Probably the most widely used dietary supplements in the US are multivitamin/mineral (MVM) ones. According to the 2007-2010 NHANES survey, 49 percent of adults said they used some supplement, and MVMs were most frequently reported by about 32 percent of adults. Led by Dr. Karen W. Andrews, scientists from the USDA, NIH, and Purdue University collaborated in an investigation of the content of such products — specifically on the accuracy of label claims and chemical analyses of the labeled pills. The rationale for the study was to provide accurate information when assessing the contribution of such supplements to Americans' diets. In many cases, the authors state, manufacturers provide more than the labeled amounts of some nutrients to ensure that the appropriate amount will still be present at the end of the supplement's shelf life. Thus, there might be the potential for an unhealthy excess of some nutrient(s) in some products. Further, since prior research on Americans' dietary habits often relied on label amounts, there is a need, the authors noted, for accurate information to improve the accuracy and consistency of such research.
In all, they examined about 400 samples from 109 different products, and their results were published in the American Journal of Clinical Nutrition. For 12 of the 18 nutrients assayed, most of the products had amounts at or above the RDAs (Recommended Dietary Allowance) on their labels. When tested, on average, the measured quantities of all except thiamin exceeded the amount listed on the label. And the relationship between the amount listed and the amount measured by assay varied among the different nutrients. The researchers then developed equations that would allow an investigator to more accurately ascertain the actual nutrient contents of an MVM supplement from the ingredient amounts on the label. In general, however, it is safe to say that for MVM supplements, you will get what you pay for, at least regarding ingredient content.
But the elephants in the room are the questions of whether supplementation with MVM is necessary or advisable for healthy adults, or for disease prevention. Research on these issues has provided conflicting answers, which might be due to misinformation on supplement contents, or to a simple lack of effect (for example, see here). So we must await further research to determine if inaccuracy of label information was responsible for at least some of the confusion about the utility of these products for health preservation or promotion.