Major fat-burning discovery

Harvard researchers discover a hormone released by exercise.

When you're taking a brisk walk on a beautiful day, what are you thinking about? The sun, the breeze, how good it feels to loosen up the stiff parts. The last thing you're thinking about as you pick up the pace is what's happening to your body chemistry.

When you exercise, your body chemistry changes in ways that we only now are coming to understand. Over the past 20 years, scientists have identified natural molecules in all of us that influence our appetite and our metabolism—and, hence, our weight. Now, researchers at Harvard Medical School and elsewhere are identifying the molecules that not only affect our weight, but also cause other health benefits of exercise.

"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.

Although Dr. Spiegelman did his studies in mice, he found that humans have irisin, too. While not yet proven, it is very likely that irisin has similar effects in humans.

"Studies like these are just plain interesting, in and of themselves," says Dr. Komaroff. "They help us to understand better how our body works. However, the discovery of irisin also could have some very practical and beneficial applications. Theoretically, irisin could become a treatment to help us maintain a healthy body weight and reduce the risk of diabetes."

Yes, other medicines with a similar promise have come and gone. However, irisin is not an unnatural pharmaceutical. Rather, it's part of our natural body chemistry. That may make it more potent and less likely to have adverse effects. So there is justifiable excitement about the discovery of irisin, and about the speed with which science is discovering the chemistry of exercise, appetite, metabolic rate and body weight.

However, our environment, and its effect on our own behavior, plays a huge role in determining how much we exercise and how much we eat, and therefore how much we weigh.

"We don't have to wait for a magic potion," says Dr. Komaroff. "We already have a proven treatment that profoundly protects our health: exercise."