Many cultures swear by the benefits of a hot bath. But scientists have only recently started to understand how passive heating (as opposed to getting hot and sweaty from exercise) improves health.
At Loughborough University, we investigated the effect of a hot bath on blood sugar control, an important measure of metabolic fitness, and on energy expended.
We recruited 14 men, who were assigned to an hour-long soak in a 40 deg C hot bath or an hour of cycling. These activities were designed to cause a 1 deg C rise in core body temperature over the course of one hour.
We measured how many calories the men burnt at each session. We also measured their blood sugar for 24 hours after each trial.
Cycling resulted in more calories being burnt, compared with a hot bath. But bathing resulted in about as many calories being burnt as a half-hour walk (about 140 calories).
The overall blood sugar response to both conditions was similar, but peak blood sugar after eating was about 10 per cent lower when participants took a hot bath, compared with when they exercised.
We also showed changes to the inflammatory response similar to that following exercise.
The anti-inflammatory response to exercise is important as it helps to protect us against infection and illness. Chronic inflammation is associated with a reduced ability to fight off diseases.
This suggests that repeated passive heating may contribute to reducing chronic inflammation, which is often present with long- term diseases such as Type 2 diabetes.
PASSIVE HEATING AS THERAPY
Passive heating for human health is a relatively new field of research, but some exciting results have emerged over the past few years.
Research in Finland, published in 2015, suggested that frequent saunas can reduce the risk of having a heart attack or stroke - at least in men.
The University of Oregon published a study last year showing that regular hot baths can lower blood pressure. To establish the effect of increasing body temperature passively as opposed to doing this 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, and a greater reduction in average arterial blood pressure. This is important as a reduction in blood pressure is 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. There is evidence to suggest there may be beneficial metabolic effects as well.
In 1999, Dr Philip Hooper of McKee Medical Centre in Colorado checked 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.
Dr Hooper thought these effects may have resulted from changes to blood flow as a result of passive heating. However, he was unable to identify a specific mechanism by which their intervention led to these benefits.
Since then, few studies have investigated the potential for passive heating to improve blood sugar control in humans.
Studies using animals may have identified how heating affects health. These studies suggest one of the key regulators of blood sugar control may be heat-shock proteins.
Heat-shock proteins are mole- cules that are made by cells in the human body in response to stresses. Their levels rise following exercise and passive heating.
In the long term, raised levels of these proteins may help the functioning of insulin and improve blood sugar control.
Conversely, heat-shock proteins have been shown to be lower in people with diabetes.
It seems that activities that increase heat-shock proteins may help to improve blood sugar control and offer an alternative to exercise. And soaking in a hot tub or taking a sauna may have health benefits for people who are unable to exercise regularly.
Hopefully, future investigations will help to establish the true potential of passive heating as a therapeutic tool.
•Dr Steve Faulkner is a research associate at Loughborough University
•This article first appeared in The Conversation at http://theconversation.com, a website which carries analysis by academics and researchers.