Absolute and Functional Iron Deficiency in the US, 2017-2020.

Importance: The prevalence of absolute and functional iron deficiency among adults in the US is unknown. Objective: To estimate the prevalence of absolute and iron deficiency and iron supplement use in the US across age, sex, and comorbidity categories. Design, Setting, and Participants: This cross-sectional study analyzed data from the National Health and Nutritional Examination Survey (NHANES) 2017 to 2020 prepandemic cycle. Participants included noninstitutionalized, civilian women and men aged 18 years or older who had available serum ferritin, iron, and unsaturated iron binding capacity measurements. Data analysis was performed from March 21, 2023, to July 5, 2024. Exposure: Absolute iron deficiency and functional iron deficiency. Main Outcomes and Measures: Absolute iron deficiency was defined as serum ferritin less than 30 ng/mL regardless of transferrin saturation. Functional iron deficiency was defined as serum ferritin greater than or equal to 30 ng/mL with transferrin saturation less than 20%. The prevalence of absolute and functional iron deficiency was estimated among all adults in the US and separately among women and men according to age category (>18 years to <50 years, 50-65 years, and ≥65 years) using recommended sample weights and sampling design factors to provide estimates representative of the national, noninstitutionalized civilian population. The 95% CIs were calculated using the Korn-Graubard method. Results: A total of 8021 US adults (mean age, 48 years; 95% CI, 47-49 years; 52% [95% CI, 50%-53%] female) were included in this analysis. An estimated 14% (95% CI, 13%-15%) of adults in the US met the criteria for absolute iron deficiency, and an estimated 15% (95% CI, 14%-17%) met the criteria for functional iron deficiency. Among US adults without anemia, heart failure, chronic kidney disease, or current pregnancy, the estimated prevalence of absolute iron deficiency was 11% (95% CI, 10%-11%), and that of functional iron deficiency was 15% (95% CI, 14%-17%). The prevalence of functional iron deficiency exceeded that of absolute iron deficiency in all US adults except women younger than 50 years. Iron supplement use ranged from 22% (95% CI, 12%-37%) to 35% (95% CI, 29%-42%) of women with iron deficiency and 12% (95% CI, 5%-21%) to 18% (95% CI, 8%-32%) of men with iron deficiency depending on age. Conclusions and Relevance: These findings suggest that absolute and functional iron deficiency affect a large proportion of American adults even in the absence of anemia, heart failure, or chronic kidney disease. Further research on the role of functional iron deficiency in adverse health outcomes and on iron deficiency screening strategies is needed.