Vitamin D Supplement Use Is Associated with Better Status, Yet Most Biochemical Vitamin D, Iron and Vitamin B12 Deficiency Is Unrecognised and Unsupplemented: A Cross-Sectional Study of a Screened Corporate Workforce in Poland

Background/Objectives: Whether self-directed supplement use reaches people who are deficient is unclear. We examined vitamin D, iron and vitamin B12 status and supplement use in a screened workforce. Methods: Cross-sectional analysis of 793 employees (75.2% women, aged 18 to 67) in Poland sampled from April to June 2026; 25-hydroxyvitamin D [25(OH)D], ferritin and vitamin B12 were measured by automated immunoassay, and supplement use was ascertained from an open inventory. Deficiency was defined as 25(OH)D < 30 ng/mL, ferritin < 30 µg/L and vitamin B12 < 200 pg/mL. Results: Deficiency was common (vitamin D 53.2%; iron 29.4%, 38.3% of women; vitamin B12 3.0%). Vitamin D users reached the target more often than non-users (77.4% versus 38.5%; adjusted odds ratio 5.5, 95% CI 3.7 to 8.3; 1.8 in a sensitivity scenario that reclassified prompted reports as use); with 22 iron and 17 vitamin B12 users, no association could be characterised. Few deficient participants listed the corresponding supplement (vitamin D 9.0%, iron 2.2%, vitamin B12 0%; 32.1% and 9.1% with a prompted item). Among deficient respondents, 70.2%, 69.2% and 86.4% (vitamin D, iron, vitamin B12) reported neither a prior diagnosis nor a supplement, and routine characteristics discriminated deficiency poorly (C-statistic 0.58 for vitamin D, 0.57 for iron within women). Conclusions: Vitamin D supplement use was associated with better status, but most biochemical deficiency occurred in people who reported neither a diagnosis nor a supplement and could not be identified from routine characteristics.