Exercise makes our fat cells burn calories.

A newly discovered hormone produced in response to exercise may be turning people’s white fat brown, a groundbreaking new study suggests, and in the process lessening their susceptibility to obesity, diabetes and other health problems. The study, published on Wednesday in Nature and led by researchers at the Dana-Farber Cancer Institute and Harvard Medical School, provides remarkable new insights into how exercise affects the body at a cellular level.

http://well.blogs.nytimes.com/2012/01/11/exercise-hormone-helps-keep-us-healthy/?hpw

...Unlike most substances birthed in the muscles, irisin does not completely remain there, the scientists noted. It apparently enters the bloodstream and surfs to fat cells, where, by providing various biochemical signals or messages, it begins turning regular fat — especially deep, visceral fat clustered around organs — into brown fat....

Brown fat, as many of us have heard, is physiologically desirable. While white fat cells are essentially inert storehouses for fat, brown fat cells are metabolically active. They use oxygen and require energy. They burn calories.

Until recently, it was thought that human adults did not have brown fat, that we lost our stores after babyhood. But beginning in 2009, a number of studies showed that grown-ups do harbor brown fat. Some people just have more than others.

And it may be that irisin, and exercise, partially determine how much brown fat each of us contains, the new study suggests....