April 30 (HealthDay News) -- A pill might one day achieve the same calorie-burning effects of vigorous exercise.

It's possible, according to a scientist who's developed a potential weight-loss drug that revs up cellular metabolism, much like what happens during heavy physical activity.

In mice, the drug does "result in protection against weight gain on high-fat and high-caloric diets," said Ronald M. Evans, an investigator at The Salk Institute in San Diego. "We're very excited by the potential extension to people."

Of course, there's one big caveat: mice aren't people, and no one knows if the drug will allow ordinary folks to eat to their heart's content without gaining weight.

At stake is a medical solution for people who want to lose weight but either will not diet and exercise properly or can't lose enough weight that way. Diet pills have existed for decades, but they have significant side effects and aren't always effective.

One possible solution is to rev up the body's metabolism, the process whereby it turns food into energy. That's where Evans enters the picture.

He has developed a drug that uses chemicals to turn on a genetic switch in the body known as PPAR-d.

Evans is scheduled to talk about the drug Monday at Experimental Biology 2007, an annual scientific program of the American Society for Biochemistry and Molecular Biology, in Washington D.C.

When given the drug in the form of a liquid or powder, the bodies of mice appear to act as if they are exercising even when they aren't, causing their metabolism to speed up, Evans explained. "You then have lower fatty acid levels in your blood, lower triglyceride levels, and lower sugar levels," he said. "They all appear to be linked."

Mice who received the drug were also able to exercise twice as long, turning into what researchers calls "marathon mice."

But what about humans?

According to Evans, the drug could indeed become a "fat pill," although "anything like this will be more effective in the context of a healthy diet and exercise. If you want to get a maximum benefit, just doing it with the drugs alone will always be somewhat of a challenge."

Several companies are testing drugs that target the genetic switch in people, Evans said.

While an effective weight-loss pill is the "holy grail" of obesity research, there are plenty of reasons to be cautious about the new finding, said Leah Whigham, a research scientist who studies nutrition at the University of Wisconsin, in Madison.

"The most obvious caution is that this work is in mice, which are much different from humans and have different energy expenditure mechanisms. It remains to be seen if this research can translate into something useful for humans," she said.

Also, mice are much more similar to each other than humans. "Something that works in all mice of a given strain might not be as effective across a population of humans with differing genetic, ethnic, cultural environmental backgrounds," she explained.

Still, Whigham said, "that doesn't mean this research isn't very exciting. It is just very preliminary at this point."