Vitamin B12 is part of the vitamin B-complex family and refers to the group of compounds that consists of cyanocobalamin, hydroxycobalamin, and related substances (1). It is essential for normal nerve function, DNA synthesis, hematopoiesis, fatty acid metabolism, and amino acid synthesis in the mitochondria. It also plays an important role in the metabolism of homocysteine and synthesis of S-adenosylmethionine (SAMe), another compound that is essential to many biological processes.
Vitamin B12 is naturally synthesized by bacteria and abundant in diets that include meat and dairy products, although a deficiency can occur in those who have malabsorption syndromes (2), take medications for gastric disorders (3) (4) or for conditions associated with insulin resistance (5), or those who follow a strict vegetarian diet (6). B12 deficiency is also more common among the elderly (7). Lack of B12 can lead to a wide variety of hematologic, neurologic, and psychiatric disorders, and may increase the risk of cardiovascular diseases (8). Low serum B12 levels have been associated with increased bone turnover and fracture risk (9). Repletion of vitamin B12 may improve response in patients resistant to antidepressants (10).
When used as homocysteine-lowering therapy, vitamin B12, folate, and vitamin B6 may have some cardiovascular benefits after coronary interventions (11), but do not appear to reduce the risk of further cardiovascular events including stroke (12) (13) (14). In addition, increased risk of in-stent restenosis with such treatment has been reported in patients after coronary stenting (15), and B12 supplementation as part of homocysteine-lowering therapy does not improve cognitive function (16) (17) (18) (19).
Clinical studies show that increased intake of vitamin B12 together with folate and vitamin B6 may lower the risk of breast (20) (21) (22) and cervical cancers (23), but has no effect on the risk of lung cancer (24). Findings from another study indicate that a combination of folate and vitamins B6 and B12 did not have an effect on overall risk of invasive cancer or breast cancer (25). However, data from a Norwegian study suggest higher cancer incidence and mortality in patients with ischemic heart disease following supplementation with vitamin B12 and folic acid (26). Further studies are needed to confirm these findings in other populations.
Supplementation with vitamin B12 intramuscular injections and oral folic acid reduces toxicity of pemetrexed chemotherapy in patients with non-small cell lung cancer (27), and may also help to improve treatment efficacy (28) (29).