Vitamin B12 is an essential micronutrient required for haematological, neurological, and reproductive health. Women of childbearing age (WCBA) represent a key at-risk group due to heightened physiological demands during pregnancy and lactation. The increasing adoption of plant-based diets in this population poses new challenges for maintaining sufficient B12 status. This thesis aimed to comprehensively investigate B12 intake, supplementation practices, awareness, and nutritional status among WCBA in the UK, and to evaluate the adequacy of current recommendations and diagnostic criteria.

A systematic review and meta-analysis of 19 studies compared biomarker profiles across dietary groups. Vegans exhibited significantly lower serum B12 (SMD -0.72; p=0.009) and higher homocysteine (SMD 0.57; p<0.001) than omnivores. Supplementation was associated with improved biomarker status. A commentary highlighted the critical importance of B12 during pregnancy and early life, noting that current UK recommendations fail to reflect modern dietary patterns. A nationwide pilot of a vitamin B12 risk awareness and practices questionnaire among vegan and vegetarian women of childbearing age (n = 121) demonstrated the tool’s feasibility and identified preliminary knowledge gaps and inconsistencies in supplementation habits, informing refinements and large-scale validation.

Analysis of pooled National Diet and Nutrition Survey (NDNS) data (2008-2019) revealed that vegetarians and vegans had markedly lower B12 intakes and biomarker concentrations than omnivores. Although supplementation improved status among vegans, over 75% of vegetarians remained below the functional serum B12 threshold (<258 pmol/L). Across all dietary groups, few women achieved the EFSA Adequate Intake (4 µg/day), and functional insufficiency was common even among those meeting the current UK RNI (1.5 µg/day). Functional markers, including low HoloTC and elevated tHcy, supported these findings.

Collectively, these findings reveal widespread suboptimal B12 status among UK WCBA and provide an evidence base for revising dietary reference values, strengthening supplementation guidance, and integrating B12 monitoring into antenatal and primary care.