I just spent an hour at the gym only to discover in an article on Forbes.com that one day soon I may not need to pump out the miles on that stationary bike or groan out those sit-ups. (Or listen to retro ’70s disco to a hip-hop beat over the loudspeaker.) If a team at Salk led by Ronald Evans is right, mice can now take a pill that will allow them to have fit little bodies without having to run around on those little metal wheels. Can humans be far behind?

As is always the case with mouse testing, no one knows. “We’re very excited by the potential extension to humans,” Evans told Forbes. The reason for his optimism is that his new chemical turns on a genetic switch called PPAR-d that apparently occurs in both mice and man. It revs up metabolism in a way similar to what happens during a heavy workout. According to Forbes.com,

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.”

When the mice exercised after taking the new drug, they could last twice as long, said Evans, turning them into “marathon mice.”

Evans is discussing his results today at the annual meetings of the American Society for Biochemistry and Molecular Biology, in Washington, D.C.

However, the purpose of the pill is not to make healthy people into superhumans; it’s to treat the obese and overweight with what may be the long-sought “cure” for fatness–a treatment that succeeds where diets and diet pills often fail. This can’t come a moment too soon to treat the rising epidemic of obesity and diseases such as type II diabetes that often result from people being overweight.

This is good news, though I admit to having a nagging voice inside my head that undoubtedly comes from my Puritan ancestors, who offered admonitions that could be summed up as “No pain, no gain.” My ancestors would have meant this literally, in the sense that rewards come only to those who work hard. Yet there is another aspect of “No pain, no gain” that might be lost with a pill that makes us skinny and fit. This is the part of the pain and gain that is not just physical, but also mental. I exercise to stay fit but also because it’s a Zen-like experience in which for a few hours a week I set aside work and various concerns and anxieties and let the blood flow. I relax my brain and stretch my muscles, and it feels good and invigorating.

Evans says that the pill will work best in humans (if it works at all) combined with exercise and a good diet–which I guess means that I can work out and get my Zen moment and also become a marathon man.

But isn’t this cheating? Under current rules, this pill would be illegal for athletes, and it almost certainly would wreck havoc with sports already battered by waves of new chemicals to make one run faster, throw harder, and jump higher. But what if, unlike steroids, this chemical has no side effects? What if it’s served up in a flashy container like Red Bull, or in chewing gum?

Or in a Big Mac?

You’ve got to wonder if the fast-food industry is cheering this latest discovery, which might come just in time to staunch the growing movement to encourage healthy eating and to remove fatty foods from public schools.

Of course, there are other reasons to avoid fatty foods high in sodium (read my blog “Killer Salt”), although there is little doubt that a fat pill will keep millions of people healthier (and leaner) than they are now. But will people truly be healthier in mind as well as body?