Vitamin D potency varies widely in dietary supplements

The potency of vitamin D in compounded and over-the counter (OTC) supplements varied widely, according to a recent research letter published in the February 11 issue of JAMA Internal Medicine.

Researchers were conducting a randomized controlled trial of vitamin D in menopausal women. “We used compounded study pills so that vitamin D and placebo pills would look the same,” Erin S. LeBlanc, MD, MPH, lead author and investigator with the Kaiser Permanente Center for Health Research in Portland, Ore., told Formulary. “As part of study quality assessment, we sent the compounded study pills to a lab for testing and found variability in the potency. This made me curious about the variability and accuracy of over-the-counter vitamin D supplements, so I tested them as well and found variability in potency.”

The researchers tested 55 bottles of OTC vitamin D pills made by 12 different manufacturers, and compounded pills made on three different occasions over 4 months. Potency variability ranged from 9% to 146% of the amount listed on the label.

“The vitamin D with only 9% potency was an outlier and we retested another pill in the bottle to be sure,” Dr LeBlanc said. “The second pill was just as low. But we think this low value shows it is possible for vitamin D pills to contain very little of the actual active ingredient. The pill with the next lowest potency was 52%.

“The amount listed on the label did not necessarily match the amount contained in the bottle,” Dr LeBlanc continued. “It takes a lot of vitamin D to result in toxic effects so the main safety issue is really for consumers with low vitamin D levels. In these people, taking pills that are less potent than they expect could pose health risks. About one-third of the pills we analyzed had less vitamin D than was listed on the label.”

The amount of vitamin D in supplements is not necessarily the amount listed on the label, the researchers concluded. “Consumers and patients may be getting more or less than they expect,” Dr LeBlanc said. “The USP verification stamp is probably the best way to get some reassurance that the amount listed on the label is close to what’s in the bottle. In our study, the verification stamp wasn’t a guarantee that every pill in the bottle contained the amount of vitamin D listed on the label, but when averaged together, the 5 pills from the USP verified bottle generally contained at least 100% of what was listed on the bottle.”

Dr LeBlanc reminds clinicians to caution patients when buying OTC vitamin D supplements.

“Vitamin D insufficiency can be harmful to health, hence supplementation is commonly prescribed. However, vitamin D supplements are not regulated by the FDA, meaning that potency might not be evaluated,” said Dr LeBlanc. “We were surprised by the variation in potency among these vitamin D pills.” ■