The calorie-burning and heat-generating brown fat found in full-grown humans is actually not quite brown; it's beige. So says a new study reported on July 12th in the journal Cell, a Cell Press publication, in which researchers fully characterize this promising obesity-fighting tissue in both mice and humans for the first time.
The findings could lead to more specific ways to address the epidemic of obesity and diabetes by giving those beige fat cells a boost, the researchers say.
"We've identified a third type of fat cell," said Bruce Spiegelman of Harvard Medical School. "There's white, brown and now there is this third type that is present in most or all human beings."
In fact, brown fat was once thought to exist only in babies, where it serves to keep them warm. More recent imaging data suggested that adults, too, maintain some brown fat.
But Spiegelman's team previously showed that the energy-burning brown fat found amongst energy-storing white fat in adults wasn't exactly the classical brown fat you see in babies. Babies' brown fat arises from muscle, but these adult brown fat cells arise from the "browning" of white fat.
In the new study, Spiegelman's team cloned beige cells taken from the fat tissue of mice to study their activity at the genetic level.
These expression profiles showed that beige cells are genetically somewhere in between white fat and brown fat. At baseline, they have low levels of uncoupling protein 1 (UCP1), a key ingredient for burning energy and generating heat, similar to white fat. But the beige cells also have a remarkable ability to ramp up their UCP1 expression, turning on an energy-burning program that is equivalent to that of classical brown fat.
The team also found that beige cells respond to a hormone known as irisin to turn on the energy-burning program. That's particularly notable because irisin is released from muscle with exercise, and it is responsible for some of the benefits that physical activity brings.
Irisin might just be the long-sought treatment aimed to increase those coveted energy-burning fat cells. In addition to the therapeutic potential, the new findings might also lead to new and better ways to characterize important differences amongst people in the numbers of beige cells they carry, Spiegelman says.