"Our muscle cells need a source of energy when they exercise," says Dr. Anthony Komaroff, a professor at Harvard Medical School. "Muscles get that energy by burning fat and sugar brought to them by the blood. That's been known for nearly a century. However, it's not the whole story. "
The hormone irisin
In January 2012, a research team led by Dr. Bruce Spiegelman, a Harvard Medical School professor, published a new study in the journal Nature. The study was done in mice, but may well apply to humans. The study showed that exercising muscle produces a hormone called irisin.
"Irisin travels throughout the body in the blood, and alters fat cells," explains Dr. Komaroff. "Body fat is stored inside fat cells. Most of these fat cells are called white fat cells, and their function is to store fat."
White fat vs. brown fat
Why do we store fat? When we eat more calories than we burn by exercise, the extra calories have to go somewhere. They're stored partly as fat. Our distant ancestors didn't eat as regularly as we do. Forty thousand years ago on the Serengeti, our ancestors were able to get a serious meal only a few times each week. In between meals, they needed some source of energy. A large part of it came from the fat they stored away after a meal.
In 2009, studies from Harvard Medical School and elsewhere discovered that humans have not only white fat cells but also brown fat cells.
"Brown fat cells don't store fat: they burn fat. If your goal is to lose weight, you want to increase the number of your brown fat cells and to decrease your white fat cells," says Dr. Komaroff.
Irisin does that, at least in mice. And those newly-created brown fat cells keep burning calories after exercise is over. But it gets better.
Irisin's other effects
We've known for some time that a regular program of moderate exercise protects us against type 2 diabetes. For example, a lifestyle program that included regular moderate exercise reduced the risk of developing type 2 diabetes by nearly 60%—more than any medicine yet invented. How does that happen? Irisin may be an important part of the answer. In addition to its effect in creating brown fat cells, it also helps prevent or overcome insulin resistance, which leads to type 2 diabetes.
https://www.health.harvard.edu/staying-healthy/major-fat-burning-discovery
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