Objective To assess serum B₁₂, folate and the associated homocysteine (Hcy) levels among women of childbearing age in the UK and examine their association with dietary intake in relation to the UK Recommended Nutrient Intakes (RNIs) for B₁₂ and folate.

Design Cross-sectional study.

Setting Data from two publicly available National Diet and Nutrition Surveys (NDNS 2000/2001 and 2008/2012) were used. These were population-based surveys of randomly selected samples of adults which were carried out in their households.

Participants Women of childbearing age (aged 19–39 years), representative of the UK population. Those who were pregnant or breastfeeding were excluded.

Outcome measures The associations between micronutrient intakes and blood levels of B₁₂, folate and Hcy were assessed by correlation and stepwise linear regression. B₁₂ intake was divided into quintiles and plotted against blood B₁₂ and Hcy concentrations to determine the threshold of any associations.

Results 299 women from the first NDNS cohort had complete intake and biomarker data. The prevalence of serum vitamin B₁₂ (≤150 pmol/L) and serum folate (≤10 nmol/L) deficiency and hyperhomocysteinemia (≥12 µmol/L) was 12.4%, 6.4% and 21.2%, respectively, despite seemingly adequate B₁₂ intakes (median 3.8 μg/day, 96% consumed more than the UK RNI of 1.5 μg/day). B₁₂ concentrations increased across all quintiles of intake with serum levels in quintiles 4 and 5 (median intake 4.9 and 7.1 μg/day, respectively) significantly higher than quintile 1. However, Hcy concentrations levelled off between quintiles 4 and 5. Comparison of micronutrient intake between the two surveys found that folate intake has reduced in the more recent cohort.

Conclusions The UK RNI for B₁₂ intake should be increased for women of childbearing age with intakes of around 5–7 μg/day likely to be associated with stable biomarker levels. B₁₂ levels should also be measured in women preconceptionally or in early pregnancy given the high rates of deficiency.