Scientists at John Hopkins University have found that while vitamin D is important for heart protection, it may have diminishing — and harmful — returns if levels in the blood rise above even the lower end of normal.

After examining data from more than 15,000 adult participants for five years, doctors found that increasing levels of vitamin D in the blood are associated with lower levels of a popular marker for cardiovascular inflammation — c-reactive protein, or CRP.

While healthy levels of inflammation were discovered in people with normal or close to normal vitamin D levels, anything higher than the low end of that normal range was associated with an increase in CRP, a factor linked to stiffening of the blood vessels and an increased risk of cardiovascular problems.

The most popular source of vitamin D is the sun, but since people these days tend to spend more time indoors or slather themselves with sunscreen, many have turned to supplements to cover the gap. And it’s those supplements that are of most concern to study leader Muhammad Amer, M.D., an assistant professor of general internal medicine at the Johns Hopkins University School of Medicine. It is also found in mackerel, herring, sardines, salmon, cod liver oil and eggs.

In the January 15 issue of the American Journal of Cardiology, he advises consulting a doctor to ensure taking extra vitamin D is necessary, adding, “Those pills could have unforeseen consequences to health even if they are not technically toxic.”

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