To understand how irisin might help the lumbering masses lose mass, it helps to remember that mammalian fat comes in (at least) two colors. Brown is the new black: It's what you want more of. Unlike the white fat that lards the thighs and jiggles dangerously across the belly, brown fat's the stuff that boosts a mammal's energy expenditure. The more of the brown adipose you have, the less of the white stuff you're likely to accumulate.

We all have some of the brown fat, but nature distributes it unevenly among us, leaving many of us wanting more.

But how to get more? An international group of scientists, led by a cell biologist from Dana-Farber Cancer Institute in Boston, believes that irisin is the key. "It has powerful effects on the browning of certain white adipose tissues," both in laboratory cell cultures and in living animals, they wrote in Nature. 

Exercise has this ability too, of course, and irisin may be at the root of that, the team, led by Bruce M. Spiegelman, found. Irisin levels rose by 65% in mice after three weeks of free-wheel running. In humans, the effect was a little less dramatic, but still good: After 10 weeks of "supervised endurance exercise  training," irisin levels doubled. Under irisin's influence, subcutaneous fat gets browner, total-body energy expenditure increases, and a cascade of changes reduces insulin resistance (which is the first step along a path leading to a Type-2 diabetes diagnosis).

Cut to the lab here, where researchers tell us they can introduce into the mammalian body a viruscarrying a synthesized gene called FNDC5, which, upon entry, creates a chain reaction leading to the elevated production of the hormone irisin. In mice bred to become chubby when fed a high-fat diet, even a short course of FNDC5 and a modest increase in irisin levels cause some weight loss; their muscles consume more oxygen, as if they had spent the last several weeks exercising; their growing insulin resistance is reversed and their glucose tolerance is improved.

To demonstrate to doubters that irisin was doing all that, researchers plied their mice-subjects with anti-FNDC5 antibodies to stop the production of the newly described hormone. Result: 10 days of swim training did nothing to improve their physiques.

Is this the long-awaited "exercise drug" that will make sedentary chubsters lean and fit? Dream on, couch potatoes; it's a long road from mice to man. "Whether long treatments with irisin an/or higher doses would cause more weight loss remains to be determined," the authors wrote.

Meantime, this research underscores findings already well-established: that regular, vigorous exercise yields significant improvements in metabolic function and other processes (such as inflammation) that obesity exacerbates -- and it might even help you lose a little weight.

And if an "injectable polypeptide" could do the same, the authors blandly suggest, the therapeutic potential of irisin could be "attractive."