TUESDAY, Jan. 6, 2015 (HealthDay News) -- 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 percent 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.
The study was published Jan. 6 in Cell Metabolism.
Cypess conducted the research while working at the Joslin Diabetes Center and Harvard Medical School. The study was funded by the U.S. 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 (PET) 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.
Much more research is needed, Cypess said. The study was small, including young men who on average had healthy body weights. Much less is known about the role of brown fat in people than in animals, he said.
In future research, he said he hopes to study larger groups, including women.
The dose given to the men was 200 milligrams (mg) a day, versus 50 mg for overactive bladder. Cypess cautioned that people should not take the drug expecting to lose weight.
While the 200-mg dose was generally well-tolerated, it did raise the heart rate to abnormally high levels, increasing it by 14 beats a minute on average, which Cypess said was too high. He hopes to investigate lower doses in the future to see if they are also effective, he said.
Connie Diekman, director of university nutrition at Washington University in St. Louis, reviewed the findings. "I would put this study into the category of interesting research that needs much more study before there is any consideration of this drug therapy as an aid to weight loss," she said.
The study is small, and "leaves many questions related to real impact on weight loss," she said.
"The subjects were all lean, so how this drug might impact an overweight subject is not clear," Diekman added.
The amount of projected weight loss is small if an individual is overweight, she said. And, as noted by the researchers, the dose used could lead to an abnormally high heart rate, which is not healthy, she said.
Cypess agreed that much more research is crucial. If more research bears out the findings on the drug, "a realistic hope is that it will be incorporated into a general plan of saying, 'Eat responsibly, exercise regularly and keep your brown fat active,'" he said.
Currently, he said, there is no practical way for people to find out how much brown fat they have, "but it is an active area of research."
To learn more about brown and white fat, see National Institutes of Health.