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The Fat That Makes You Skinny

While many of us struggle to keep the amount of fat in our bodies to a minimum, there is one type of fat that we may want to encourage more of. Brown fat (also called brown adipose tissue) is our friend. It has the ability to burn calories at a great rate, particularly when it is stimulated by exposure to cold temperatures.

As opposed to white fat, whose purpose is to store calories, brown fat essentially sucks white fat from the body to use for fuel, leading to a reduction in overall body fat. Researchers are now looking into ways of stimulating the body to replace white fat with brown. 

Both human babies and animals are born with a significant amount of brown fat in the body. In infants it is concentrated around the upper back and trunk area to provide insulation and heat-generating activity to keep an infant warm, as they do not have the ability to shiver to generate heat. It was originally thought that this type of fat was no longer present in adulthood, but researchers have found residual amounts that are activated when people are exposed to cold and when they exercise.

Interestingly, those who are obese have been shown to have little or no brown fat. It appears in the greatest amounts in people who are thin, which is perhaps why they are slender in the first place. Women have more brown fat than men, and young people have more than older people. It can increase the metabolism by up to 80 percent and generates warmth. The presence of brown fat can be seen around the lower neck, the clavicle and along the spine on a PET scan when the subject is put in a cold room.

Until very recently, researchers did not know what particular mechanism caused the brown fat to be activated. However, scientists from the University of California, San Francisco (UCSF) have discovered a protein that spurs brown fat into action. Dr. Yuriy Kirichok, associate professor of physiology, and colleagues found that the protein called uncoupling protein 1 (UCP1) causes the mitochondria in the cells of brown fat to burn energy and generate heat. There are more mitochondria in brown fat cells than in the body’s other cells (including white fat cells), so they have greater potential for energy burning.

Those who have low levels of brown fat have also been shown to have low bone mineral density. Dr. Clifford Rosen, professor of medicine at Boston’s Tufts University School of Medicine was shocked by the state of the bones in his mouse study subjects. “The animals have the worst bone density we have ever seen,” he said. “I see osteoporotic bones all the time, but, oh my God, these are the extreme.”

So brown fat has the possibility of not only aiding in weight loss, but may also aid in maintaining good bone health. Researchers still have a lot of work to do, but they intend to take what they have discovered about brown fat and develop therapies that may be able to help people generate more of their own brown fat in the future.


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