Metabolic Functions and Dietary Determinants of Vitamin B₁₂: A Review Focused on the Indian Population

Vitamin B₁₂ (cobalamin) is an essential water-soluble micronutrient required for neurological integrity, red blood cell formation, DNA synthesis, and one-carbon metabolism. As it is naturally present only in animal-source foods, individuals consuming predominantly plant-based diets, particularly vegetarians and vegans, are at increased risk of deficiency. Recent evidence highlights potential microbial or fermented plant-derived sources such as edible algae, mushrooms, and fermented foods, although their bioactive cobalamin content remains inconsistent. Relevant peer-reviewed literature was identified through databases including PubMed, Scopus, and Google Scholar, covering studies published between 1998 and 2025, and synthesised to examine sources, metabolism, deficiency epidemiology, and health implications of vitamin B₁₂. Globally, vitamin B₁₂ deficiency affects diverse population groups, ranging from approximately 6–10% in elderly populations in developed countries to up to 34% in children in developing regions, and is strongly influenced by dietary patterns, socioeconomic status, age-related malabsorption, and chronic medication use. In India, the burden is especially high, with prevalence estimates ranging from 30% to over 60% across different population groups, particularly among vegetarians. Maternal deficiency additionally poses risks for neurodevelopmental impairments in exclusively breastfed infants. Biochemical pathways linking vitamin B₁₂ to methionine synthase and L-methylmalonyl-CoA mutase underpin its critical roles in haematological, neurological, and methylation-related processes. This review summarises current evidence on vitamin B₁₂ sources, metabolism, deficiency epidemiology, determinants, and clinical consequences, with a particular focus on vulnerable Indian and vegetarian populations. The review underscores the need for targeted dietary guidance, fortification strategies, and public-health interventions to address the persistent burden of vitamin B₁₂ deficiency.