(Photo: Orin Zebest)
This is because of the way that brown fat reacts to cold temperatures. Brown fat is adipose tissue that can burn and create heat. When a person exercises, s/he produces the hormone irisin, which converts white fat to brown fat.
Until a few years ago, scientists thought that humans have brown fat only in infancy. As they age, they lose their brown fat and shiver as a substitute means of staying warm. Now we know that adults retain brown fat.
The researchers for this study had test subjects either exercise vigorously, exercise leisurely or lie on a bed in a cold room. They found that the people who exercised vigorously and those that lay shivering in bed produced high levels of irisin. Thus they both metabolized brown fat.