This exact connection between exercise and weight loss has been a bit of a mystery even to the medical community, but a new study, published in Nature and led by researchers at the Dana-Farber Cancer Institute and Harvard Medical School, has helped clarify exactly what's going on at a cellular level when you go for a run or hit the elliptical. In a nutshell -- exercise causes your muscles to tell your white fat to turn into brown fat.
What’s that? You didn’t know your muscles could talk to your fat cells? And what the heck is the difference between white fat and brown fat? We’ll explain.
“The more white fat you get, the more likely you are to develop health problems like diabetes and heart disease,” Cypess says.
OK, so what about this mysterious brown fat? “Brown fat stores calories not for future use as an energy supply, but as fuel to burn off when it’s activated,” Cypess explains. He likens white fat to a huge gas tanker storing an enormous amount of fuel, while brown fat is a Lamborghini — it has gas in it because it needs it to run. Brown fat burns off calories specifically to generate heat, says Cypess, and it can do that in response to cold, or, it seems, in response to muscular activity (though further research is needed to prove this, says Cypess).
What’s exciting about the Dana Farber study is that, for the first time, researchers discovered that when you exercise, your muscles release a hormone the researchers named “irisin” (after Iris, the Greek messenger goddess). And what does irisin do? Cypess describes it like this: “It goes to the fat that is under the skin and induces it to grow more brown fat.”