Why the 'exercise pill' isn't likely to eliminate the gym any time soon

— -- Recent headlines touting the benefits of an "exercise pill" have teased the idea that it could be possible to skip the gym and stay fit.

But a new study released this month, which takes a closer look at how the drug GW501516 acts on the body's metabolism, shows the alleged benefits aren't all that new and the potential risks could be significant.

The study, published in the journal Cell Metabolism revealed additional information about the way the chemistry of the drug works, giving outside researchers a chance to assess the claims that this "exercise pill" could be a substitute for physical activity.

The intention of the research, according to one of the study authors, was not to create a pill that healthy people could take to mimic, or substitute for, exercise.

"It was never our intention to encourage the replacement of exercise with any exercise mimetics," said Dr. Michael Downes, a senior staff scientist at the Salk Institute and the co-author of the paper, in an email to ABC News.

"We believe exercise is one of the best solutions to combat many human disease conditions, and nothing can fully replace exercise for its many health benefits."

Despite the well-known risks, some unscrupulous marketers still promote the experimental drug, which has been commonly referred to as "endurobol" and sold on the black market for years as an exercise supplement. It has been a known performance-enhancing drug that at least one major anti-doping agency has warned athletes not to use.

To test the effects of the drug during the study, the authors gave a group of mice a higher dose of GW501516 and for a longer period -- eight weeks instead of four -- than in previous studies. They used sedentary mice and their results showed some promise. At the end of the eight weeks, the sedentary mice that had GW501516 outperformed the sedentary mice who did not receive it; the medicated mice were able to run for about 50 percent more time before "hitting the wall" -- reaching exhaustion or passing out -- compared to their counterparts.

The way GW501516 works, researchers found, is that it promotes the burning of fat instead of glucose for energy. This prolongs the time mice can exercise by sparing glucose, which in turn delays fatigue. The byproduct is that, in normal exercise conditions, the body starts using fatty acid stores instead of relying on burning glucose for energy, leading to weight loss and increased endurance.

The same trick worked even when the mice weren't exercising, the researchers found, leading to the idea that taking GW501516 could mimic the effects of regular exercise.

But there are many reasons to be cautious about the pill. Past animal studies have suggested that the drug is associated with a host of health issues, including cancer risks.

In fact, the story goes all the way back to 1992, when Ligand Pharmaceuticals and GlaxoSmithKline started developing GW501516 with the goal of potentially treating metabolic diseases such as diabetes and hyperlipidemia.

They published the first scientific article in 2001, with promising results. But in 2007, the pharmaceutical company abandoned clinical trials after animal studies showed that even low doses led to rapidly developing cancers in animals.

But the end of clinical trials did not prevent the pill from being developed illegally and sold on the black market as a doping agent, according to the World Anti-Doping Agency (WADA). The agency added the drug to its prohibited list and helped develop a test in 2009 to detect it so that athletes participating in high profile competitions would be discouraged from taking the unproven medication. They also released a safety warning about GW501516 due to its potentially cancer-causing properties.

Research has continued on GW501516, and similar drugs, to study its effects on metabolism and relation to cancers over the past decade, with mixed results. In fact, Downes said, in some trials the drug has even appeared to be protective against cancer. But, he added, "further work is needed to bring clarity to this issue."

Until additional research is done and the results are in, most experts in the field agree that it would be a mistake for the public to experiment with such drugs.

Dr. Max Mehlman, professor of health law and bioethics at Case Western Reserve University School of Law, said further study on humans is unlikely in the near future –- and that anyone taking it now is putting their health at serious risk.

"People are taking a big risk when they take pills that have not been adequately studied in humans," Mehlman told ABC News. "No data on human safety now and all we have are safety risks in animal studies."

Dr. Margarita Abi Zeid Daou is a fourth year psychiatry resident at Vanderbilt University Medical Center and a resident in the ABC News Medical Unit.