If you’ve read health headlines recently, you may think that a pill has been developed to replace exercise, but this isn’t really the case. Dr. Ronald M. Evans of the Salk Institute in San Diego, a noted researcher, has spent many years studying the effects of a cellular protein known as PPAR -delta. Having previously shown that this protein causes fat burning, he and his research group have now discovered that PPAR-delta can be provoked by drugs to increase the production of type 1 muscle fibers rather than the less useful type 2 fibers. This is significant because type 1 fibers contain more mitochondria, the tiny furnace that provides the muscle with its energy. Type 2 fibers, by contrast, are more prevalent in the muscles of folks who are obese, sedentary, or have diabetes.

Evans and his group found that when given a drug known as Aicar, mice increased their exercise endurance by 44 percent in just 4 weeks. When the test mice were given GW1516, they were able to run 75% longer, provided that the drug was combined with exercise. The results were just published in the reputable journal Cell.

Many in the news media have jumped on these findings and begun to speculate that exercise might soon be obsolete, but there are several reasons to be cautious. In the first place, though these mouse proteins are similar to human proteins, mice are not people, and it is not possible to test these drugs for long term safety in mice, since they only live for little more than a year. Even if these pills appear to help heart function, our most essential muscle, there needs to be more study as to how they affect other essential organs including the kidney, liver, and brain.

Exercise itself has many immeasurable effects on the metabolism, and it also has a positive psychological effect that may not be reproducible by a pill. When you exercise, the brain releases endorphins, happy hormones that make us feel good.

Of course GW1516 or Aicar or drugs like them might end up being useful  for patients with disabilities who  cannot exercise, as well as for people who can exercise but need help getting started or building up after months of inactivity. When human trials start, it might be interesting to study these drugs in astronauts , who are faced with long periods of confinement in space.

But there are other concerns. We already have a problem with performance-enhancing drugs in sports, and this could add to it. And it will always be impossible to accurately measure or replace all the hormones released in response to actual exercise, as well as the feeling of wellbeing that ensues.

Marc Siegel MD is an internist and associate professor of medicine at the NYU School of Medicine. He is a Fox News Medical Contributor and writes a health column for LA Times, where he examines TV and movies for medical accuracy. Dr. Siegel is the author of False Alarm: the Truth About the Epidemic of Fear (Wiley 2005) and Bird Flu: Everything You Need to Know About the Next Pandemic (Wiley 2006). Read more at www.doctorsiegel.com