Revisiting Vitamin B12 Deficiency: A Clinician's Guide For the 21st Century

by Brian J. Wentworth, Andrew P. Copland

Vitamin B12 (cobalamin) deficiency is a common disorder encountered across various medical and surgical disciplines. Traditional diagnosis has relied on serum cobalamin quantification; however, accumulating evidence suggests that a significant proportion of cases are missed without additional workup. This review discusses the various etiologies of B12 deficiency, provides a practical approach to diagnosis, and summarizes the available nutritional and medical literature regarding management.

Brian J. Wentworth MD, Fellow Physician, PGY-4, University of Virginia Health System, Division of Gastroenterology & Hepatology Andrew P. Copland MD, Assistant Professor of Medicine, University of Virginia Health System, Division of Gastroenterology & Hepatology, Charlottesville, VA


Vitamin B12 (cobalamin) is a water-soluble vitamin that serves as cofactor for three major cellular reactions converting:

  • 1. Methylmalonic acid (MMA) to succinyl coenzyme A,
  • 2. Homocysteine to methionine, and
  • 3. 5-methyltetrahydrofolate to tetrahydrofolate.

The first reaction is a key step in the tricarboxylic acid (TCA) or Krebs cycle within the mitochondria to generate energy (adenosine triphosphate), while the latter two reactions ensure unimpeded DNA synthesis. In addition, vitamin B12 (B12) is essential for myelin synthesis and maintenance within the nervous system and also plays a role in bone marrow erythropoiesis.1,2

B12 deficiency is quite common. Estimates range from 40% to 80% in developing nations;3 surprisingly, approximately 6% of people aged less than 60 years and nearly 20% of adults older than 60 years are B12 deficient in the United Kingdom and the United States.4 Despite its high prevalence, however, B12 deficiency often remains undiagnosed and may present subtly in patients. An understanding of the basic physiology of B12 absorption will help the clinician contextualize how deficiency may develop. Appreciating the limitations of current diagnostic strategies is key to effective clinical practice.


Vitamin B12 is one of the essential vitamins as it cannot be synthesized by human metabolism. Bacteria and archaebacteria synthesize B12 through aerobic and anerobic pathways, respectively.5 Human colonic flora are also able to produce B12, yet its location distal to the terminal ileum prevents absorption.6 To achieve an adequate daily intake of 2.4µg for adults (2.6µg for pregnant women and 2.8µg for lactating women), humans must obtain B12 from animal products including meat, seafood, dairy, and fortified cereals.7 Interestingly, ≤1% of free cobalamin is absorbed at the epithelial border in the terminal ileum. The remainder is stored in the liver and muscles, with a half-life of 1-4 years.6

In food, B12 is protein-bound. As food reaches the stomach, gastric parietal cells secrete pepsinogen and intrinsic factor (IF). Pepsin, the activated form of pepsinogen, cleaves food-bound B12 allowing it to bind to haptocorrin (R-binder). In the small bowel, pancreatic proteases break this B12-haptocorrin complex, forming a new B12-IF complex. The B12-IF complex travels to the terminal ileum where it is absorbed via the receptor complex cubam.6,8 After absorption, B12 binds to either haptocorrin for transport to the liver, or transcobalamin to form holotranscobalamin, which facilitates incorporation into cells.8 In contrast, synthetic or unbound B12 does not require pepsin to bind to IF and 1-2% can be passively absorbed throughout the GI tract without intrinsic factor or the presence of an ileum.9

Both enteral nutrition (EN) and parental nutrition (PN) are able to provide adequate daily requirements for B12, assuming the patient is on daily PN10 or receives the volume of EN needed to provide the daily requirement. A recent review of 62 enteral formulas determined on average each product provided > 200% of the recommended daily amount (doses of 1500 and 2000 Kcal/day).11 Although jejunal feeding bypasses the stomach, the passively absorbed synthetic B12 in commercial products is adequate to prevent deficiency.8


In addition to inadequate B12 intake, there are numerous steps in the B12 absorptive pathway where disease may strike (Table 1). Gastric parietal cell loss secondary to autoantibodies (autoimmune gastritis) or surgical removal causes loss of hydrochloric acid and intrinsic factor production.12 Autoimmune gastritis (AIG) has a prevalence of 2.5-12% without sex preference; all ages may be affected,13 but a large series reported a median age range of 70-80 years. It is associated with the presence of autoantibodies to parietal cells and/or intrinsic factor. Risk factors for development of AIG include a history of autoimmune disease (particularly thyroid disorders), northern European heritage, HLA DRB1*03 and DRB1*04 genotypes, and age over 30.14 Over time, pernicious anemia may develop, which is defined as the presence of anemia, low serum B12, gastric body atrophy (with resultant atrophy of oxyntic glands and hypochlorhydria), and the presence of autoantibodies. The duration from onset of AIG to development of pernicious anemia is not well described in the literature, but some reports suggest a latency of as long as 20 years.2

Intestinal malabsorption of food-bound B12 has several physiologic mechanisms, including ileal resection or active inflammation, pancreatic insufficiency, congenital defects (Table 1), and an altered intestinal microbiome.4 Small-intestinal bacterial overgrowth (SIBO) has increased in prevalence over time and may interfere with protein-bound B12 absorption due to competitive inhibition by abnormal ileal flora.15

Some medications can also interfere with B12 absorption. Chronic use (2+ years) of acid-suppressing medications, including H2 receptor antagonists (H2RAs) and proton-pump inhibitors (PPIs), are associated with a higher likelihood of deficiency. The proposed mechanism involves a loss of gastric acid required to activate pepsinogen to pepsin in the stomach, disabling the cleavage of B12 from its associated R-protein.16 Long-term metformin use has also been associated with B12 deficiency; however, a true estimate of effect size remains elusive.17 Unlike acid suppressants, the mechanism for B12 deficiency is less well understood for metformin, and may relate to interference of calcium-dependent membrane action necessary for B12-IF complex absorption in the terminal ileum.18 Recreational nitrous oxide (N2O) use in adolescent and young adult population may also precipitate B12 deficiency with high dose or chronic abuse. N2O irreversibly oxidizes the cobalt ion of B12, interfering with its ability to be a cofactor to methionine synthase, leading to downstream impairment of myelin production.19

Clinical Manifestations

The sequelae of B12 deficiency in adults ranges widely in severity. Given the hepatic storage of inactive B12, onset to overt deficiency may take up to 10 years.2 Mild deficiency may present only as fatigue. As B12 deficiency becomes more severe, skin hyperpigmentation, glossitis, cardiomyopathy and infertility can be seen.2,4 Thrombosis, including atypical presentations such as cerebral venous sinus thrombosis, may occur as a result of hyper-homocysteinemia induced by severe B12 deficiency.20 Bone marrow involvement is common and pancytopenia may develop in severe deficiency. Megaloblastic anemia is most frequently seen, although patients with AIG may initially demonstrate iron deficiency (gastric acid is necessary for duodenal iron absorption), before B12 deficiency is diagnosed.20 Neurologic dysfunction is not uniform and can present with demyelination of the posterior and lateral tracts of the spinal cord. Demyelination of these neurons causes both peripheral and truncal weakness as well as paresthesias and a loss of vibration, pressure, and touch sensation. Progressive neurologic damage with untreated B12 deficiency includes spastic ataxia, anosmia, ageusia, and optic atrophy.4,20 Peripheral neuropathy may also be seen, and in those with diabetes, it can be difficult to distinguish from diabetic polyneuropathy.21 Finally, at its most severe, B12 deficiency may cause a dementia-like presentation termed "megaloblastic madness" with depression, mania, irritability, paranoia, delusions, and frank psychosis with hallucinations.4,20 Clinicians need to be aware that concomitant anemia in the presence of neurologic signs may be absent in up to 20% of cases and delayed diagnosis can lead to progressive and irreversible damage.4


Making the diagnosis of B12 deficiency requires attention to the limitations of current laboratory assays. Serum B12 levels are often the first test performed, however these are subject to both false negatives and false positives. A severely low level (<100 µg/mL) is often associated with signs and symptoms of deficiency. Significant variation exists between various laboratory assays and B12 levels may be spuriously normal or falsely high in patients with anti-intrinsic factor antibodies as intrinsic factor is often used in the U.S. as the assay-binding protein.20 Thus, clinicians should consider the clinical context when interpreting serum levels and be careful to avoid direct comparison between two different values from independent laboratories (Table 2).

Elevated B12

An elevated serum B12 level is common. Prevalence ranges from 7-18% in hospitalized patients22 and does not necessarily exclude an underlying deficiency. The principle reason for a high level typically stems from an imbalance in B12 plasma binding proteins (haptocorrin, transcobalamin) related to either increased synthesis or decreased clearance. In liver disease, damaged hepatocytes release B12 in addition to abnormal hepatic clearance of haptocorrin. Elevated B12 levels may be seen in various solid and hematological cancers, mostly secondary to high haptocorrin production. Additionally, renal dysfunction leads to poor B12 clearance.8,22

Methylmalonic Acid and Homocysteine

When clinical manifestations are subtle, measurement of serum methylmalonic acid (MMA) and homocysteine (HCys) can be helpful as they reflect key cellular pathways involving B12. Both MMA and HCys are elevated in >98% of patients with B12 deficiency; HCys will also be elevated in folate deficiency. Both levels decrease rapidly after treatment and can be used to ensure adequate B12 supplementation.20 Limitations of MMA and HCys include falsely elevated levels in the presence of renal dysfunction.20 variation in pregnancy without validated reference ranges,24 and short-term fluctuations of MMA and HCys in both normal and deficient individuals.25 There also is new evidence that polymorphisms in the gene HIBCH affect MMA levels irrespective of B12 status.24

Determining Etiology

Identifying the cause of B12 deficiency aids in directing treatment. A detailed clinical history often reveals an obvious etiology such as vegetarian or vegan diets or patients with either gastric or ileal resections. The cumbersome Schilling test, involving administration of radioactive B12 and measuring fractional urine excretion, has been phased out. Non-invasive assessment for AIG currently relies on detection of serum autoantibodies to parietal cells (PCAs) and intrinsic factor (IFAs). The combination of PCA and IFA often improves the characteristics of this testing12 (Table 3). However, the gold standard for diagnosis of AIG is endoscopy with biopsy. Elevated fasting serum gastrin and low serum pepsinogen may also be used to support diagnosis if uncertainty remains.14


No guidelines exist to assist clinicians with identification of patients at increased risk for deficiency and guide screening intervals. Nonetheless, clinicians should be aware of the high prevalence in certain key patient populations (Table 4). Expert opinion regarding several of these conditions suggests annual screening with a CBC and possibly serum B12, MMA, and HCys.


Treatment of B12 deficiency has traditionally centered on increasing oral intake of food-bound B12 and intramuscular (IM) injection of the synthetic vitamin. Cyanocobalamin is the preferred form of B12 in the U.S., while hydroxocobalamin is primarily used in Europe; the latter formulation has been noted to have better retention and thus may be dosed less frequently.24 Both are readily converted to the biologically active adenosylcobalamin and methylcobalamin.24 Approximately 10-15% of a standard 1000µg IM B12 injection is retained, allowing for rapid replacement.24,26 Guidelines from the British Society for Haematology recommend thrice weekly injections for two weeks in patients without neurologic deficits, with extension to three weeks or until clinical improvement if neurologic symptoms are present.2 Injections may then be tapered to weekly for a month, then monthly in perpetuity if an irreversible cause is present. Improvement in MMA and HCys levels is seen within one-week; neurologic symptoms may take 6-12 weeks (sometimes with transient paradoxical worsening). Hematologic abnormalities may take up to eight weeks to normalize.2,20

Oral replacement has become more popular in recent years given the cost, convenience, and pain associated with injection. For a similar 1000µg dose (as compared to IM), only 0.5-4% is absorbed.24 A Cochrane review of the available evidence found no difference between serum B12 levels in patients taking either IM or oral formulations (most commonly 1000µg/day). Outcomes related to signs and symptoms of deficiency or quality of life were not reported in the trials reviewed.26 Oral supplementation should ideally be administered in a fasting state as it is less effectively absorbed when taken with a meal.

Although there is some evidence for high dose oral supplementation in patients with known malabsorption or severe deficiency, most experts recommend IM administration. Treatment should be continued indefinitely if the etiology of malabsorption is irreversible - in patients with pernicious anemia who discontinue supplementation, neurologic symptoms recur as soon as 6 months; megaloblastic anemia can return within a few years.24,27 A prophylactic daily oral dose of 1000µg B12 may be reasonable for patients having undergone bariatric surgery; in fact, this is recommended by the American Society for Metabolic and Bariatric Surgery.2 Interestingly, despite the high prevalence in Crohn's disease, the recent American College of Gastroenterology28 and American Gastroenterology Association guidelines29 do not address specific recommendations regarding B12 deficiency.

Other less common administration routes include sublingual30 and intranasal,31 although the data supporting these modalities is derived from small cohorts of patients without severe clinical manifestations (or anemia in the sublingual cohort). There is anecdotal experience with successful subcutaneous (SQ) administration, however rigorous comparisons to IM have not been published. SQ injection is a preferred administration route by some patients at our institution, as they report less injection site pain as compared to IM. Table 5 provides a condensed summary of B12 repletion strategies.


B12 deficiency is common, yet under diagnosed, as clinical manifestations may be subtle. Serum B12 levels can be problematic and clinicians should consider obtaining MMA and HCys to assist with diagnosis. Treatment can prevent irreversible neurologic damage. Fortunately, there are many therapeutic options for treating B12 deficiency and maintaining adequate B12 reserves.

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