DEAR DOCTOR K: You frequently write that exercising regularly may be the best thing you can do to improve your health. I don’t doubt that’s true. But have scientists figured out what it is about exercise that promotes health?
DEAR READER: You’ve asked a very interesting question. Perhaps you’re thinking, as do some of my patients, that exercise leads the body to produce certain natural chemicals that promote health. And that if you could make pills out of those health-promoting chemicals, maybe you wouldn’t need to exercise.
Actually, research here at Harvard in the past few years may have made a step in that direction. The research is primarily in mice, but it probably applies in humans as well.
When you exercise, your body chemistry changes. Muscles need a source of energy when they exercise; they get that energy by burning fat and sugar. That’s old news, but it’s only part of the story.
The scientists here discovered that when muscle exercises, it releases a
hormone called irisin that travels through the blood to fat cells. There are
three types of fat cells: white, brown and beige. White fat cells store fat. Brown and beige fat cells burn fat.
We store fat in white fat cells for the same reason we store electricity in batteries. When we eat more calories than we burn through exercise, the extra calories have to go somewhere. They’re stored partly as fat. Our distant ancestors in prehistoric times didn’t eat as regularly as we do. So when they found a meal, they really pigged out. In between meals, they got lots of their energy from white fat cells.
Brown fat cells don’t store fat; they burn it. If your goal is to lose weight, you want to increase the number of your brown fat cells and to decrease the number of your white fat cells. That’s exactly what irisin does: It turns white fat cells into brown fat cells. And those newly created brown fat cells keep burning calories after exercise is over.
That’s not all that irisin does. It also helps prevent insulin resistance, a condition that leads to Type 2 diabetes. Type 2 diabetes, in turn, greatly increases the risk of getting heart disease and stroke — two of the most important causes of premature death.
Although Dr. Bruce Spiegelman did his studies in mice, he found that humans have irisin, too. It is very likely, though not proven, that in humans irisin has effects similar to those found in mice.
Studies like these are just plain interesting, in and of themselves. 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.
I doubt irisin will have all of the beneficial effects of exercise, because I’ll bet it’s only part of the way that exercise improves health. And whether it someday becomes a medicine or not, we still have exercise itself — and exercise is free.
(Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.)
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