Fat people have less than thin people. Older people have less than younger people. Men have less than younger women.

It is brown fat, actually brown in color, and its great appeal is that it burns calories like a furnace. A new study finds that one form of it, which is turned on when people get cold, sucks fat out of the rest of the body to fuel itself. Another new study finds that a second form of brown fat can be created from ordinary white fat by exercise.