A type of fat that converts stored energy into heat. Scientists believe that brown fat may hold the key to developing successful methods for losing weight.
Fat, also known as adipose tissue, contains two distinct types of cells. Brown fat cells have very high concentrations of mitochondria, the cells’ “powerhouses,” which give them their distinctive color. They convert stored energy into heat. White fat cells are responsible for storing energy in the form of triglycerides.
Until very recently, brown fat cells were thought to be metabolically active only in hibernating animals and babies – after infancy, scientists thought, brown fat lost its mitochondria and became in function and appearance much like white fat. However, three studies in The New England Journal of Medicine changed all that. In one study, researchers exposed five healthy adults to cold temperatures and then studied their bodies with positron-emission technology (PET) scans. They found substantial amounts of brown fat and took biopsy specimens of it, which determined that the deposits were indeed metabolically active.
In another study, scientists used PET and computerized tomography (CT) scans to look for brown fat in human adults. They also found that active brown fat is present in adults, more frequently in women than in men. The amount of brown fat was inversely correlated with body-mass index, suggesting that it plays a role in adult human metabolism.
In a third study, scientists subjected 24 healthy men to mildly cold temperatures and then measured their brown fat activity using PET and CT scans. Their conclusions matched those of the earlier studies, and they also found that brown fat activity was reduced in obese or overweight men.
One exciting avenue of research is finding ways to turn skin cells and other types of cells into brown fat cells. In a study reported in the journal Nature in 2008, scientists successfully turned immature muscle cells into brown fat by activating a protein switch in the young cells. The same research group, in a study reported in Nature in 2009, took connective tissue cells from mice, turned them into brown fat using the same switch, and transplanted them back into mice. Imaging tests showed that the converted tissue was burning energy, just like naturally occurring brown fat.
This line of research suggests two possible future therapies for obesity. For example, people could take a drug that would promote the growth and activity of brown fat. Alternatively, doctors could take other cells from a person’s body, add the protein switch to turn them into brown fat cells, and transplant them back into the same person.
What remains to be seen is whether brown fat can actually cause weight loss. Researchers calculate that 50 grams of brown fat could burn roughly 300–500 calories a day, or increase the average person’s resting metabolic rate by 20%. However, the body tends to maintain a metabolic set point, keeping body weight constant. This means that activating brown fat might make a person lose weight initially, but then people would begin to eat more – and take in more calories – to compensate for the loss.