Many cultures swear by the benefits of a hot bath. But only recently has science began 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 (number of calories burned). We recruited 14 men to take part in the study. They were assigned to an hour-long soak in a hot bath (40˚C) or an hour of cycling. The activities were designed to cause a 1˚C rise in core body temperature over the course of one hour.
We measured how many calories the men burned in each session. We also measured their blood sugar for 24 hours after each trial.
Cycling resulted in more calories being burned compared with a hot bath, but bathing resulted in about as many calories being burned as a half-hour walk (around 140 calories). The overall blood sugar response to both conditions was similar, but peak blood sugar after eating was about 10% 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, but 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.
Exciting new field of research
Passive heating for human health is a relatively new field of research, but some exciting results have emerged over the past few years.
Research from Finland, published in 2015, suggested that frequent saunas can reduce the risk of having a heart attack or stroke – at least in men. 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.