Ask the Doctors

Dear Doctor: Can a hot bath really burn as many calories as a brisk walk? If so, give me the hot bath any day!

Dear Reader: For readers who missed this story, scientists at Loughborough University in England were concerned about people who can't (or won't) exercise. They wondered if there might be a passive alternative, like a long soak in hot water, with similar health benefits. To find out, 14 men were split into two groups -- those who were lean, and those who were overweight. All were healthy non-smokers with no signs of cardiovascular disease. None of the men did more than 90 minutes of physical exercise per week, so all were considered inactive. The researchers targeted changes to markers of chronic inflammation, insulin resistance and glucose control, which are key factors in metabolic syndrome, an indicator of potential heart disease, diabetes and stroke.

The men were assigned to one of two activities -- relaxing in a 104-degree bath for one hour, or riding a stationary bike set at pre-determined resistance and speed for one hour. In the hours after the trial, the men ate similar meals. Blood was drawn several times, both before and after each trial. Changes to core body temperature and muscle temperature were monitored.

It's not all that surprising that the men assigned to an hour of stationary cycling burned significantly more calories than the bathers. However, it was discovered that while lounging in that hot water, the bathers burned 140 calories, or the equivalent of a 30-minute walk. Even more intriguing was the fact that while both groups had similar blood sugar trajectories before, during and after a subsequent meal, the blood sugar levels among the bathers peaked at a level that was 10 percent lower than among the cyclists. And when it came to anti-inflammatory response, which is one of the immune system's first lines of defense, the passive bathers did as well as the active cyclists.

The idea that a hot bath can offer benefits similar to those of exercise sounds counterintuitive, but the results of a couple of other studies suggest that passive heating is an important new area of research. In 2015, scientists in Finland saw a connection between the frequent use of saunas, an important part of Finnish culture, and a decrease in the risk of heart attack or stroke in men. A year later, researchers at the University of Oregon found passive heat therapy resulted in lowered blood pressure and decreased arterial stiffness in both women and men.

But don't trade your running shoes for an inflatable bath pillow just yet. Two of the three studies looked solely at men, so whether that research translates to women is not yet known. And all three studies had fairly small sample sizes. While the area of passive heat is indeed exciting and appears promising, we need more information to corroborate the existing findings.

Our take is it's a good idea to stick to the goal of at least 30 minutes per day of physical activity, which offers a host of long- and short-term physical and mental health benefits. And as you relax in a long, hot bath afterward, know that it is more than just an indulgence.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

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