It's long been thought that adults have just one type of fat: the white/yellow, lumpy fat that gets blamed for spare tires. White fat stores any extra calories you eat for your future energy needs, but if you have too much of it, it's also associated with health problems such as insulin resistance and heart disease. As it turns out, though, it's not the only kind of fat in our bodies.
Humans also have brown fat, located in deposits in the neck and around collarbones. Although people have an abundance of it when they're kids, we have far less as adults, and some of us don't have any at all — studies have found that thin people are more likely to have brown fat than people who are overweight or obese, but no one is sure why that is, at least not without further investigation [source: Doheny].
Brown fat doesn't behave the same way as white fat. Brown fat gets its color and name from its iron-rich mitochondria, and scientists describe its function as more similar to muscle than how we've always thought fat behaved. For instance, brown fat is a good insulator because it burns white fat for energy. Burns? Yes. While white fat stores energy and is associated with weight gain, brown fat is thermogenic. It burns energy for heat and may play a role in weight loss. Under the right circumstances, an average healthy person at a healthy weight could burn as many as 250 calories in a single day just because of their brown fat stores, and as it turns out, all that's needed is a cold environment to activate brown fat. The bigger our brown fat stores, the bigger the benefit; just 2 ounces (56 grams) of brown fat activity could burn as many as 8 to 9 pounds per year [sources: Cypress, Raloff].