A drug already used to treat overactive bladder may also someday help control weight by boosting the metabolic powers of brown fat, a small study suggests. While white fat stores energy, brown fat burns energy to generate body heat. In the process, it can help maintain body weight and prevent obesity, at least in animals, previous studies have shown. In the new study, researchers gave 12 healthy, lean young men a high dose of the drug mirabegron (Myrbetriq), and found that it boosted their metabolic rate.

The drug “activates the brown fat cells to burn calories and generate heat,” said study researcher Dr. Aaron Cypess. He is section head of translational physiology at the U.S. National Institute of Diabetes and Digestive and Kidney Diseases. When the activity of the drug peaked, “the metabolic rate went up by 13 per cent on average,” Cypess said. That translates to about 203 calories, he said. However, Cypess said that doesn’t necessarily mean the men would burn an extra 203 calories a day over the long-term.

The researchers don’t yet know how long the calorie-burning effect might last, as they didn’t follow the men over time. The researchers projected the three-year weight loss would be about 22 pounds. Cypess conducted the research while working at the Joslin Diabetes Center and Harvard Medical School. The study was funded by the US National Institutes of Health, with no drug company involvement.

The men, whose average age was 22, took a single dose of the drug in one session and took a single dose of a placebo in another, serving as their own comparisons. The researchers measured metabolic rate by scans, including positron emission tomography and CT scans. The effects of the drug on fat-burning, Cypess said, would be “mild to moderate if sustained.” The drug works by activating what is known as a beta 3-adrenergic receptor, found on the surface of brown fat cells. It is also found on the urinary bladder cells, and the drug works to calm an overactive bladder by relaxing muscle cells there, he said.

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