comparable with those of exercise.