WASHINGTON — When it comes to vitamins, it’s possible to have too much of a good thing.

Those who regularly gulp down a bunch of antioxidants could cut the effectiveness of their workouts, especially if they are training for an endurance sport.

Researchers at the Norwegian School of Sports Sciences in Oslo followed 54 active adults as they went through a tough 11-week training program.

Half of the participants took 1,000 milligrams of vitamin C and 235 milligrams of vitamin E every day. The remaining people took placebos.

After the rigorous workouts, all were more fit. But those who were on the placebos were creating far more mitochondria in their muscle cells.

That’s important because, mitochondria convert nutrients into energy that enables cells to function. And increasing the size and quantity of mitochondria is key for building endurance and improving athletic performance.

Most of the volunteers in the Norwegian study were runners and cyclists. But the findings provide a message for everyone — that while antioxidants are important, more is not necessarily better.

The study was published recently in The Journal of Physiology.

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