Brown fat makes you thin.

From New Scientist:

In the war on our waistlines, fat is the enemy. It is fat, or adipose tissue, that gives us our beer bellies and our love handles, our man boobs and our muffin tops. And when plastic surgeons sculpt people into slenderness, it is fat tissue they suck up and throw out with the clinical waste.

It seems odd, then, that a type of fat tissue could also be the key to weight control. Not ordinary white adipose tissue, but a special kind called brown fat.

In some mammals, brown fat turns the energy obtained from food into heat, burning up calories without the animal expending any effort. It used to be thought that adult humans had no brown fat, but a raft of new evidence indicates that this is wrong, and it is in fact present and functional in at least some individuals. Differences in the amount of brown fat each person has may help to explain why some of us are slim while others are overweight, and why many of us pile on the pounds as we age.

Researchers are experimenting with various ways to increase the amount or activity of our brown fat, either pharmaceutically or even surgically, by extracting ordinary white fat through liposuction, transforming it into brown fat and re-implanting it. A mere 50 grams of brown fat - well within the range of what some of us already have - could dissipate around 500 calories a day. "I exercise on an elliptical trainer and it's pretty hard for me to burn up 500 calories," says Ronald Kahn, head of obesity research at Harvard Medical School's Joslin Diabetes Center. "If I could do it without working and do it every day, it would be pretty great."

Brown fat's role in heat generation, also known as thermogenesis, has been extensively studied by animal physiologists. It turns out that brown fat cells have unusual mitochondria, the tiny structures found in almost all cells that release energy from food. In the vast majority of cells this energy is either stored or used to power cellular processes. The mitochondria in brown fat, however, contain a protein called thermogenin (or uncoupling protein 1), which causes energy to dissipate as heat. "This is a tissue whose sole purpose is burning energy," says Francesco Celi, a researcher at the US National Institutes of Health.