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.
As well as the cardiovascular effects of passive heating, there is evidence to suggest that there may be beneficial metabolic effects as well – such as better control of blood sugar. The first study, conducted by Philip Hooper of McKee Medical Center, Colorado, in 1999, investigated the effect of three weeks of hot-tub therapy in patients diagnosed with type 2 diabetes. The results showed improvements in body weight, blood sugar control and a reduced dependence on insulin.
Hooper thought these effects may result from changes to blood flow as a result of passive heating, but he was unable to identify a specific mechanism by which their intervention led to these benefits.
It seems that activities that increase heat shock proteins may help to improve blood sugar control and offer an alternative to exercise. These activities—such as soaking in a hot tub or taking a sauna—may have health benefits for people who are unable to exercise regularly. Hopefully our future investigations, coupled with those of other groups worldwide, will help to establish the true potential of passive heating as a therapeutic tool.