Vitamin B₁₂ (cobalamin) is an essential cofactor for two metabolic pathways. It is obtained principally from food of animal origin. Cobalamin becomes bioavailable through a series of steps pertaining to its release from dietary protein, intrinsic factor-mediated absorption, haptocorrin or transcobalamin-mediated transport, cellular uptake, and two enzymatic conversions (via methionine synthase and methylmalonyl-CoA-mutase) into cofactor forms: methylcobalamin and adenosylcobalamin. Vitamin B₁₂ deficiency can masquerade as a multitude of illnesses, presenting different perspectives from the point of view of the hematologist, neurologist, gastroenterologist, general physician, or dietician. Increased physician vigilance and heightened patient awareness often account for its early presentation, and testing sometimes occurs during a phase of vitamin B₁₂ insufficiency before the main onset of the disease. The chosen test often depends on its availability rather than on the diagnostic performance and sensitivity to irrelevant factors interfering with vitamin B₁₂ markers. Although serum B₁₂ is still the most commonly used and widely available test, diagnostics by holotranscobalamin, serum methylmalonic acid, and plasma homocysteine measurements have grown in the last several years in routine practice. The lack of a robust absorption test, coupled with compromised sensitivity and specificity of other tests (intrinsic factor and gastric parietal cell antibodies), hinders determination of the cause for depleted B₁₂ status. This can lead to incorrect supplementation regimes and uncertainty regarding later treatment. This review discusses currently available knowledge on vitamin B₁₂, informs the reader about the pitfalls of tests for assessing its deficiency, reviews B₁₂ status in various populations at different disease stages, and provides recommendations for interpretation, treatment, and associated risks. Future directions for diagnostics of B₁₂ status and health interventions are also discussed.

5-Methyltetrahydrofolate (5-MTHF) is the predominant form of folate and a strong determinant of homocysteine concentrations. There is evidence that suboptimal 5-MTHF availability is a risk factor for cardiovascular disease independent of homocysteine. The analysis of folates remains challenging and is almost exclusively limited to the reporting of “total” folate rather than individual molecular forms. The purpose of this study was to establish the reference intervals of 5-MTHF in plasma and red cells of healthy adults who had been prescreened to exclude biochemical evidence of functional deficiency of folate and/or vitamin B₁₂. Functional folate and vitamin B₁₂status was assessed by respective plasma measurements of homocysteine and methylmalonic acid in 144 healthy volunteers, aged 19–64 years. After the exclusion of 10 individuals, values for 134 subjects were used to establish the upper reference limits for homocysteine (13 μmol/L females and 15 μmol/L males) and methylmalonic acid (430 nmol/L). Subjects with values below these cutoffs were designated as folate and vitamin B₁₂replete and their plasma and red cell 5-MTHF reference intervals determined,N=126: 6.6–39.9 nmol/L and 223–1041 nmol/L, respectively. The application of these intervals will assist in the evaluation of folate status and facilitate studies to evaluate the relationship of 5-MTHF to disease.

Gestational diabetes mellitus (GDM) can adversely affect the health of the developing fetus. Women of South Asian origin are particularly at risk of developing GDM. Insulin resistance (IR) contributes to the etiology of GDM, and although studies have shown associations of vitamin B₁₂ (B₁₂) and folate status with GDM and IR, only a limited number of B₁₂ and folate markers have been used. We used a comprehensive panel of B₁₂ and folate markers to examine their association with IR in pregnant women with diet-controlled GDM and normal glucose tolerance (NGT). In this cross-sectional study, 59 British-Bangladeshi women (24 GDM and 35 NGT) with a mean age of 29 y, BMI (in kg/m²) 26.7 and gestational age 33 wk were recruited. Serum total B₁₂, holotranscobalamin, folate, methylmalonic acid, plasma homocysteine, 5-methyltetrahydrofolate, and red cell folate (RCF) were measured along with other parameters. The independent sample t-test and chi-squared test were used to assess differences in markers between GDM and NGT women. Spearman's test was used to look for correlations. A simple multiple regression analysis was used to investigate if markers of B₁₂ and folate status predicted IR, using the HOMA-IR and adjusting for age, GDM status, and BMI. There were no differences in concentrations of B₁₂ and folate markers between GDM and NGT women. In Spearman's analysis HOMA-IR correlated negatively with total serum B₁₂ (P < 0.001) and holotranscobalamin (P < 0.05), and positively with BMI (P < 0.001), blood pressure (P < 0.05) and triglycerides (P < 0.05) in all women. MMA did not correlate with any of the B₁₂ markers. In regression analysis, total B₁₂ (β = −0.622, P = 0.004), RCF (β = 0.387, P = 0.018), and BMI (β = 0.024, P < 0.001) were the significant predictors of HOMA-IR variance. Significant associations between markers of B₁₂ and folate status with HOMA-IR were found during the third trimester in British-Bangladeshi women. B₁₂ markers correlated poorly with each other.

Aims: A growing body of evidence suggests that ethnicity and race influence vitamin B ₁₂ metabolism and status yet clinical awareness of this is poor, causing doubts regarding diagnosis and treatment. Moreover, deficiency and insufficiency cut-offs are universally applied for this test in most diagnostic settings. The objective of this study was to assess serum vitamin B ₁₂ concentrations in Black, Asian and White primary care patients in London, UK, particularly in patients of Black or Black British ethnic origin and establish if there is a need for specific reference ranges. Methods: Serum B ₁₂ results from 49 414 patients were processed between January 2018 and November 2019 using the Architect assay (Abbott Diagnostics) at St. Thomas’ Hospital, London, UK. Age, sex and ethnicity data were collected from the laboratory Health Informatics Team. Results: Black patients (n=13 806) were found to have significantly higher serum vitamin B ₁₂ concentration across all age groups and both sexes, especially Nigerian patients (median B ₁₂ 505 pmol/L,IQR: 362–727, n=891), compared with Asian and White ethnic groups (p652 pmol/L) (adjusted OR 3.38, 95% CI 3.17 to 3.61, p<0.0001). Conclusions: It is likely that a combination of genetic and acquired/environmental factors are responsible for the ethnic differences in serum B ₁₂ . This suggests that there is a need for ethnic-specific reference ranges with indications for the incorporation of age and sex too.