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updated 2/11/2013 7:41:34 PM ET 2013-02-12T00:41:34

The actual dose of vitamin D you get from a supplement can vary widely from the amount listed on the label, a new study finds.

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The amount of vitamin D found in pill tested in the study ranged from 9 to 146 percent of the amount listed on the label, the researchers said.

Inaccurate supplement labels pose the biggest worry for people who have low vitamin D levels, said study researcher Dr. Erin LeBlanc, an investigator with the Kaiser Permanente Center for Health Research in Portland, OR. "If they are consistently taking a supplement with little vitamin D in it, they could face health risks," LeBlanc said in a statement.

One bottle tested in the study was certified by the U.S. Pharmacopeial Convention — an independent organization that sets standards for dietary supplements, and certifies supplements for companies that allow their products to be tested. The amount of vitamin D in USP-certified pills was generally very close to the amount stated on the label. According to the USP, pills should contain between 90 to 120 percent of the dose stated on the label.

"There are not many manufacturers that have the USP mark, but it may be worth the extra effort to look for it," LeBlanc said. Another company that certifies supplements is NSF International.

In the study, the researchers analyzed 12 bottles of vitamin D supplements (each made by a different manufacturer), bought at 5 different stores in Portland, Oregon, and tested five pills from each bottle. The labeled dose on the bottles ranged from 1000 international units (IUs) to 10,000 IUs.

All five pills met the USP standard in just 25 percent of the bottles.  

When the researchers calculated the average amount of D in the five pills, this dose was generally closer to the labeled amount. Still one third of the bottles did not meet the USP standard, the researchers said.

Dr. Pieter Cohen, an assistant professor of medicine at Harvard Medical School and a general internist at Cambridge Health Alliance in Boston who was not involved in the study, has patients take D supplements to boost low vitamin D levels. But he worries about the accuracy of supplement dosages. "It leaves patients in the dark about how to replete their vitamin D," Cohen said. Last month, a study by Cohen and colleagues found that some dietary supplements contain high amounts of caffeine, even though caffeine is not listed on the label.

Supplements are not inspected before they come to market to make sure they contain accurate amounts of nutrients. But current laws state that the dosages in supplements should match the amount listed on the label. The Food and Drug Administration can reprimand companies if their products do not meet this requirement.  The new study suggests that "even the minimal laws that do exist aren’t being followed or enforced," Cohen, said.

Cohen suspects that over-the-counter medications, which are regulated by the FDA, would be much less variable in their doses. "If you looked at 12 bottles of aspirin, I suspect that every single one would be accurately labeled," Cohen said.

The new study is published today (Feb. 11) in the journal JAMA Internal Medicine.

Pass it on: The dose of vitamin D you get from a supplement may differ from what's on the label.

Follow Rachael Rettner on Twitter@RachaelRettner, or MyHealthNewsDaily@MyHealth_MHND. We're also onFacebook&Google+.

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