Unlike white fat, which stores calories, brown fat burns calories like a furnace. Its main job seems to be to keep the body warm.
People have deposits of brown fat between the shoulder blades and along the spine, around the heart, on the side of the neck, and near the collarbones. But those stores are tiny compared to the pounds of white fat most people carry. Researchers have wondered if adding more brown fat might jump-start a body’s sluggish metabolism.
To test that theory, researchers took a tenth of a gram of brown fat from the backs of male mice and injected it into other mice that were the same sex and age.
Scientists have tried brown fat transplants before but they haven’t worked very well, says researcher Laurie J. Goodyear, PhD, head of the Section on Integrative Physiology and Metabolism at Harvard’s Joslin Diabetes Center in Boston.
Goodyear thinks that was partly because of where the brown fat was placed in the body and how long researchers waited to see results.
This time, Goodyear and her team placed the brown fat in the gut, a place it’s not normally found. Studies have shown that fat in the abdomen, especially fat around the liver, influences insulin resistance and also the release of blood fats called triglycerides.
“These effects were really very pronounced and very dramatic,” Goodyear says.
Encouraged by their results, the team next gave brown fat transplants to mice on high-fat diets. In both people and mice, high-fat diets reliably lead to weight gain and drive up blood sugar, setting the stage for type 2 diabetes.
Brown fat appeared to blunt some of the effects of the high-fat diet. Transplanted mice maintained their weight and blood sugar better than those who didn’t get transplants. And the more brown fat they got, the stronger the benefits seemed to be.
How is brown fat working? Further testing on transplanted mice showed they had higher levels of proteins and molecules that control how the body handles blood sugar.
The study is published in the Journal of Clinical Investigation.
Human Testing on the Horizon
How soon could the technique be tried in humans?
“I think it’s a ways off,” Goodyear says, “but I think that the studies that we’ve done here really support the idea that it could work.”
Other researchers say the study is interesting, but they aren’t yet convinced that brown fat will someday become a treatment for obesity or diabetes.
“Its purpose is to produce heat and fight cold exposure. It’s certainly not there to combat obesity. It has no purpose in evolution to do that,” says Andre Carpentier, MD, a professor of diabetes and physiology at the University of Sherbrooke in Quebec, Canada.
Carpentier studies brown fat in people, but he was not involved in the current research.
Carpentier says if more studies show that brown fat can safely be increased in the body, it could one day become a tool to fight weight gain and diabetes.
But he says it’s important to keep in mind that even under the best circumstances, the effects of brown fat are likely to be modest.
“You may end up burning a little bit more calories at the end of your day, but it’s not going to be anything close to what you can achieve by doing exercise and diet,” Carpentier says.