Dietary Supplement Use Explains More Variance in Nutritional Biomarker Concentrations than 24-Hour Dietary Intake from Food Only: Evidence from a Cross-Sectional Survey of the United States Population (NHANES 2003–2006)

Background: Nutritional biomarkers are used as objective measures of nutritional status to inform public health strategies. The relationship between dietary intake and biomarkers varies across population subgroups due to biological, demographic, and behavioral factors. Understanding these differences is important for identifying nutritional interventions. Objectives: We evaluated how sociodemographic, lifestyle, and physiologic factors modify the association between vitamin-specific dietary intake (food and supplements) and selected water- and fat-soluble vitamin biomarkers among US adults aged ≥20 y. Methods: Data were used from the 2003 to 2006 National Health and Nutrition Examination Survey for selected biomarkers [serum folate (SFOL), red blood cell folate (RBF), pyridoxal-5'-phosphate, vitamins B-12, A (VIA), C (VIC), E (VIE), and 25-hydroxyvitamin D] and either vitamin-specific dietary intake (24-h dietary recall) or self-reported supplement use (past 30 d). Demographic, lifestyle, and physiologic variables were: age, sex, race and Hispanic origin, education, poverty level, serum cotinine, alcohol use, physical activity, body mass index, estimated glomerular filtration rate, and serum C-reactive protein. Associations between vitamin-specific intake and biomarkers were assessed using both bivariate analyses and multivariable linear regression models accounting for the clustered sampling design. Results: ) from 3% (VIA) to 21% (SFOL) and dietary intake explained from 0.8% (VIE) to 8.8% (VIC) of the biomarker variability. Intake models indicated more confounding due to sociodemographic and physiologic variables. Supplement-use models demonstrated heterogeneous effects across subgroups (i.e., effect modification by age, sex, and race and Hispanic origin). After full adjustment, both dietary intake and supplement use were associated with biomarkers for all vitamins except for vitamin A intake and all supplement-use associations changed direction except for SFOL and RBF. Conclusions: These findings highlight the need to include supplement use and account for heterogeneous demographic effects when interpreting intake-biomarker associations in models.