Millions May Be Taking Vitamin D Unnecessarily, Analysis Suggests

Health experts disagree on safe levels needed to prevent illness

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For his part, Heaney noted that taking vitamin D and other nutrients may be akin to changing the oil in your car. "If you don't change the oil, your car runs well now, but it may break down sooner," he said.

In terms of side effects, Kramer said, too much vitamin D can increase the risk of kidney stones, but in general, it's a well-tolerated supplement. The upper safe limit for daily intake is 4,000 international units, according to the U.S. Office of Dietary Supplements, though most people take a much lower dose.

Results of Kramer's study are published in the Oct. 24 issue of the journal PLoS ONE.

More information

To learn more about vitamin D, visit the U.S. government's Office of Dietary Supplements.

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