While the FDA does not regulate the potency of vitamin D supplements, companies can comply with the standards of the United States Pharmacopeial Convention, which requires that pills have at least 90% to 110% of the listed potency. Recently, results of a study were published online in JAMA Internal Medicine. The study found that pills from bottles made by a single manufacturer, but in varying lots had potencies as low as 9%, and as high as 140% of the listed dose. When the dosages of five pills from 5 separate bottles were averaged, the researchers found that only two-thirds met the U.S. Pharmacopeial Convention standard. High-performance liquid chromatography was used to analyze pills in 55 bottles of vitamin D bought at five stores in Portland, Oregon. Dr. Erin S. LeBlanc, lead author of the study, and researcher at Kaiser Permanente in Portland suggests looking for the U.S. Pharmacopeial Convention seal when purchasing vitamins. She said, “If you have a bottle with the U.S.P. stamp on it, you can feel reassured that what’s listed on the label is actually in the bottle.”   nytimes.com    2/19/13

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