Potency of vitamin D supplements often varies widely from what is listed on the label, according to results from a study of pills from 12 different manufacturers.
The study was authored by Erin S. LeBlanc, MD, MPH, from the Kaiser Permanente Center for Health Research, Portland, Oregon, and colleagues and published online February 11 in JAMA Internal Medicine.
In previous research that focused on vitamin D in menopausal women, Dr. LeBlanc and colleagues found that compounded vitamin D3 (cholecalciferol) supplements varied significantly in potency and that only a third of the examined compounded study pills met US Pharmacopeial (USP) Convention standards.
"This variability in compounded cholecalciferol pills led us to additionally investigate over-the-counter (OTC) cholecalciferol pills, in which we also found variability," the authors write.
In this study, the investigators randomly tested the potency of 5 pills from 15 sealed bottles of OTC cholecalciferol dietary supplements (1000 IU, 5000 IU, and 10,000 IU) they purchased at 5 stores in Portland, Oregon.
They found the pills contained from 52% to 135% of the dose listed on the bottles. "When averaged over 5 pills, two-thirds of bottles met USP Convention standards for OTC cholecalciferol solution, which state that contents should be within 90% to 120% of the stated dose," the authors write. In about a quarter of the bottles, all 5 pills met the standards.
The investigators next checked a single pill from 5 bottles with the same lot number and found that potency ranged from 57% to 138% of what was on the label.
They then evaluated a single pill from 5 bottles with different lot numbers. Those pills ranged from 9% to 140% of the stated dose, with mean potencies over the 5 lots ranging from 89% to 105%.
Finally, the researchers analyzed compounded study cholecalciferol pills (1000 IU and 50,000 IU) that had been produced on 3 occasions over the course of several months. Those analyses were done over varied times (0 - 6 months) after compounding. They found that the 50,000-IU tablets contained 52% to 105% of the anticipated dose, whereas the 1000-IU tablets contained 23% to 146% of the expected dose.
"In our test, just over one-half of the OTC pills and only one-third of compounded pills met USP Convention standards," they write.
They say that although lack of accuracy in vitamin D dosing may not cause harm in most people, such deviations could render the supplements less effective. They also point out that variability from pill to pill may undermine the validity of vitamin D trials that use compounded pills to blind participants.
"As more people take vitamin D supplements, it is critical that health care providers and patients understand that cholecalciferol potency may vary widely," they conclude, adding that they agree with calls for increased regulation of dietary supplements.
This study was supported by Kaiser Permanente. The authors have disclosed no relevant financial relationships.
JAMA Intern Med. Published online February 11, 2013. Abstract
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Cite this: Vitamin D Supplements Found to Vary Widely in Potency - Medscape - Feb 11, 2013.