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Vitamin B12 has the largest and most complex chemical structure of all the vitamins. It is unique among vitamins in that it contains a metal ion, cobalt. For this reason cobalamin is the term used to refer to compounds having vitamin B12 activity. Methylcobalamin and 5-deoxyadenosylcobalamin are the forms of vitamin B12 used in the human body (1). The form of cobalamin used in most nutritional supplements and fortified foods, cyanocobalamin, is readily converted to 5-deoxyadenosylcobalamin and methylcobalamin in the body. In mammals, cobalamin is a cofactor for only two enzymes, methionine synthase and L-methylmalonyl-coenzyme A mutase (2).


Cofactor for methionine synthase

Methylcobalamin is required for the function of the folate-dependent enzyme, methionine synthase. This enzyme is required for the synthesis of the amino acid, methionine, from homocysteine. Methionine in turn is required for the synthesis of S-adenosylmethionine, a methyl group donor used in many biological methylation reactions, including the methylation of a number of sites within DNA, RNA, and proteins (Figure 1) (3). Aberrant methylation of DNA and proteins, which causes alterations in chromatin structure and gene expression, are a common feature of cancer cells. Inadequate function of methionine synthase can lead to an accumulation of homocysteine, which has been associated with increased risk of cardiovascular disease and age-related neurodegenerative disease (e.g., Alzheimer’s disease).

Figure 1. Vitamin B12 and Homocysteine Metabolism. Methionine synthase is a vitamin B12-dependent enzyme that catalyzes the formation of methionine from homocysteine using 5-methyltetrahydrofolate (5-methyl TH4), a folate derivative, as a methyl donor. Another pathway catalyzed by betaine homocysteine methyltransferase also remethylates homocysteine to methionine using betaine as a methyl donor (not shown here). Methionine, in the form of S-adenosylmethionine, is required for most biological methylation reactions, including DNA methylation.

Cofactor for L-methylmalonyl-coenzyme A mutase

5-Deoxyadenosylcobalamin is required by the enzyme that catalyzes the conversion of L-methylmalonyl-coenzyme A to succinyl-coenzyme A (succinyl-CoA), which then enters the citric acid cycle (Figure 2). Succinyl-CoA plays an important role in the production of energy from lipids and proteins and is also required for the synthesis of hemoglobin, the oxygen-carrying pigment in red blood cells (3).

Figure 2. Metabolic Pathway Requiring 5-Deoxyadenosylcobalamin. 5-deoxyadenosylcobalamin is required by L-methylmalonyl-CoA mutase, which converts L-methylmalonyl-CoA to succinyl-CoA.


In healthy adults, overt vitamin B12 deficiency is uncommon, mainly because total body stores can exceed 2,500 μg, daily turnover is slow, and dietary intake of only 2.4 μg/day is sufficient to maintain adequate vitamin B12 status (see RDA) (4). Among elderly individuals, vitamin B12 deficiency is more common, primarily because of impaired intestinal absorption that can result in marginal to severe vitamin B12 deficiency in this population. In certain regions of the world, marginal or subclinical vitamin B12 deficiency may be fairly common in other age groups as well, including children, premenopausal women, and pregnant women, primarily due to low intake of animal-source foods (reviewed in 5).

Causes of vitamin B12 deficiency

Absorption of vitamin B12 from food requires normal function of the stomach, pancreas, and small intestine. Stomach acid and enzymes free vitamin B12 from food, allowing it to bind to R-protein (formerly known as transcobalamin I and now called haptocorrin), found in saliva and gastric fluids. In the alkaline environment of the small intestine, R-proteins are degraded by pancreatic enzymes, freeing vitamin B12 to bind to intrinsic factor (IF), a protein secreted by specialized cells in the stomach (parietal cells). Receptors on the surface of the terminal ileum (the distal portion of the small intestine) take up the IF-B12 complex only in the presence of calcium, which is supplied by the pancreas (6). Vitamin B12 can also be absorbed by passive diffusion, but this process is very inefficient — only about 1% to 2% of the oral vitamin B12 dose is absorbed passively (2). The prevalent causes of vitamin B12 deficiency are (1) an autoimmune condition known as pernicious anemia, and (2) a disorder called food-bound vitamin B12 malabsorption. Both conditions have been associated with a chronic inflammatory disease of the stomach known as atrophic gastritis.  

Atrophic gastritis

Atrophic gastritis is thought to affect 10%-30% of people over 60 years of age (7). The condition is frequently associated with the presence of autoantibodies directed towards stomach cells (see Pernicious anemia) and/or infection by the bacteria, Helicobacter pylori (H. pylori) (8). H. pylori infection induces chronic inflammation of the stomach, which may progress to peptic ulcer disease, atrophic gastritis, and/or gastric cancer in some individuals. Diminished gastric function in individuals with atrophic gastritis can result in bacterial overgrowth in the small intestine and cause food-bound vitamin B12 malabsorption. Vitamin B12 levels in serum, plasma, and gastric fluids are significantly decreased in individuals with H. pylori infection, and eradication of the bacteria has been shown to significantly improve vitamin B12 serum concentrations (9).

Pernicious anemia

Pernicious anemia has been estimated to affect approximately 2% to 3% of individuals over 65 years of age (5). Although anemia is often a symptom, the condition is actually the end stage of an autoimmune inflammation of the stomach known as autoimmune atrophic gastritis, resulting in destruction of stomach cells by one's own antibodies (autoantibodies). Progressive destruction of the cells that line the stomach causes decreased secretion of acid and enzymes required to release food-bound vitamin B12 (5). Antibodies to intrinsic factor (IF) bind to IF and prevent formation of the IF-B12 complex, further inhibiting vitamin B12 absorption. Absorption of vitamin B12 is inhibited from both dietary sources and from the bile (enterohepatic circulation) and thus depletion of vitamin B12 body stores can occur relatively quickly. About 20% of the relatives of pernicious anemia patients also have the condition, suggesting a genetic predisposition. It is also thought that H. pylori infection could be involved in initiating the autoimmune response in a subset of individuals (10). Further, co-occurrence of autoimmune atrophic gastritis with other autoimmune conditions, especially autoimmune thyroiditis and type 1 diabetes mellitus, has been reported (11, 12).

Treatment of pernicious anemia generally requires intramuscular injections of vitamin B12 to bypass intestinal absorption. High-dose oral supplementation may be another treatment option, because consuming 1,000 μg (1 mg) per day of vitamin B12 orally should result in the absorption of about 10 μg/day (1% of dose) by passive diffusion. However, the effect of oral vitamin B12 varies among patients (13), and the available randomized controlled trials comparing the two treatment regimens are considered to be of low quality (14). Large-scale, well-designed trials with proper randomization and blinding are needed to determine whether high-dose oral supplementation is as effective as intramuscular injection for increasing vitamin B12 status and alleviating clinical symptoms of vitamin B12 deficiency.

Food-bound vitamin B12 malabsorption

Food-bound vitamin B12 malabsorption is defined as an impaired ability to absorb food- or protein-bound vitamin B12; individuals with this condition can fully absorb the free form (15). While the condition is the major cause of poor vitamin B12 status in the elderly population, it is usually associated with atrophic gastritis, a chronic inflammation of the lining of the stomach that ultimately results in the loss of glands in the stomach (atrophy) and decreased stomach acid production (see Atrophic gastritis). Because stomach acid is required for the release of vitamin B12 from the proteins in food, vitamin B12 absorption is diminished. Decreased stomach acid production also provides an environment conducive to the overgrowth of anaerobic bacteria in the stomach, which further interferes with vitamin B12 absorption (3). Because vitamin B12 in supplements is not bound to protein, and because intrinsic factor (IF) is still available, the absorption of supplemental vitamin B12 is not reduced as it is in pernicious anemia. Thus, individuals with food-bound vitamin B12 malabsorption do not have an increased requirement for vitamin B12; they simply need it in the crystalline form found in fortified foods and dietary supplements.

Other causes of vitamin B12 deficiency

Other causes of vitamin B12 deficiency include surgical resection of the stomach or portions of the small intestine where receptors for the IF-B12 complex are located. Conditions affecting the small intestine, such as malabsorption syndromes (celiac disease and tropical sprue), may also result in vitamin B12 deficiency. Because the pancreas provides critical enzymes, as well as calcium required for vitamin B12 absorption, pancreatic insufficiency may contribute to vitamin B12 deficiency. Since vitamin B12 is found predominantly in foods of animal origin, strict vegetarian and vegan diets can result in vitamin B12 deficiency (2, 16). Moreover, alcoholics may experience reduced intestinal absorption of vitamin B12 (2), and individuals with acquired immunodeficiency syndrome (AIDS) appear to be at increased risk of deficiency, possibly related to a failure of the IF-B12 receptor to take up the IF-B12 complex (3). Further, long-term use of acid-reducing drugs and the anti-diabetes drug metformin and repeated exposure to nitrous oxide have been implicated in vitamin B12 deficiency (see Drug interactions).

Inherited disorders of vitamin B12 absorption

Rare cases of inborn errors of vitamin B12 metabolism have been reported in the literature (reviewed in 6). Imerslund-Gräsbeck syndrome is an inherited vitamin B12 malabsorption syndrome that causes megaloblastic anemia and neurologic disorders of variable severity in affected subjects. Similar clinical symptoms are found in individuals with hereditary IF deficiency (also called congenital pernicious anemia) in whom the lack of IF results in the defective absorption of vitamin B12. Additionally, mutations affecting vitamin B12 transport and intracellular metabolism have been identified (17).

Symptoms of vitamin B12 deficiency

Vitamin B12 deficiency results in impairment of the activities of vitamin B12-requiring enzymes. Impaired activity of methionine synthase results in elevated blood concentrations of homocysteine, while impaired activity of L-methylmalonyl-CoA mutase results in increased concentrations of a metabolite of methylmalonyl-CoA called methylmalonic acid (MMA) in blood and urine. While individuals with mild vitamin B12 deficiency may not experience symptoms, blood homocysteine and/or MMA may be elevated (18). While elevated MMA blood concentration is considered the specific indicator of deficiency (19), other clinical assays are often used as biomarkers of vitamin B12 status — sometimes in combination. These include blood concentrations of total cobalamin, holo-transcobalamin (vitamin B12 bound to transcobalamin, one of the two carrier proteins in blood; called 'active vitamin B12'), and homocysteine (5). Yet, there is no ‘gold standard’ blood test, and the manifestation of clinical symptoms is important in the diagnosis of vitamin B12 deficiency (13).

Megaloblastic anemia

Diminished activity of methionine synthase in vitamin B12 deficiency inhibits the regeneration of tetrahydrofolate (THF) and traps folate in a form that is not usable by the body (Figure 3), resulting in symptoms of folate deficiency even in the presence of adequate folate. Thus, in both folate and vitamin B12 deficiencies, folate is unavailable to participate in DNA synthesis. This impairment of DNA synthesis affects the rapidly dividing cells of the bone marrow earlier than other cells, resulting in the production of large, immature, hemoglobin-poor red blood cells (macrocytosis), as well as effects on white blood cells, including abnormal (hypersegmented) neutrophils and reduced overall cell counts (pancytopenia). The resulting anemia is known as megaloblastic anemia and is the symptom for which the disease, pernicious anemia, was named (3). Supplementation with folic acid will provide enough usable folate to restore normal red blood cell formation. However, if vitamin B12 deficiency is the cause, it will persist despite the resolution of the anemia. Thus, megaloblastic anemia should not be treated with folic acid until the underlying cause has been determined (20).

Figure 3. Vitamin B12 and Nucleic Acid Metabolism. 5,10-Methylene tetrahydrofolate (TH4) is required for the synthesis of nucleic acids, while 5-methyl TH4 is required for the formation of methionine from homocysteine. Methionine, in the form of S-adenosylmethionine, is required for many biological methylation reactions, including DNA methylation. Methylene TH4 reductase is a flavin-dependent enzyme required to catalyze the reduction of 5,10-methylene TH4 to 5-methyl TH4.

Neurologic symptoms

The neurologic symptoms of vitamin B12 deficiency are myriad and include numbness and tingling of the hands and, more commonly, the feet (peripheral neuropathy); difficulty walking (gait ataxia); memory loss and other cognitive impairments; disorientation; alterations in mood, including depression and anxiety; and dementia that can resemble Alzheimer’s disease (13). Although the progression of neurologic complications is generally gradual, such symptoms may not be reversed with treatment of vitamin B12 deficiency, especially if they have been present for a long time. Neurologic complications are not always associated with megaloblastic anemia and are the only clinical symptom of vitamin B12 deficiency in about 25% of cases (21). Although vitamin B12 deficiency is known to damage the myelin sheath covering cranial, spinal, and peripheral nerves, the biochemical processes leading to neurologic damage in vitamin B12 deficiency are not yet fully understood (22).

Gastrointestinal symptoms

Tongue soreness, appetite loss, and constipation have also been associated with vitamin B12 deficiency. The origins of these symptoms are unclear, but they may be related to the stomach inflammation underlying some cases of vitamin B12 deficiency and to the progressive destruction of the lining of the stomach (21).

The Recommended Dietary Allowance (RDA)

The RDA for vitamin B12 was last revised by the Food and Nutrition Board (FNB) of the US Institute of Medicine (now the National Academy of Medicine) in 1998 (Table 1). Because of the increased risk of food-bound vitamin B12 malabsorption in older adults, the FNB recommended that adults over 50 years of age get most of the RDA from fortified food or vitamin B12-containing supplements (21).

Table 1. Recommended Dietary Allowance (RDA) for Vitamin B12
Life Stage Age Males (μg/day) Females (μg/day)
Infants  0-6 months  0.4 (AI 0.4 (AI) 
Infants  7-12 months  0.5 (AI)  0.5 (AI) 
Children  1-3 years  0.9  0.9 
Children 4-8 years  1.2  1.2 
Children  9-13 years  1.8  1.8 
Adolescents  14-18 years  2.4  2.4 
Adults  19-50 years  2.4  2.4 
Adults  51 years and older  2.4*  2.4* 
Pregnancy  all ages  2.6 
Breast-feeding  all ages  2.8
*Vitamin B12 intake should be from supplements or fortified foods due to the age-related increase in food-bound malabsorption.

Disease Prevention

Cardiovascular disease

As mentioned above, chronic atrophic gastritis and infection by H. pylori can cause deficiency in vitamin B12 secondary to malabsorption disorders (see Causes of vitamin B12 deficiency). Some studies, but not all, have found H. pylori infection or chronic atrophic gastritis to be associated with adverse cardiovascular events, including myocardial infarction and stroke (reviewed in 23-25).

Homocysteine and cardiovascular disease

Observational studies indicate that even moderately elevated blood concentrations of homocysteine raise the risk of cardiovascular disease (CVD), although randomized controlled trials of homocysteine-lowering therapy have generally not translated to reductions in adverse cardiovascular outcomes (26, 27). The mechanism by which homocysteine might increase CVD risk remains the subject of a great deal of research (28). The amount of homocysteine in the blood is regulated by at least four vitamins: folate, vitamin B6, riboflavin, and vitamin B12 (see Figure 1). An early analysis of the results of 12 randomized controlled trials showed that folic acid supplementation (0.5-5 mg/day) had the greatest lowering effect on blood homocysteine concentrations (25% decrease); co-supplementation with folic acid and vitamin B12 (500 μg/day) provided an additional 7% reduction (32% decrease) in blood homocysteine concentrations (29). The results of a sequential supplementation trial in 53 men and women indicated that after optimization of folate status with folic acid supplements, vitamin B12 became the major determinant of plasma homocysteine levels (30). In addition, in a population of older adults (age >60 years) exposed to folic acid fortification, the main determinants of plasma homocysteine were renal function and vitamin B12 (31). It is thought that the elevation of homocysteine levels might be partly due to vitamin B12 deficiency in individuals over 60 years of age. Two studies found blood methylmalonic acid (MMA) levels to be elevated in more than 60% of elderly individuals with elevated homocysteine levels. In the absence of impaired kidney function, an elevated MMA level in conjunction with elevated homocysteine suggests either a vitamin B12 deficiency or a combined vitamin B12 and folate deficiency (32). Thus, it appears important to evaluate vitamin B12 status, as well as kidney function, in older individuals with elevated homocysteine levels prior to initiating homocysteine-lowering therapy. For more information regarding homocysteine and CVD, see the article on folate.

Intervention studies

Although increased intake of folic acid and vitamin B12 is effective in decreasing blood concentrations of homocysteine, whether these B vitamins lower risk of CVD remains controversial. Several randomized, placebo-controlled trials have been conducted to determine whether homocysteine-lowering through folic acid, vitamin B12, and vitamin B6 supplementation reduces the incidence of CVD. A 2017 meta-analysis of data from 15 trials, including more than 71,000 participants at risk of or with existing CVD, showed that B-vitamin supplementation had no significant effect on risk of myocardial infarction (heart attack) or all-cause mortality but reduced risk of stroke by 10% (RR, 0.90; 95% CI, 0.82-0.99) (33). This analysis excluded trials in those with end-stage renal disease, who are at high risk of cardiovascular events and mortality. Two recent meta-analyses that pooled data from patients with end-stage renal disease found no benefit of B-vitamin supplementation on stroke outcomes (34, 35). Some have raised concern over potential toxicity of high-dose supplemental cyanocobalamin in individuals with impaired kidney function (34, 36); alternate supplement formulations of vitamin B12, such as methylcobalamin and hydroxycobalamin, have been suggested for this patient population.

A meta-analysis of 12 clinical trials measuring flow-mediated vasodilation (FMD; a surrogate marker of vascular health) in response to homocysteine reduction revealed that B-vitamin supplementation was accompanied by improved FMD in short-term (<8 weeks) but not in long-term studies conducted in subjects with preexisting vascular diseases (37). Some of the studies included in these meta-analyses did not use vitamin B12, and folate administration on its own has shown a protective role on vascular function and stroke risk (38, 39). Also, the high prevalence of malabsorption disorders and vitamin B12 deficiency in elderly individuals might warrant the use of higher doses of vitamin B12 than those used in these trials (40).


Folate is required for synthesis of DNA, and there is evidence that decreased availability of folate results in strands of DNA that are more susceptible to damage. Deficiency of vitamin B12 traps folate in a form that is unusable by the body for DNA synthesis. Both vitamin B12 and folate deficiencies result in a diminished capacity for methylation reactions (see Figure 3). Thus, vitamin B12 deficiency may lead to an elevated rate of DNA damage and altered methylation of DNA, both of which are important risk factors for cancer. A series of studies in young adults and older men indicated that increased levels of homocysteine and decreased levels of vitamin B12 in the blood were associated with a biomarker of chromosome breakage in white blood cells (reviewed in 41). In a double-blind, placebo-controlled study, the same biomarker of chromosome breakage was minimized in young adults who were supplemented with 700 μg of folic acid and 7 μg of vitamin B12 daily in cereal for two months (42).

Breast cancer

While results from two early case-control studies suggested higher intakes of vitamin B12 might help protect against breast cancer (43, 44), nested case-control studies and prospective cohort studies in different populations have found no associations between breast cancer incidence and dietary or total intake of vitamin B12 (dietary plus supplemental) (45-51) or blood concentrations of vitamin B12 (52-54). However, a nested case-control study within the Women’s Health Study (participants ≥45 years at baseline) reported a positive association between the highest quintile of total vitamin B12 intake (>10.5 μg/day) and breast cancer compared to the lowest quintile (≤4.3 μg/day; RR, 1.44; 95% CI, 1.02-2.04), but this study did not find an association between plasma concentrations of vitamin B12 and breast cancer (55). A more recent nested case-control study within the Nurses’ Health Study II reported a positive association between plasma concentrations of vitamin B12 and breast cancer risk; participants in this study were mostly premenopausal women (56). While additional studies are needed to determine whether elevated vitamin B12 status may be related to cancer development, including evaluating whether it might be a potential marker of cancer, it is important for observational studies to adequately control for potential confounders, such as age, menopausal status, alcohol use, and intake of the B-vitamin folate.

Neural tube defects

Neural tube defects (NTD) may result in anencephaly or spina bifida, which are often fatal congenital malformations of the central nervous system. The defects arise from failure of the embryonic neural tube to close, which occurs between the 21st and 28th days after conception, a time when most women are unaware of their pregnancy (57). Randomized controlled trials have demonstrated 60 to 100% reductions in NTD cases when women consumed folic acid supplements in addition to a varied diet during the month before and the month after conception. The homocysteine-lowering effect of folic acid may play a critical role in reducing the risk of NTD (58, 59). Homocysteine accumulates in the blood when there is inadequate folate and/or vitamin B12 for effective functioning of the methionine synthase enzyme (see Function). However, disruption in folate metabolism may be the underlying cause of folic acid-responsive NTD, and elevated homocysteine may simply be coincidental.

While the etiology of NTD is multifactorial, some studies have found that blood concentrations of homocysteine or holo-transcobalamin to be lower in NTD-affected infants and mothers compared to non-affected controls (reviewed in 59). Polymorphisms in various genes involved in vitamin B12 metabolism, including the genes encoding the intrinsic factor-cobalamin receptor (60) and the holo-transcobalamin receptor (61), have been associated with increased NTD risk in some studies. However, it is not known whether vitamin B12 supplementation, in addition to folic acid supplementation, may be beneficial in the prevention of NTD (62) — randomized controlled trials are needed to address this question.

Cognitive decline, dementia, and Alzheimer's disease

The occurrence of vitamin B12 deficiency prevails in the elderly population and has been frequently associated with Alzheimer's disease (reviewed in 63, 64). One study found lower vitamin B12 levels in the cerebrospinal fluid of patients with Alzheimer's disease than in patients with other types of dementia, though blood levels of vitamin B12 did not differ (65). The reason for the association of low vitamin B12 status with Alzheimer's disease is not clear. Vitamin B12 deficiency, like folate deficiency, may lead to decreased synthesis of methionine and S-adenosylmethionine (SAM), thereby adversely affecting methylation reactions. Methylation reactions are essential for the metabolism of components of the myelin sheath of nerve cells, as well as for synthesis and metabolism of neurotransmitters (22). Other metabolic implications of vitamin B12 deficiency include the accumulation of homocysteine and methylmalonic acid, which might contribute to the neuropathologic features of dementia (63). Indeed, elevated homocysteine is now recognized as a strong risk factor for cognitive impairment and dementia, including Alzheimer’s disease (66-68).

Observational studies

A large number of cross-sectional and prospective cohort studies have associated elevated blood homocysteine concentrations with measures of poor cognitive scores and increased risk of dementia, including Alzheimer's disease (reviewed in 67, 69). A case-control study of 164 patients with dementia of Alzheimer's type included 76 cases in which the diagnosis of Alzheimer's disease was confirmed by examination of brain cells after death. Compared to 108 control subjects without evidence of dementia, subjects with dementia of Alzheimer's type and confirmed Alzheimer's disease had higher blood levels of homocysteine and lower blood levels of folate and vitamin B12. Measures of general nutritional status indicated that the association of increased homocysteine levels and diminished vitamin B12 status with Alzheimer's disease was not due to dementia-related malnutrition (70). In a sample of 1,092 men and women without dementia followed for an average of 10 years, those with higher plasma homocysteine levels at baseline had a significantly higher risk of developing Alzheimer's disease and other types of dementia. Specifically, those with plasma homocysteine levels greater than 14 μmol/L had nearly double the risk of developing Alzheimer's disease (71). A study in 650 elderly men and women reported that the risk of elevated plasma homocysteine levels was significantly higher in those with lower cognitive function scores (72). A prospective study in 816 elderly men and women reported that those with hyperhomocysteinemia (homocysteine levels >15 μmol/L) had a significantly higher risk of developing Alzheimer's disease or dementia. Although raised homocysteine levels might be partly due to a poor vitamin B12 status, the latter was not related to risk of dementia or Alzheimer's disease in this study (73). Several meta-analyses of observational studies have found elevated blood concentrations of homocysteine to be linked with an increased risk of dementia (74) or Alzheimer’s disease (68, 75-78).

A systematic review of 35 prospective cohort studies assessing the association between vitamin B12 status and cognitive deterioration in older individuals with or without dementia at baseline did not support a relationship between serum vitamin B12 concentrations and cognitive decline, dementia, or Alzheimer’s disease (79). More recent meta-analyses of prospective cohort studies in older adults have not found a relationship between blood cobalamin concentration or dietary vitamin B12 intake and cognitive decline or dementia (80, 81). Nevertheless, studies utilizing more specific biomarkers of vitamin B12 status, including measures of holo-transcobalamin and methylmalonic acid, have shown more consistent results and a trend towards associations between poor vitamin B12 status and faster cognitive decline and increased risk of Alzheimer’s disease (82-86). Interestingly, two longitudinal studies in older adults (>60 years) found lower blood concentrations of holo-transcobalamin (and cobalamin) at baseline were associated with increased brain volume loss over a period of five (87) and six years (88).

Intervention studies

High-dose B-vitamin supplementation has been proven effective for treating hyperhomocysteinemia in elderly individuals with or without cognitive impairment. However, homocysteine-lowering trials have produced equivocal results regarding the prevention of cognitive deterioration in this population. A recent systematic review and meta-analysis of 21 randomized, placebo-controlled trials examining the effect of B-vitamin supplementation found the supplementation decreased homocysteine concentrations and improved measures global cognitive function but had no effect on individual measures of executive function, information processing speed, or episodic memory (89). This analysis pooled results from trials of participants with mild cognitive impairment (6 trials) and trials of participants without cognitive impairment (15 trials). A separate meta-analysis that pooled results of trials in older adults with mild cognitive impairment found B-vitamin supplementation (6 to 24 months, depending on the trial) had no benefits on various cognitive parameters, including executive function, information processing speed, and episodic memory (90). The trials of this meta-analysis employed different dosages of B vitamins, with vitamin B12 included in four of five trials.

A two-year, randomized, placebo-controlled study in older adults (>70 years) with mild cognitive impairment reported that a daily regimen of 800 μg of folic acid, 500 μg of vitamin B12, and 20 mg of vitamin B6 significantly reduced the rate of brain atrophy compared to placebo treatment (0.5% vs. 3.7%) (91, 92). Compared to placebo, B-vitamin supplementation in these older subjects also improved a measure of executive function, a secondary outcome of the trial (93). Interestingly, greater benefits were seen in those with high compared to low homocysteine concentrations at baseline, suggesting the importance of lowering homocysteine levels in prevention of brain atrophy and cognitive decline (91-93). The authors attributed the changes in homocysteine levels primarily to vitamin B12 (92). Yet, very few clinical trials have looked at the effect of vitamin B12, solely, on cognitive endpoints. A randomized, double-blind, placebo-controlled trial in 191 older adults (≥75 years) with moderate vitamin B12 deficiency (serum cobalamin concentrations of 107-210 pmol/L) without anemia provided participants with 1 mg/day of cyanocobalamin for one year (94). Compared to baseline, the vitamin B12 supplementation increased serum cobalamin and holo-transcobalamin levels and decreased blood homocysteine concentrations, but had no effect on measures of cognitive function, including assessments of memory, processing speed, and executive function (94). It is important to note that the participants in this trial were not experiencing clinical symptoms of vitamin B12 deficiency.

While it is known that B-vitamin supplementation effectively treats hyperhomocysteinemia, there is a need for well-designed, large trials to evaluate the effect of B-vitamin supplementation on long-term outcomes, such as age-related cognitive decline and the incidence of Alzheimer’s disease. The authors of one trial (95) included in the above-mentioned meta-analysis (89) noted that little cognitive decline was observed in the placebo group over their two-year trial, which may have limited their power to detect an effect of the B-vitamin intervention (95).


A few observational studies have found higher dietary intakes of vitamin B12 to be associated with lower risk of developing depression, especially among women. One study found as many as 30% of patients hospitalized for depression were deficient in vitamin B12 (96). A cross-sectional study of 700 community-living, physically disabled women over the age of 65 found that vitamin B12-deficient women were twice as likely to be severely depressed as non-deficient women (97). A population-based study in 3,884 elderly men and women with depressive disorders found that those with vitamin B12 deficiency were almost 70% more likely to experience depression than those with normal vitamin B12 status (98). A recent meta-analysis of 12 observational studies found that higher dietary intake of vitamin B12 was associated with a 23% lower risk of depression compared to lower intakes, but stratifying the data revealed a significant protective association in females, not in males (99). Similarly, a meta-analysis of nine observational studies in older adults found low serum vitamin B12 concentration to be associated with increased risk for depression, with significant association only in women (100). More recently, a longitudinal study among 3,849 community-living older adults in Ireland found that low vitamin B12 serum concentrations (<185 pmol/L) at baseline were associated with a 51% increased risk of developing symptoms of depression over a four-year period compared to those with normal concentrations (>258-601 pmol/L) (101).

The reasons for a link between vitamin B12 deficiency and depression are not clear but may involve a shortage in S-adenosylmethionine (SAM). SAM is a methyl group donor for numerous methylation reactions in the brain, including those involved in the metabolism of neurotransmitters whose deficiency has been related to depression (102). Severe vitamin B12 deficiency in a mouse model showed dramatic alterations in the level of DNA methylation in the brain, which might lead to neurologic impairments (103). This hypothesis is supported by several studies that have shown supplementation with SAM improves depressive symptoms (104-107).

Increased blood concentration of homocysteine is another nonspecific biomarker of vitamin B12 deficiency that has been linked to depressive symptoms in the elderly (108). However, few studies have examined the relationship of vitamin B12 status, homocysteine concentrations, and the development of depression over time. In a randomized, placebo-controlled intervention study with more than 900 older participants experiencing psychological distress, daily supplementation with folic acid (400 μg) and vitamin B12 (100 μg) for two years did not reduce the occurrence of symptoms of depression despite significantly improving blood folate, vitamin B12, and homocysteine levels compared to placebo (109). Additionally, a randomized, double-blind, placebo-controlled trial in 2,588 healthy older adults with mild hyperhomocysteinemia found that daily supplementation with 500 μg of vitamin B12 and 400 μg of folic acid for two years reduced blood homocysteine concentrations but had no benefit on depressive symptoms in those with or without depressive symptoms at baseline (110). In a long-term randomized, double-blind, placebo-controlled study among sufferers of cerebrovascular accidents at high risk of depression, daily supplementation with 2 mg of folic acid, 25 mg of vitamin B6, and 500 μg vitamin B12 significantly lowered the risk of major depressive episodes during a seven-year follow-up period compared to placebo (111). A randomized, double-blind, placebo-controlled trial in 153 older adults taking the antidepressant citalopram found daily supplementation with 500 μg vitamin B12, 2 mg of folic acid, and 25 mg of vitamin B6 for one year did not reduce the severity of depressive symptoms but increased the response to drug treatment and helped to prevent symptom relapse (112), suggesting B-vitamin supplementation might provide some benefit as an adjunct therapy.

Although it cannot yet be determined whether vitamin B12 deficiency plays a causal role in depression, it may be beneficial to screen for vitamin B12 deficiency in older individuals as part of a medical evaluation for depression.


High blood concentrations of homocysteine may affect bone remodeling by increasing bone resorption (breakdown), decreasing bone formation, and reducing bone blood flow. Another proposed mechanism involves the binding of homocysteine to the collagenous matrix of bone, which may modify collagen properties and reduce bone strength (reviewed in 113). Alterations of bone biomechanical properties can contribute to osteoporosis and increase the risk of fractures in the elderly. Since vitamin B12 is a determinant of homocysteine metabolism, it was suggested that the risk of osteoporotic fractures in older subjects might be enhanced by vitamin B12 deficiency. A meta-analysis of four observational studies, following a total of 7,475 older individuals for 3 to 16 years, found a weak association between an elevation in vitamin B12 of 50 pmol/L in blood and a reduction in fracture risk (114). Moreover, a US national study of 2,806 older women (≥50 years) found increased plasma concentration of methylmalonic acid — the metabolic indicator of vitamin B12 deficiency — to be associated with increased risk of lumbar spine osteoporosis (115).

Randomized controlled trials evaluating the role of vitamin B12 in bone health have used vitamin B12 in combination with other B-vitamins. A randomized controlled trial in 167 older adults (≥50 years) found two-year supplementation with vitamin B12 (10 μg/day), along with folic acid (200 μg/day), vitamin B6 (10 mg/day), riboflavin (5 mg/day), and vitamin D (10 μg/day), had no effect on bone mineral density (total hip, femoral neck, or lumbar spine) compared to the control group only receiving vitamin D (10 μg/day ) (116). However, in a subanalysis of participants with low vitamin B12 status at baseline (n= 101), the B-vitamin supplementation slowed the decline in bone mineral density (total hip and femoral neck but not lumbar spine) (116). A randomized, placebo-controlled trial in 559 elderly individuals with low serum levels of folate and vitamin B12 and at increased risk of fracture evaluated the combined supplementation of very high doses of folic acid (5 mg/day) and vitamin B12 (1.5 mg/day). The two-year study found that the supplementation improved B-vitamin status, decreased homocysteine concentrations, and reduced risk of total fractures compared to placebo (117). However, a multicenter study in 5,485 subjects with cardiovascular disease or diabetes mellitus showed that daily supplementation with folic acid (2.5 mg), vitamin B12 (1 mg), and vitamin B6 (50 mg) lowered homocysteine concentrations but had no effect on fracture risk compared to placebo (118). Another small, randomized, double-blind trial in 93 individuals with low vitamin D status found no additional benefit of B-vitamin supplementation (50 mg/day of vitamin B6, 0.5 mg/day of folic acid, and 0.5 mg/day of vitamin B12) on markers of bone health over a one-year period beyond that associated with vitamin D and calcium supplementation. Yet, the short duration of the study did not permit a conclusion on whether the lowering of homocysteine through B-vitamin supplementation could have long-term benefits on bone strength and fracture risk (119). In a randomized, double-blind, placebo-controlled trial in 2,919 participants (≥65 years) with elevated blood concentrations of homocysteine, supplementation with vitamin B12 (500 μg/day) and folic acid (400 μg/day) for two years did not decrease the risk of osteoporotic fracture despite reductions in homocysteine (120, 121). However, stratification of the data revealed a protective effect against osteoporotic fracture in participants older than 80 years (120). Further, a secondary analysis of a trial in women at high risk for or with existing cardiovascular disease reported that high-dose supplemental B-vitamins (1 mg/day of vitamin B12, 50 mg/day of vitamin B6, and 2.5 mg/day of folic acid) for 7.3 years had no effect on risk of non-vertebral fractures (122). In general, the available data from intervention trials do not support the use of supplemental B-vitamins to prevent osteoporotic fractures.


Food sources

Only certain bacteria and archaea can synthesize vitamin B12 (123, 124). Vitamin B12 is present in animal products, such as meat, poultry, fish (including shellfish), and to a lesser extent, dairy products and eggs (1). Strict vegetarians who eat no animal products (vegans) need supplemental vitamin B12 to meet their requirements. A few plant-source foods, such as certain fermented beans and vegetables and edible algae and mushrooms, may contain bioactive vitamin B12 (125). Together with B-vitamin fortified food (e.g., cereal, nutritional yeast) and supplements, these foods might contribute, though modestly, to prevent vitamin B12 deficiency in individuals consuming vegetarian diets. Individuals over the age of 50 years should augment their dietary intake with vitamin B12 from supplements or fortified foods because of the increased likelihood of food-bound vitamin B12 malabsorption with advanced age. 

Most people do not have a problem obtaining the RDA of 2.4 μg/day of vitamin B12 from food. According to a US national survey, the average dietary intake of vitamin B12 is 5.9 μg/day for adult men and 3.8 μg/day for adult women. Men and women 60 years or older have average dietary intakes of 5.6 μg/day and 3.7 μg/day, respectively (126). However, consumption of any type of vegetarian diet dramatically increases the prevalence of vitamin B12 deficiency in individuals across all age groups (127, 128). Some foods with substantial amounts of vitamin B12 are listed in Table 2, along with their vitamin B12 content in micrograms (μg). For more information on the nutrient content of specific foods, search USDA's FoodData Central.

Table 2. Some Food Sources of Vitamin B12
Food Serving Vitamin B12 (μg)
Clams (steamed) 3 ounces 84.1
Mussels (steamed) 3 ounces 20.4
Mackerel (Atlantic, cooked, dry-heat) 3 ounces* 16.2
Crab (Alaska king, steamed) 3 ounces 9.8
Nutritional yeast 1 tablespoon 8.8
Beef (lean, plate steak, cooked, grilled) 3 ounces 7.0
Salmon (chinook, cooked, dry-heat) 3 ounces 2.4
Rockfish (cooked, dry-heat) 3 ounces 1.4
Milk (2%) 8 fluid ounces 1.3
Turkey (roasted) 3 ounces 0.8
Brie (cheese) 1 ounce 0.5
Egg (poached) 1 large 0.4
Chicken (light meat, roasted) 3 ounces 0.3
*A 3-ounce serving of meat or fish is about the size of a deck of cards.


Vitamin B12 is available over-the-counter as a single-nutrient supplement and also as a component of multivitamin and vitamin B-complex supplements (129). Cyanocobalamin is the principal form used in oral supplements in the United States, while hydroxycobalamin is used primarily in Europe. Other forms, including methylcobalamin and adenosylcobalamin, are available as well (130). Sublingual vitamin B12 (placed under the tongue until dissolved) is available over-the-counter. Some studies suggest that sublingual cyanocobalamin (131-133) and sublingual methylcobalamin (134, 135) are effective at increasing vitamin B12 status. Further, cyanocobalamin and hydroxocobalamin are available by prescription in an injectable form to treat pernicious anemia, and cyanocobalamin is available as a prescription nasal spray.



No toxic or adverse effects have been associated with large intakes of vitamin B12 from food or supplements in healthy people. Doses as high as 2 mg (2,000 μg) daily by mouth or 1 mg monthly by intramuscular (IM) injection have been used to treat pernicious anemia without significant side effects (136). When high doses of vitamin B12 are given orally, only a small percentage can be absorbed, which may explain the low toxicity (4). Because of the low toxicity of vitamin B12, no tolerable upper intake level (UL) has been set by the US Food and Nutrition Board (21). Some have raised concern over potential toxicity of high-dose cyanocobalamin supplements in those with impaired kidney function (renal disease) (34, 36); alternate supplement formulations of vitamin B12, such as methylcobalamin and hydroxycobalamin, have been suggested for this patient population.

Drug interactions

A number of drugs reduce the absorption of vitamin B12. Proton-pump inhibitors (e.g., omeprazole, esomeprazole, and lansoprazole), used to treat Zollinger-Ellison syndrome and gastroesophageal reflux disease (GERD), markedly decrease stomach acid secretion required for the release of vitamin B12 from food but not from supplements. Long-term use of proton-pump inhibitors has been found to decrease blood vitamin B12 concentrations. However, vitamin B12 deficiency does not generally develop until after at least two-to-three years of continuous therapy (137-139). A recent systematic review and meta-analysis of 25 observational studies found proton-pump inhibitor use was associated with a small increased risk of vitamin B12 deficiency; however, the studies included in the pooled analysis were too heterogeneous to inform an association (140). Another class of gastric acid inhibitors known as histamine2 (H2)-receptor antagonists (e.g., cimetidine, famotidine, and ranitidine), often used to treat peptic ulcer disease, has also been found to decrease the absorption of vitamin B12 from food. It is not clear whether the long-term use of H2-receptor antagonists could cause overt vitamin B12 deficiency (141, 142). Individuals taking drugs that inhibit gastric acid secretion should consider taking vitamin B12 in the form of a supplement because gastric acid is not required for its absorption.

Other drugs found to inhibit vitamin B12 absorption from food include cholestyramine (a bile acid-binding resin used in the treatment of high cholesterol), chloramphenicol and neomycin (antibiotics), and colchicine (medicine for gout treatment). Metformin, a medication for individuals with type 2 diabetes mellitus, was found to decrease vitamin B12 absorption possibly by tying up free calcium required for absorption of the IF-B12 complex (143). However, the clinical significance of this is unclear (144). It is not known whether calcium supplementation can reverse vitamin B12 malabsorption; therefore, calcium supplementation is not currently prescribed for the prevention or treatment of metformin-induced vitamin B12 deficiency (145). Nevertheless, use of metformin may decrease vitamin B12 status: a meta-analysis of four short-term intervention trials (up to 4 months’ duration) in patients with type 2 diabetes found that metformin use decreased serum cobalamin concentrations by 57 pmol/L, which could have clinical implications if such patients have suboptimal vitamin B12 status (146). Previous reports that megadoses of vitamin C destroy vitamin B12 have not been supported (147) and may have been an artifact of the assay used to measure vitamin B12 status (21).

Nitrous oxide, a commonly used anesthetic, oxidizes and inactivates vitamin B12, thus inhibiting both of the vitamin B12-dependent enzymes, and can produce many of the clinical features of vitamin B12 deficiency, such as megaloblastic anemia and neuropathy. This is of particular risk in chronic recreational users of nitrous oxide. Since nitrous oxide is commonly used for surgery in the elderly, some experts feel vitamin B12 deficiency should be ruled out prior to its use or prophylactic vitamin B12 supplementation implemented before and after exposure to the gas (7, 18).

Large doses of folic acid given to an individual with an undiagnosed vitamin B12 deficiency can correct megaloblastic anemia without correcting the underlying vitamin B12 deficiency, leaving the individual at risk of developing irreversible neurologic damage (21). For this reason, the Food and Nutrition Board of the National Academy of Medicine advises that all adults limit their intake of folic acid (supplements and fortification) to 1,000 μg (1 mg) daily. There is no upper level set for reduced forms of folate (i.e., forms other than folic acid) found in foods.

Linus Pauling Institute Recommendation

A varied diet should provide enough vitamin B12 to prevent deficiency in most individuals 50 years of age and younger. Strict vegetarians and women planning to become pregnant should take a multivitamin supplement daily or eat fortified cereal, which would ensure a daily intake of 6 to 30 μg of vitamin B12 in a form that is easily absorbed. Higher doses of vitamin B12 supplements are recommended for patients taking medications that interfere with its absorption (see Drug interactions).

Older adults (>50 years)

Because vitamin B12 malabsorption and vitamin B12 deficiency are more common in older adults, the Linus Pauling Institute recommends that adults older than 50 years take 100 to 400 μg/day of supplemental vitamin B12.

Authors and Reviewers

Originally written in 2000 by: 
Jane Higdon, Ph.D. 
Linus Pauling Institute
Oregon State University

Updated in March 2003 by: 
Jane Higdon, Ph.D. 
Linus Pauling Institute 
Oregon State University

Updated in August 2007 by: 
Victoria J. Drake, Ph.D. 
Linus Pauling Institute 
Oregon State University

Updated in January 2014 by: 
Barbara Delage, Ph.D. 
Linus Pauling Institute 
Oregon State University

Updated in October 2023 by: 
Victoria J. Drake, Ph.D. 
Linus Pauling Institute 
Oregon State University

Reviewed in November 2023 by: 
Joshua W. Miller, Ph.D. 
Professor and Chair, Department of Nutritional Sciences 
Rutgers, The State University of New Jersey

Copyright 2000-2024  Linus Pauling Institute


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