Leptin is a hormone made by fat cells that tells you when you’re full. If you didn’t have leptin—and some people don’t—you’d eat too way too much, get real fat and almost certainly develop diabetes. Leptin acts like a natural appetite suppressant. Kids born without the hormone can top 100 pounds before they hit kindergarten. And mice without leptin are sluggish balls of blubber. But how does leptin work its dietary wonders?

To find out, scientists took mice that don’t respond to leptin because they lack the receptor to which it binds. They then added the leptin receptor to one small group of brain cells. The result: the enhanced mice eat less and spend twice as much time running around. Even better, being able to sense leptin returns their blood sugar to normal levels, staving off diabetes. The study appears in the June 2nd issue of the journal Cell Metabolism.

Could be that these brain cells, in response to leptin, convince muscles to remove more sugar from the blood, or they might be telling the liver to stop dumping sugar into the bloodstream in the first place. Either way you get healthier mice—and maybe someday healthier people, too. Thanks to researchers who saw a difficult problem and leapt in.

—Karen Hopkin