At high doses, “there are some studies that do show harm,” says Davis, of the NIH Office of Dietary Supplements. Participants in the VITAL study, which included a daily dose of 2,000 IUs, did not appear to have any major side effects. But some pills are sold at much higher amounts, as high as 50,000 IUs to be taken weekly.
And some pills contain the less-accessible form of vitamin D produced by plants, called D2, not the version made by the human body, called D3.
One paper published in 2018 in Cancer Research examined 19 studies of vitamin D’s association with prostate cancer. That analysis found that higher levels of vitamin D were associated with an increased risk of prostate cancer, although the researchers say this could be due to detection bias — perhaps the men with high vitamin D were more health conscious and more likely to get screened for the disease.
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Rosen, from the Maine Medical Center, doesn’t think the supplements will make a dramatic difference in health, but agrees that a 2,000-IU supplement won’t do harm either. “When I go and talk to doctors I say, ‘How many [of you] are taking vitamin D?’ The hands go up,” he says. Most of them. When he asks why, the doctors respond, “ ‘Eventually it’s going to be proven that it prevents something.’ ” Belief in vitamin D runs deep.
That said, there are certain populations that doctors already know may need supplements to reach even minimally adequate levels: babies who are exclusively breastfed, some people with dark skin, some who live in northern latitudes and people who don’t have much exposure to daylight because they live in institutions such as nursing homes or hospitals.
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In the end, people have to consider their own personal circumstances, she says. Herman doesn’t immediately suggest supplements when a patient appears to have inadequate levels of vitamin D. Often these patients are overweight, which could be affecting their vitamin D levels. “I usually recommend they get more physically active and get in the sun first,” she says. “If people could master the basics, they wouldn’t have to worry about supplements.”