A drug used to treat people with overactive bladder can also boost the calorie-burning capacity of the body's brown fat, new findings show.

Unlike its cousin "white fat," which stores calories, brown fat actually burns calories, helping babies and hibernating mammals to stay warm. In 2009, five research teams reported finding functional brown fat in adult humans.

Now, investigators hope that cranking up the metabolic activity of brown fat could help people lose weight, as well as bring other metabolic benefits. Previous studies have shown that exposing someone to cold can activate brown fat, but the new study shows that a medicine may have the same effect.

"I would say the results are promising, but there's a lot that we still have to figure out," said Dr. Aaron Cypess, head of the Diabetes, Endocrinology, and Obesity Branch at the National Institute of Diabetes and Digestive and Kidney Diseases. Cypess conducted the study while at the Joslin Diabetes Center in Boston, and was also one of the researchers to identify brown fat in adults for the first time.

"We don't know if the drug is effective when taken chronically," either at a dose of 50 milligrams daily, which is the amount used for treating people with overactive bladder, or a dose of 200 mg daily, which the researchers used in their study, Cypess said. "Since all drugs have side effects, it's not ethical for us to recommend its use for weight loss until we have more information on both efficacy and safety."

The medication, mirabegron, works by activating receptors called beta-3 adrenergic receptors. Both brown and white fat cells, as well as bladder tissue, have these receptors. Researchers have been trying to develop a drug that activates these receptors for over two decades, Cypess said. The idea was that stimulating these receptors might burn white fat, he said. But many of the experimental medications tested had cardiovascular side effects, and in clinical trials, none were found to be effective. [Lose Weight Smartly: 7 Little-Known Tricks that Shave Pounds]

In the new study, Cypess and his team screened 15 healthy men for brown fat activity. The 12 men with detectable amounts of brown fat then underwent two imaging scans, at least 48 hours apart, that let researchers see the activity levels in their brown fat. On one day, the men received a placebo, and on the other, they were given 200 mg of mirabegron.

The scans showed that brown fat took up significantly more glucose when the men received mirabegron treatment. In other words, the drug seemed to prompt the fat cells to use more energy. On average, study participants burned 203 more calories per day when on the drug, compared with days they took the placebo, an increase of 13 percent. This would translate to a weight loss of about 11 pounds (5 kilograms) in one year, according to the researchers.

The drug did have some side effects — the study participants' heart rate increased by an average of 14 beats per minute when they took the drug, and their systolic blood pressure increased by 11 mmHg. However, Cypess and his colleagues noted, these increases were less than those seen with other drugs that boost metabolic rate, such as ephedrine.

Cypess said he and his colleagues hope that activating brown fat may have benefits beyond just burning more calories, for example, helping to treat people with fatty liver disease. Researchers don't yet know everything about what brown fat does in the body, such as what hormones it may release, he said.

"We're certainly excited about the energy-burning capacity, but we're thinking about the more broad physiological basis of what brown fat may be able to do," he said.

Copyright 2015 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.