How much vitamin D is too much?

JOHNS HOPKINS (US) — Although vitamin D is essential to good health, researchers say too much may damage blood vessels.

Muhammad Amer of the Johns Hopkins University School of Medicine says his findings show that increasing levels of vitamin D in the blood are linked with lower levels of a marker for cardiovascular inflammation: c-reactive protein, or CRP.

“Clearly vitamin D is important for your heart health, especially if you have low blood levels of vitamin D,” Amer says. “It reduces cardiovascular inflammation and atherosclerosis, and may reduce mortality, but it appears that at some point it can be too much of a good thing.”

Amer and colleague Rehan Qayyum examined data from more than 15,000 adult participants in the continuous National Health and Nutrition Examination Survey, a nationally representative sample, from 2001 and 2006.

They found healthier, lower levels of inflammation in people with normal or close to normal vitamin D levels. But beyond blood levels of 21 nanograms per milliliter of 25-Hydroxyvitamin D—considered the low end of the normal range for vitamin D—any additional increase in vitamin D was associated with an increase in CRP, a factor linked to stiffening of the blood vessels and an increased risk of cardiovascular problems.

“The inflammation that was curtailed by vitamin D does not appear to be curtailed at higher levels of vitamin D,” says Amer, whose report is published in the American Journal of Cardiology.

Vitamin D is often called the “sunshine vitamin” because its primary source is exposure to the sun. It is found in very few foods, though commercially sold milk is usually fortified with it. As people spend more and more time indoors and slather their bodies with sunscreen, concern is rising that many are vitamin D-deficient, Amer notes.

As a result, many doctors prescribe vitamin D supplements, and many consumers, after reading news stories about the vitamin’s benefits, dose themselves. Older women often take large doses to fight and prevent osteoporosis.

Amer says consumers should exercise caution before taking supplements and physicians should know the potential risks. Each 100 international unit of vitamin D ingested daily produces about a 1nanogram per milliliter increase in 25-Hydroxyvitamin D levels in the blood.

“People taking vitamin D supplements need to be sure the supplements are necessary,” Amer says. “Those pills could have unforeseen consequences to health even if they are not technically toxic.”

Amer and Qayyum, both assistant professors in the Division of General Internal Medicine at Johns Hopkins, say the mechanism that accounts for the loss of cardiovascular benefits at higher doses is not clear.

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