Q. What is brown fat?
A. Brown fat has been getting a lot of attention as a type of fat that helps you burn energy rather than simply storing it. Unlike typical “white” fat cells, which serve as storage depots for fat, brown fat cells burn the fats they contain in order to produce heat when the body is cold. “What makes it brown is iron,” says Aaron Cypess, an endocrinologist at Joslin Diabetes Center and Beth Israel Deaconess Medical Center. Iron is found in blood and in mitochondria, parts of the cell that burn energy, and brown fat has more of both.
Rodents use brown fat to keep warm, as do human newborns, who lack the ability to warm themselves by shivering. But only in the past several years has brown fat been detected in adult humans. Cypess says that more brown fat is detected in lean people than obese people, and more young people than old people. Recent studies have found that brown fat activates in cold temperatures, causing people to burn more calories, and that both shivering and exercise seem to convert white fat to brown fat.
Cypess says that research is still trying to uncover whether the small amounts of brown fat that adults carry can actually have an effect on weight, or whether we could grow more brown fat to slim down. Another question is whether brown fat, like white fat, releases chemical signals that affect the rest of the body, beyond its heat-generating role.