You might be surprised to learn that, from the point of view of physiology, not all fat is the same. And you might be even more surprised to learn that not all fat is bad. While many of us struggle to keep the amount of fat in our bodies to a sensible minimum, there’s one type of fat that we might actually want some more of… Seriously!
When it comes to weight management, brown fat (also called brown adipose tissue) is our friend in that it has the ability to burn calories at a very high rate, particularly when it is stimulated by exposure to cold temperatures. Unlike white fat, the purpose of which is to store energy (calories) for future use, brown fat essentially sucks white fat from the body to use for fuel, leading to a reduction in overall body fat. That’s why researchers are now exploring ways to encourage the body to replace its stores of 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 generate heat, keeping babies warm until they develop the ability to shiver. While it was long believed that brown fat isn’t present in the bodies of adults, researchers have found residual amounts that are activated when people are exposed to cold and when they exercise. It can actually increase the metabolism by up to 80 percent!
Interestingly, those who are obese have been shown to have little or no brown fat. It actually appears in the greatest amounts in people who are thin, which is perhaps why they are slender in the first place. Women generally have more brown fat than men, and young people have more than older people. 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 understand the mechanisms that cause 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 may play a role in helping us maintain healthy bones in addition to helping us manage our weight. While researchers still have a lot of work to do, the hope is that they will be able to build on what they’ve learned about brown fat to develop therapies to help people generate more of their own brown fat in the future.