Irisin: The Muscle Hormone Linked to the Health Benefits of Exercise

When people exercise and irisin levels climb the body is better able to convert white fat to brown fat and glucose tolerance improves.


Scientists at Dana-Farber Cancer Institute have isolated a muscle hormone which may be responsible for the many health benefits of exercise. The scientists have named the hormone irisin after the Greek messenger Goddess, Iris.

Irisin levels rise when people exercise. One effect of this is to convert white fat, the common type of fat that everyone knows too well, into brown fat. While white fat is used to store energy (calories), brown fat seems to be more involved in burning it. Irisin also improves glucose tolerance and causes insulin levels to rise, suggesting that it may be helpful in treating diabetes.

In the last few years, brown fat has begun to increasingly intrigue scientists. In many ways, it physically resembles muscle more than it does typical fat. While little is known about it, it seems to play a major role in maintaining normal weight. One study found that the more brown fat a person has, the lower their BMI will be. Another study found that mice with a genetic abnormality that eliminated their brown fat became obese even without overeating.

Brown fat is common in infants but was once thought to disappear in adults. Recent studies have shown that it merely decreases. It's found mainly in the neck and upper chest of adults. In infants, brown fat is an important regulator of body temperature, helping to keep the infant warm. Unlike white fat, when brown fat is metabolized (burned), most of its energy content is converted into heat. Adults don't seem to need this system of keeping warm.

It's not yet known precisely what function brown fat serves in adults. So far, most studies of brown fat have been done in mice, because adult mice have more brown fat than adults humans do.

In the current study, the Dana-Farber researchers injected small amounts of irisin into the muscles of sedentary adult mice that were both obese and pre-diabetic. Within 10 days of treatment, the mice showed better control of blood sugar and insulin levels -- possibly preventing the onset of full-blown diabetes -- and also lost a small amount of weight. The researchers suspect that longer therapy would have led to greater weight loss.

There are a host of possible uses for a naturally produced substance that can normalize sugar metabolism and help people lose weight. The researchers think that clinical trials of irisin could begin in as little as two years.

Irisin was named after a messenger Goddess because it's a chemical messenger. It's normally present as part of a larger protein in a muscle cell's outer membrane, where it lies dormant and inactive. Exercise (and, most likely, other unknown factors) cause this protein to be split, releasing irisin, which exits the muscle cell and carries its message to other cells of the body. Ultimately, some white fat cells are told to convert to brown fat cells and islet cells of the pancreas are told to produce more insulin.

The discovery is likely to be portrayed by some media outlets as promising exercise in a pill. While that's certainly possible, new health discoveries rarely work out that way. They didn't in the 1980s, anyway. Those were heady times for people who loved junk food. Left-handed sugar promised a calorie-free sugar substitute for all sweets. And sucrose polyester (olestra, Olean) promised the same for high-fat foods. It looked as if people would soon be able to eat cookies, cake, and chips to their heart's content and never gain a pound. Of course, that didn't work out. Life is rarely so simple. Irisin probably won't be, either.

The irisin study was published online by Nature on January 11, 2012.

Image: Christopher Edwin Nuzzaco/Shutterstock.

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