Association between vitamin D and iron status in women of reproductive age: preliminary (cross-sectional) analysis of the UK National Diet and Nutrition Survey (NDNS)

Vitamin D and iron deficiencies are highly prevalent globally, with evidence of a complex interaction (1) . However, the extent to which these micronutrients interact, and whether this changes according to age and sex, remains uncertain. Understanding the association between vitamin D and iron could help address these nutrient insufficiencies concurrently, rather than as two independent public health concerns. The aim of this study was to determine the association between vitamin D (serum 25-hydroxyvitamin D (25(OH)D)) and iron (haemoglobin and ferritin) status in men and women of reproductive age. Secondary analysis of Years 9-11 (2016/17-2018/19) of the UK National Diet and Nutrition Survey (NDNS) Rolling Programme was conducted. Concentrations of blood biomarkers in adults aged 19-49 years were compared to UK Scientific Advisory Committee on Nutrition (SACN) guidance. Spearman’s bivariate correlations were performed to investigate associations between vitamin D and iron status using IBM SPSS Statistics (2024). Descriptive analyses found the percentage of men and women with serum 25(OH)D concentrations below the SACN threshold for increased risk of deficiency (<25nmol/L) (2) , were 18.4% and 17.9%, respectively. Moreover, 62.0% of men and 49.8% of women had insufficient 25(OH)D concentrations (<50nmol/L). For ferritin, 17.0% of women had concentrations below the 15µg/L deficiency threshold, compared to 0.0% of men (3) . Similarly, for haemoglobin, 8.7% of women were below the 120g/L diagnostic deficiency threshold compared to 2.3% of men (<130g/L) (3) . Haemoglobin (g/L) and 25(OH)D (nmol/L) concentrations were not associated in men (rs(172)=-0.019, p=0.802) or women (rs(244)=0.034, p=0.595). However, despite no association found between ferritin (µg/L) and 25(OH)D (nmol/L) in men (rs(175)=-0.022, p=0.766), a significant correlation was observed in women (rs(259)=0.184, p=0.003), albeit with a small effect size. Furthermore, a Kruskal-Wallis test showed a significant difference when comparing median ferritin concentrations by vitamin D status (deficient <25.00nmol/L; insufficient 25-49.99nmol/L; replete ≥50nmol/L) in women (H(2)=8.888, p=0.012). Post-hoc testing revealed no significant difference between ferritin concentrations in women classified as deficient or insufficient (p=0.687 – Bonferroni correction p=1.000), whereas, there was a significant difference between the ferritin concentrations of women who were vitamin D deficient and replete (p=0.015 – Bonferroni correction p=0.046), or insufficient and replete (p=0.015 – Bonferroni correction p=0.046). A significant correlation was found between 25(OH)D and ferritin concentrations in women; moreover, a significant difference was identified when comparing ferritin concentrations by vitamin D status. No associations were found between haemoglobin and 25(OH)D concentrations. In conclusion, these results suggest a potential association between vitamin D and iron status; and that vitamin D adequacy or deficiency/insufficiency may influence concentrations of ferritin, as a marker of iron, and/or vice versa. To further explore these associations between vitamin D and iron status, further analysis will adjust ferritin concentrations by C-reactive protein concentrations and investigate whether seasonality influences observed associations.