Avoid vitamins E and C while training.
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Avoid vitamins E and C while training.

If you're training for a 10K or long-distance bike race, you might want to steer clear of vitamin C and E supplements. A new study published in the Journal of Physiology found that these pills mess with your muscles' ability to get stronger through training.

A team of researchers in Norway gave 54 healthy men and women either vitamin E and C supplements or dummy pills for 11 weeks. During this time, the participants completed an intense training program of mostly running. The researchers administered fitness tests, blood tests, and muscle biopsies at the beginning and end of the study. The post-trial tests revealed that those in placebo group had normal production of new muscle mitochondria – the cells' energy centers – just as would be expected after an extended bout of endurance training. However, the participants taking supplements did not generate new muscle mitochondria like they should have, suggesting the vitamin C and E pills had interfered.

According to the researchers, these supplements seem to get in the way of normal, beneficial cellular adaptations in response to exercise. It's already well known that exercise churns up free radicals, or oxidative stress, and that vitamins C and E are potent antioxidants that fight against free radical damage. Although antioxidant protection is a good thing in many parts of the body, such as in skin cells, where unchecked free radicals can cause premature aging, stopping oxidative stress in hardworking, growing muscles may be a big negative. The researchers say more studies are needed to confirm these results, but for now, use vitamin C and E supplements with caution, especially if you're gearing up for a big endurance event.