Potency of Vitamin D Supplements Varies Widely From Labels

The contents of vitamin D supplements tested in a study ranged so widely that researchers argue they should be subjected to increased oversight and regulation.

OTC vitamin D supplement pills tested by researchers contained anywhere from 9% to 140% of their labeled dose, according to the results of a study published online on February 11, 2013, in JAMA Internal Medicine.

The researchers purchased 15 bottles of OTC vitamin D₃ (cholecalciferol) supplements (1000 IU, 5000 IU, and 10,000 IU per pill) made by 12 different manufacturers at stores in Portland, Oregon. Results of high performance liquid chromatography testing of 5 pills from each bottle found that the pills contained from 52% to 135% of the expected dose.

After averaging the contents of the 5 pills from each bottle, the researchers found that two-thirds of the bottles met US Pharmacopeial (USP) Convention standards for OTC cholecalciferol, which hold that the pill contents should be within 90%-120% of the stated dose. In one-fourth of the bottles, all 5 pills met the USP standards. Pills from the 1 manufacturer in the study that was USP verified were extremely accurate (101.7%) and all 5 pills tested were within 10% of the expected dose.

The researchers then tested 1 pill from each of 5 bottles with the same lot number from 4 different manufacturers. This time, the potency of the pills ranged from 57% to 138% of the expected amount. When the 5 pills from each lot were averaged, their potencies ranged from 99.7% to 107% of the expected amount. Only the 5000-IU pills from the USP-verified manufacturer had pills from the same lot that all tested within 90%-120% of the expected potency. (The 1000-IU pills from the same manufacturer varied from 97% to 135% of the expected dose.)

Next, the researchers tested 1 pill from each of 5 bottles with different lot numbers from 4 different manufacturers. Here, the potency of the pills ranged from 9% to 140% of the expected dose. Mean potencies for a single manufacturer over 5 different lots ranged from 89% to 105%. Again, only the 5000-IU pills from the USP-verified manufacturer had pills from different lots that were all within 90%-120% of the expected dose. (The 1000-IU pills from this manufacturer ranged from 70% to 140% of the expected potency.)

The researchers also analyzed compounded cholecalciferol tablets (1000 IU and 50,000 IU) between 0 and 6 months after compounding. The compounded 50,000-IU cholecalciferol tablets ranged from 52% to 105% of the expected dose, and the 1000-IU compounded tablets ranged from 23% to 146% of the expected dose. Just one-third of the tablets were within 10% of the expected dose, which is the USP Convention standard for compounded pills. Variation in the tablets’ storage time did not account for the potency differences.

The researchers note that, overall, the cholecalciferol content of OTC and compounded pills in their study was extremely variable, ranging from 9% to 146% of the expected potencies. Just over half of the OTC pills and just one-third of the compounded pills met USP Convention standards. They also note that the 1 USP-verified manufacturer in the study was, in general, more accurate than the other manufacturers.

Although inaccurate dosing with vitamin D supplements may not harm most consumers, they note, it can make supplementation less effective and make dose adjustments inaccurate in inconsistent users, and may pose the potential of harm to women with severe vitamin D deficiencies. Based on the results of their study, the researchers argue that regulation of dietary supplements should be increased.