Hormones aren't just for sex—they help control everything from the times when we feel hungry to the timing of our heart beats. Dozens have been described, but there is now a new one on the scene. It might help explain some of the health benefits of exercise and point the way to preventing obesity and diabetes. The find was described online Wednesday in Nature (Scientific American is part of Nature Publishing Group).

Exercise has myriad benefits for the body and brain, but many of the triggers for these improvements have so far been somewhat of a mystery.

"There has been a feeling in the field that exercise 'talks to' various tissues in the body," Bruce Spiegelman, a cell biologist at Dana-Farber Cancer Institute and co-author of the new study, said in a prepared statement. "But the question has been, how?"

Speigelman and his colleagues found that exercise—in both mice and humans—starts a cascade of signaling changes, including the production of a never-before-described hormone. They dubbed the new hormone irisin, as a nod to the Greek messenger goddess Iris for its ability to send information to surrounding body tissue.

And the messages irisin carries are not trivial—they seem to effect positive changes in the body. An increase in irisin helps turn white fat into the more beneficial and metabolically active brown fat, which burns more calories. It also seems to make the body more sensitive to glucose, an important capability for keeping diabetes at bay.

In the study, the researchers discovered that exercise increases the body's production of a metabolism-regulating protein, which in turn stimulates expression of a protein that can produce the new hormone, found to reside in the outer membranes of muscle cells.

The effects of exercise on the hormone's production seem to be long-lived. Even after 12 hours of rest, mice that had been on a three-week jogging regimen had 65 percent more irisin in their blood than unexercised mice. And people who had gotten 10 weeks of endurance exercise training had double the amount of irisin in their blood than those who had not.

But could this hormone, the scientists wondered, mimic some of the effects of exercise—without subjects having to hit the treadmill? To find out, they injected a batch of obese, pre-diabetic mice that had been fed a high-fat diet with just about as much of an irisin boost as they would get from a workout. After 10 days of injections, the irisin-boosted mice had shed a little weight and become more sensitive to glucose—all without exercise. And a later dissection showed that the hormone spike didn't seem to have any negative biological effects.

"It is likely that irisin is responsible for at least some of the beneficial effects of exercise on the browning of adipose tissues and increase in energy expenditure," Speigelman and his colleagues noted in their paper. This find might help explain some of the "afterburn" of extra calories after vigorous activity.

Even if the hormone proves safe for humans to take as a supplement, it won't replace all the benefits of going to the gym. But it might help people fight obesity and remain more sensitive to glucose, thus fighting off diabetes.

"It's exciting to find a natural substance connected to exercise that has such clear therapeutic potential," Pontus Bostrom, a postdoctoral researcher at Dana Farber and co-author on the new paper, said in a prepared statement. The researchers are now also investigating possible effects of the exercise-based hormone on other diseases, including neurodegenerative conditions, and have licensed the finding to Ember Therapeutics (a company co-founded by Spiegelman) for drug development.