Marked heterogeneity in micronutrient composition of multivitamin-mineral supplements

Background: Dietary research commonly assesses multivitamin (MVI) supplement use through broad categorical questions that assume products are compositionally similar. This study quantified the variability in micronutrient composition among best-selling MVI products to assess whether treating "multivitamin use" as a single exposure category is defensible. Methods: The 100 best-selling multivitamin products on Amazon, the dominant U.S. online supplement retailer, were screened on July 24, 2026. After excluding duplicate listings and products that were not general adult multivitamin supplements, 64 formulations remained. For each product, the labeled percent Daily Value (%DV) of 25 micronutrients was recorded directly from the Supplement Facts panel. Heterogeneity was characterized along three axes: composition breadth (number of nutrients included), per-nutrient content (inclusion rate, median, interquartile range, and range), and megadosing (defined a priori as ≥1,000% DV). Results: Composition breadth ranged from 7 to 24 of 25 nutrients (median 19.5). Only vitamin D, folate, and vitamin B12 were present in all 64 products, whereas choline, phosphorus, and iron were present in 22% or fewer. Labeled content varied widely; vitamin B12 ranged from 100% to 33,333% DV (median 408%). Thirty products (47%) contained at least one megadosed nutrient, concentrated in the B-complex vitamins and biotin. Conclusion: Best-selling multivitamins vary markedly in which nutrients they contain, in how much, and in whether they megadose. "Multivitamin use" is a compositionally heterogeneous exposure; studies should specify formulations or stratify by composition.