Supplements are big business — roughly $28 billion dollars are spent on vitamins, herbs, and nutritional supplements in the U.S. each year. But this growing nutritional industry is largely unregulated. A study of the potency of both over-the-counter (OTC) and compounded vitamin D supplements found that they vary enormously and frequently do not match the amount listed on the label. The same may be true of other types of supplements as well.

Dr. Erin S. LeBlanc, MD, MPH, and colleagues at the Kaiser Permanente Center for Health Research in Portland, Oregon analyzed the vitamin D content of five pills from each of 15 bottles of OTC cholecalciferol supplements. (Cholecalciferal, or vitamin D3, is the most active form of vitamin D in the body.) Bottles containing 1,000 IU, 5,000 IU, and 10,000 IU were purchased for the study as well as compounded vitamin D pills.

The biggest worry is for someone who has low levels of vitamin D in their blood. If they are consistently taking a supplement with little vitamin D in it, they could face health risks.

The researchers took five pills from the same bottle, pills from five bottles with the same lot number, pills from five bottles with different lot numbers, and compounded tablets (supplements custom mixed in a pharmacy) to get a broad sample of what was available. Lab analysis showed that the amount of vitamin D in the supplements ranged from 9% to 146% of the amount stated on the label. The range of variation occurred among different brands, different manufacturers, and among different pills in the same bottle.

About half of the OTC pills and one-third of the compounded pills met US Pharmacopeial (USP) Convention standards which mandate that the contents of a pill should fall within 90% to 120% of the dose listed on the label. One manufacturer of the OTC pills was USP verified, and those pills generally showed less variation.

The huge variation in content among vitamin D supplements is not necessarily harmful to consumers, but these deviations may make the supplements less effective. "We were surprised by the variation in potency among these vitamin D pills. The biggest worry is for someone who has low levels of vitamin D in their blood. If they are consistently taking a supplement with little vitamin D in it, they could face health risks," said Dr. LeBlanc in a statement.

Another concern about the variability in vitamin D is the compounded pills used in vitamin D research because the inconsistency may threaten the validity of vitamin D trials.

The United States Pharmacopeial Convention (USP) is a scientific nonprofit organization that sets standards for the identity, strength, quality, and purity of dietary supplements. Some manufacturers of dietary supplements voluntarily participate in their quality verification program, and their products contain a USP verification mark on the label.

"The USP verification mark may give consumers some reassurance that the amount of vitamin D in those pills is close to the amount listed on the label," according to Dr. LeBlanc. "There are not many manufacturers that have the USP mark, but it may be worth the extra effort to look for it."

Health care professionals and consumers need to understand that the Food and Drug Administration (FDA) does not regulate dietary supplements, so potency can be highly variable as shown in this study of vitamin D. The authors agreed with calls for increased regulation of dietary supplements.

The study was published online in the Journal of the American Medical Association Internal Medicine.