If exercise generates free radicals and free radicals are bad, why is exercise good?

Free radicals -- forms of oxygen that are missing an electron -- are actually not all bad.

Although free radicals can destroy cell membranes and DNA, scientists have discovered that they have a ''good" side, said Dr. John McDonald, a neurologist at Johns Hopkins. For instance, they help with cell signaling, the process of chemical communication that tells a cell, among other things, when to activate certain genes.

Exercise does generate free radicals -- but it also produces molecules that sop up the free radicals, William J. Evans, an exercise physiologist at the University of Arkansas for Medical Sciences, said in an e-mail.

In other words, regular exercise actually ''trains the anti-oxidant system," boosting production of natural anti-oxidants, said Dr. David Systrom, director of the Cardiopulmonary Exercise Laboratory at Massachusetts General Hospital.

Which leads to one of the hottest questions in exercise research: Should athletes take extra anti-oxidants, particularly vitamins C, E, and beta-carotene?

Some data suggest that performance and muscle recovery are enhanced with the supplements. But other data suggest that ''anti-oxidants can become pro-oxidants -- in other words, free radicals -- when taken to excess," Systrom said. ''Nobody knows where to draw the line."

JUDY FOREMAN

E-mail health questions to Foreman@globe.com.