Vitamin B (B12; also known as cobalamin) is a B vitamin that has an important role in cellular metabolism, especially in DNA synthesis, methylation and mitochondrial metabolism. Clinical B12 deficiency with classic haematological and neurological manifestations is relatively uncommon. However, subclinical deficiency affects between 2.5% and 26% of the general population depending on the definition used, although the clinical relevance is unclear. B12 deficiency can affect individuals at all ages, but most particularly elderly individuals. Infants, children, adolescents and women of reproductive age are also at high risk of deficiency in populations where dietary intake of B12-containing animal-derived foods is restricted. Deficiency is caused by either inadequate intake, inadequate bioavailability or malabsorption. Disruption of B12 transport in the blood, or impaired cellular uptake or metabolism causes an intracellular deficiency. Diagnostic biomarkers for B12 status include decreased levels of circulating total B12 and transcobalamin-bound B12, and abnormally increased levels of homocysteine and methylmalonic acid. However, the exact cut-offs to classify clinical and subclinical deficiency remain debated. Management depends on B12 supplementation, either via high-dose oral routes or via parenteral administration. This Primer describes the current knowledge surrounding B12 deficiency, and highlights improvements in diagnostic methods as well as shifting concepts about the prevalence, causes and manifestations of B12 deficiency.

Contradictory findings have been recently published on the evaluation of genetic polymorphisms of methylenetetrahydrofolate reductase (MTHFR 677 C-->T) and methionine synthase reductase (MTRR 66 A-->G) as risk factors for having a child with Down syndrome (DS); however, the influence of polymorphisms of methionine synthase (MTR 2756 A-->G) and of MTHFR 1298 A-->C has never been evaluated. In this study, the risk of being a DS case or having a DS child (case mother) was studied by multiple logistic regression analysis of the independent and combined genotypes and of plasma homocysteine, folates, and vitamin B12 in 92 DS cases and 140 control subjects as well as in 63 case mothers and 72 age-matched control mothers from Sicily. (The MTHFR 677 T allele frequency was not different in DS cases and case mothers, compared to the respective control groups). After adjustment for age, total homocysteine (t-Hcys) and MTR 2756 AG/GG genotype were significant risk factors for having a DS child, with odds ratio (OR) of 6.7 (95% CI: 1.4-32.0, P = 0.016) and of 3.5 (95% CI: 1.2-10.9, P = 0.028), respectively. By comparison, MTR 2756 AG/GG genotype increased significantly the risk of being a DS case, with an OR of 3.8 (95% CI: 1.4-10.5, P = 0.009). The double heterozygosity MTR 2756 AG/MTRR 66 AG was the single combined genotype that was a significant risk factor for having a DS child, with an OR estimated at 5.0 (95% CI: 1.1-24.1), after adjustment for t-Hcys. In conclusion, our results provide evidences that homocysteine and MTR genetic polymorphism are two potent risk factors for mothers to have a DS child in Sicily.

Folate, an essential nutrient found naturally in foods in a reduced form, is present in dietary supplements and fortified foods in an oxidized synthetic form (folic acid). There is widespread agreement that maintaining adequate folate status is critical to prevent diseases due to folate inadequacy (e.g., anemia, birth defects, and cancer). However, there are concerns of potential adverse effects of excess folic acid intake and/or elevated folate status, with the original concern focused on exacerbation of clinical effects of vitamin B-12 deficiency and its role in neurocognitive health. More recently, animal and observational studies have suggested potential adverse effects on cancer risk, birth outcomes, and other diseases. Observations indicating adverse effects from excess folic acid intake, elevated folate status, and unmetabolized folic acid (UMFA) remain inconclusive; the data do not provide the evidence needed to affect public health recommendations. Moreover, strong biological and mechanistic premises connecting elevated folic acid intake, UMFA, and/or high folate status to adverse health outcomes are lacking. However, the body of evidence on potential adverse health outcomes indicates the need for comprehensive research to clarify these issues and bridge knowledge gaps. Three key research questions encompass the additional research needed to establish whether high folic acid or total folate intake contributes to disease risk. 1) Does UMFA affect biological pathways leading to adverse health effects? 2) Does elevated folate status resulting from any form of folate intake affect vitamin B-12 function and its roles in sustaining health? 3) Does elevated folate intake, regardless of form, affect biological pathways leading to adverse health effects other than those linked to vitamin B-12 function? This article summarizes the proceedings of an August 2019 NIH expert workshop focused on addressing these research areas.