When it comes to vitamin D, it’s possible to get too much a good thing. That’s the key finding of new research by Johns Hopkins University scientists that shows blood levels of the "sunshine vitamin" that are higher than experts recommend offer no benefit and may, in fact, be harmful.
The study, published online in the American Journal of Medicine, cautioned against taking high doses of vitamin D supplements, which some have recommended as a way to protect against everything from high blood pressure to hardening of the arteries to diabetes.
"Healthy people have been popping these pills, but they should not continue taking vitamin D supplements unchecked," said lead researcher Muhammad Amer, M.D., an assistant professor of internal medicine at the Johns Hopkins University School of Medicine. "At a certain point, more vitamin D no longer confers any survival benefit, so taking these expensive supplements is at best a waste of money."
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Dr. Amer stressed that some people — seniors, postmenopausal women, and those with kidney disease — can benefit from higher blood levels of the vitamin, which is vital to bone health. Such groups may need to take supplements. But the new research shows vitamin levels higher than the top of the range suggested by the Institute of Medicine aren’t helpful for most people.
The study is based on a review of medical records for more than 10,000 participants in the National Health and Nutrition Examination Survey (NHANES) from 2001 to 2004. They compared that information with data from the National Death Index through 2006.
The results showed blood levels of 21 nanograms per milliliter of 25-Hydroxyvitamin D — the top of the range that the IOM considers "adequate" — and at the low end of "normal" cut the risk of death from all causes and cardiovascular disease specifically in half. Above 21 nanograms, the protective effect appears to wear off and may even be harmful.
The primary source of vitamin D is the sun, but it is also found in milk. Past research has shown increasing levels of vitamin D in deficient people can lower cardiovascular risks, but that levels higher than the IOM guideline are associated with an increased risk.
People should consult with their doctors, Dr. Amer said, before starting vitamin D supplements and have their blood levels checked.
"Most healthy people are unlikely to find that supplementation prevents cardiovascular diseases or extends their lives," he added. "There are a lot of myths out there and not enough data."
The study was funded, in part, by the National Heart, Lung, and Blood Institute.
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