Exercise is good for you. Antioxidants are good for you. But put them together and it’s not as good as you’d think. Because a recent study shows that some vitamins block the beneficial effects of exercise.

A good workout not only gets your heart pumping, it makes your body better able to process glucose and decreases your risk of diabetes. And it does so, in part, by firing up your cells’ mitochondria. Stoking those mitochondrial flames boosts your metabolism. But it also throws off so-called free radicals, which are usually considered harmful. So adding antioxidants—which get rid of free radicals—should make your workout even healthier, right?

Well, no. Forty men took part in a four-week training program. Half the group also received daily doses of Vitamins C and E. The researchers discovered that exercise on its own improves insulin sensitivity, which keeps diabetes at bay. But taking the vitamins erased that gain—findings that appear in the May 12th issue of the Proceedings of the National Academy of Sciences.

So you actually need a burst of potentially damaging radicals to reap some of the important rewards of a good sweat. Which means that even on a molecular level: no pain, no gain.

—Karen Hopkin