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