A survey of over-the-counter vitamin D tablets in the USA has revealed very wide variations in pill potency, with different tablets containing as little as 9% or as much as 146% of the stated dose.
While this variability may not cause harm in most cases, it could be problematic in women with severe vitamin D deficiency, warn the study authors.
"As more people take vitamin D supplements, it is critical that healthcare providers and patients understand that cholecalciferol potency may vary widely," write Erin LeBlanc (Kaiser Permanente Center for Health Research, Portland, Oregon, USA) and co-authors in JAMA Internal Medicine.
LeBlanc et al started their study after a recent trial examining vitamin D in menopausal women found that compounded vitamin D3 supplements varied in potency. In this trial, just one in three of the study pills met the US Pharmacopeial (USP) Convention standard, which requires that compounded pills contain 90-110% of the active ingredient.
To investigate further, the team randomly selected five pills from 15 sealed bottles of over-the-counter cholecalciferol dietary supplements (1000 IU, 5000 IU, and 10,000 IU) purchased from five stores.
Analysis revealed that pills contained between 52% and 135% of the expected dose; when averaged over five pills, two-thirds of bottles met USP standards and in just one-quarter of bottles all pills met the standards.
Of note, the one USP-verified manufacturer produced pills that were highly accurate (101.7%), with all pills tested within 10% of the expected dose.
The team then randomly selected one pill from five bottles with the same lot number. In this analysis, potency ranged from 57% to 138% of the stated amount. Analysis of pills from five bottles with different lot numbers revealed even greater variability, with potency ranging from 9% to 140% of the stated dose.
In both instances, only pills from the USP-verified manufacturer tested within 90-110% of the expected amount in every case.
Finally, LeBlanc et al tested compounded tablets, finding that the 50,000 IU pills contained 52-105% and the 1000-IU pills contained 23-146% of the expected dose. Only one-third of pills were within 10% of the expected dose, meeting the USP standard.
Variations in storage time did not explain potency differences, note the authors. They say that pill variability may cause supplements to be less effective - and dose adjustments inaccurate - in inconsistent users.
"Pill variability may also threaten validity of vitamin D trials that use compounded pills to blind participants," they observe. "On the basis of our study, we agree with a recent editorial calling for increased regulation of dietary supplements."
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