Good news for couch potatoes. There may be a pill that lets them watch their TV and get their exercise, too—without moving a muscle. Scientists have found a drug that mimics the effects of a workout by, among other things, increasing the body's ability to burn fat.

The study shows the pill can also increase endurance; lab mice that took it ran more than 40 percent longer on a treadmill than their untreated peers.

"It's tricking the muscle into 'believing' it's been exercised daily," says Ronald Evans, a developmental biologist at the Salk Institute for Biological Studies, in La Jolla, Calif., and co-author of a study published in Cell. "It proves you can have a pharmacologic equivalent to exercise."

In addition to supercharging stamina, the drug, called AICAR, may also be useful in treating debilitating muscular disorders such as muscular dystrophy as well as metabolic diseases such as diabetes, because it also appears to help the body use and remove sugar from the blood more effectively.

Researchers say that AICAR—which is in clinical trials to treat some heart ailments—in essence works by reprogramming muscle, switching it from sugar-burning, fast twitch muscle—which is better for speed and power—into fat-burning, slow-twitch muscle that does not tire as easily.

The key to this transformation is a protein called PPARdelta, which Evan's team previously showed could create so-called high-endurance "marathon mice" if it genetically engineered the animals were genetically engineered to make a lot of it. But, another experimental drug that targeted only PPARdelta had some metabolic benefits, including lowering fatty acids and blood sugar, but it only boosted endurance in mice that were running regularly.

Enter AICAR, which targets a protein called adenosine monophosphate-activated protein kinase (AMPK). AMPK is produced when cells need more energy--as they do when we're exercising--and triggers increased levels of PPARdelta. As a result, researchers thought that AICAR could kick off the process inlieu of exercise.

After four weeks of treatment with AICAR, the mice that took it could run on treadmills nearly 1.5 times as long as untreated animals—and without any training.

Sound too good to be true? It may be. Laurie Goodyear, a n associate professor at Harvard Medical School and an investigator at the Harvard-affiliated Joslin Diabetes Center in Boston, applauded the findings but says no pill can duplicate all of the benefits that exercise has on the body.

Warning to athletes who think popping the pill might give them a leg up on the competition: Evans has developed urine and blood tests designed to detect the drug—so don't get any ideas.