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Vitamin B12 (Cobalamin)

Vitamin B12 supports DNA synthesis, erythropoiesis and nervous-system function through methionine synthase and methylmalonyl-CoA mutase.[1][2][3] Deficiency can cause anemia or neurologic injury, and neurologic disease may occur with a normal blood count.

For reconstruction, the question is which bowel segment was used and how much functional terminal ileum remains. A limited ileal conduit, augmentation or ileal ureter does not necessarily abolish B12 absorption. Deficiency can emerge years later; post-cystectomy surveillance includes annual B12 measurement.[27][28] See Renal Function & Metabolic Surveillance for the follow-up framework and Vitamin B12 Supplementation for the treatment companion.

Biochemistry and Absorption​

B12 compounds include cyanocobalamin, hydroxocobalamin, methylcobalamin and adenosylcobalamin. The active coenzymes are:[1][3][5]

  • Methylcobalamin: supports remethylation of homocysteine to methionine and one-carbon metabolism.
  • Adenosylcobalamin: supports conversion of methylmalonyl-CoA to succinyl-CoA; impaired activity raises methylmalonic acid (MMA).

Gastric acid and pepsin release food-bound B12. It binds haptocorrin, then intrinsic factor after pancreatic proteases act in the duodenum. The intrinsic-factor–B12 complex is absorbed in the terminal ileum. A small fraction of a large oral dose is absorbed passively without intrinsic factor, explaining why high-dose oral replacement can work in selected patients with malabsorption.[3][6][17]

Dietary Sources and Requirements​

Animal foods and fortified foods or supplements provide B12. The US adult RDA is 2.4 μg/day, increasing to 2.6 μg/day in pregnancy and 2.8 μg/day during lactation. No tolerable upper intake level has been established because toxicity is low.[7] An older-adult cohort associated dairy-derived intake with better B12 status.[8]

Causes of Deficiency​

MechanismExamples and implications
Reduced absorptionAutoimmune gastritis/pernicious anemia, gastric surgery, extensive ileal disease or resection, some bariatric procedures, celiac disease and pancreatic insufficiency
Insufficient intakeVegan or restricted diets without adequate fortified foods/supplements, food insecurity and malnutrition
Medication-associated riskLong-term metformin, proton pump inhibitors and H2 blockers; assess the individual exposure and other risk factors
Functional inactivationNitrous oxide can inactivate B12 even when the total serum concentration is not low. Before surgery in patients with known or suspected B12 deficiency or gastrointestinal malabsorption, communicate this risk to the anesthesia team; postoperative myelopathy after nitrous oxide exposure has been described in a systematic review of cases.[32]
Multiple contributing factorsOlder age, gastric disease and restricted intake often coexist; pregnancy changes requirements and interpretation of tests

Consider periodic B12 assessment during long-term metformin use (ADA 2026, grade B), particularly with anemia or peripheral neuropathy; ADA's discussion advises annual testing after more than four years or with another risk such as previous small-bowel surgery.[34] Evaluate additional dietary or malabsorptive causes.[4][9][26]

Clinical Manifestations​

  • Hematologic: megaloblastic anemia, macrocytosis and hypersegmented neutrophils; severe deficiency may cause pancytopenia or a thrombotic-microangiopathy mimic.[4][10]
  • Neurologic: paresthesias, impaired vibration and proprioception, sensory ataxia, weakness and spasticity. Subacute combined degeneration affects the posterior and lateral spinal columns; peripheral or autonomic neuropathy may coexist.[9][10]
  • Neuropsychiatric: cognitive or mood change can occur, but these nonspecific symptoms also require consideration of other causes.[11][12]

Normal hemoglobin and MCV do not exclude neurologic B12 deficiency. Recovery may be incomplete when treatment is delayed. Obtain diagnostic blood samples before replacement when feasible, but do not delay treatment in a patient with suspected megaloblastic anemia and neurologic symptoms, especially subacute combined degeneration. NICE also regards deficiency as highly likely after total gastrectomy or complete terminal ileal resection in patients not receiving replacement.[10][16][19][26]

Diagnosis​

Interpret symptoms, anatomy, diet, medicines and supplements together; no single biomarker resolves every case.[17][19]

TestInterpretation
Total B12NICE uses <180 pg/mL as confirmed deficiency and 180–350 pg/mL as indeterminate; use validated local laboratory thresholds when they differ.[4][26]
Active B12 (holotranscobalamin)An alternative initial test; NICE prefers it in pregnancy. NICE treats below 25 pmol/L as confirmed deficiency and 25–70 pmol/L as indeterminate. Availability and assay ranges vary.[26]
MMAUseful with indeterminate B12 and compatible symptoms. Renal impairment can raise MMA independently; apply the assay reference range and clinical context.[10][19] For an indeterminate initial result after surgery that may cause deficiency or with a neurologic or hematologic condition that may deteriorate rapidly and substantially affect quality of life, NICE advises considering B12 replacement; if MMA is also ordered, start replacement while awaiting its result.[26]
HomocysteineLess specific: folate deficiency and renal dysfunction also increase it.[9][10]

For suspected deficiency caused by recreational nitrous oxide use, NICE recommends plasma homocysteine or serum MMA as the initial test; address ongoing exposure.[26] A serum value above the usual threshold does not explain away convincing neurologic findings or the effects of recent supplementation.

Identify the Cause​

Review the operation report for the remaining bowel anatomy. In IBD, AGA advises B12 monitoring with extensive ileal disease or previous ileal surgery.[13]

When autoimmune gastritis is suspected, check intrinsic-factor antibodies. A negative result does not exclude it; persistent suspicion may warrant additional specialist testing. Investigate other causes, including celiac disease, when the explanation remains unclear.[26]

Treatment and Follow-up​

Before treating megaloblastic anemia with high-dose folic acid alone, assess B12; folic acid can correct B12-related anemia while untreated neurologic disease may continue.[35] Choose replacement according to the cause, severity and likelihood of adherence. Parenteral treatment is favored for important neurologic disease, severe symptomatic deficiency or an unreliable oral response. Initial loading and later maintenance are different phases: a monthly maintenance injection alone is not an adequate initial treatment instruction for neurologic deficiency.[16][17][19]

NICE recommends lifelong IM replacement for autoimmune gastritis, total gastrectomy or complete terminal ileal resection. For other malabsorption, consider IM treatment; if oral replacement is chosen, use at least 1 mg/day and assess response. Partial ileal use in urinary reconstruction is not equivalent to complete terminal ileal resection.[26]

High-dose oral cyanocobalamin is commonly given at 1–2 mg/day.[19] Cyanocobalamin and hydroxocobalamin injection schedules depend on formulation and local protocol; see the supplementation companion. There is no established general clinical superiority of methylcobalamin over other replacement forms.[4][17][19]

What the Oral-versus-IM Evidence Shows​

EvidenceFinding and limit
Cochrane 2018: 3 RCTs, 153 participantsLow-certainty evidence for similar short-term biochemical correction with high-dose oral and IM treatment. Follow-up was only 3–4 months; the review did not provide usable clinical neurologic or quality-of-life outcome evidence.[14]
OB12: 283 adults aged ≥65 years, 52-week pragmatic RCTOral 1 mg/day for 8 weeks met the short-term noninferiority criterion. After changing to 1 mg/week, oral treatment did not establish noninferiority at 52 weeks. That maintenance regimen is not equivalent to continuing 1 mg/day. At baseline, 153/283 (54.1%) reported at least one symptom; symptom severity was not graded, limiting extrapolation to severe neurologic disease.[29]
2024 pernicious-anemia cohort: 26 patientsPatients began oral cyanocobalamin 1 mg/day; biochemical B12 deficiency was corrected in 23/26 (88.5%) within 1 month. The authors report correction in all participants by their last available assessment during 12 months of follow-up, but three died and one was lost before the final visit.[15]

Follow clinical response and adherence, rather than serum B12 alone. Arrange earlier review for severe disease; NICE generally uses an initial follow-up at 3 months and 1 month in pregnancy or breastfeeding. Symptoms may begin to improve within 2 weeks but can take up to 3 months, and NICE advises against repeating the initial diagnostic test in patients receiving IM treatment.[19][26]

Elevated Vitamin B12​

An unexpectedly high result can reflect supplementation, liver disease, renal dysfunction or increased binding proteins, including in myeloid disorders. It needs interpretation and is not a diagnosis of B12 toxicity or cancer.[22][23][25]

Among hospital patients without B12 supplementation or known causes of high B12, persistent B12 ≥1,000 pg/mL (n = 144) was associated with incident solid cancer over 60 months versus two values <1,000 pg/mL (n = 344; adjusted HR 5.90, 95% CI 2.79–12.45).[20] In a UK primary-care cohort (n = 757,185), B12 >1,000 pmol/L was associated with higher one-year incident cancer than B12 150–600 pmol/L (adjusted incidence rate ratio 4.72, 95% CI 3.99–5.58).[21] In a 22-cohort meta-analysis, high serum B12 (>600 pmol/L) was associated with all-cause mortality across 10 contributing cohorts (adjusted HR 1.50, 95% CI 1.29–1.74).[24]

Review supplement or injection exposure, symptoms, CBC, renal function and liver tests; tailor further evaluation to the findings and keep routine cancer screening current. There is no established universal imaging panel for an isolated high result, and indicated replacement should not be stopped solely because a post-treatment serum level is high.[17][19][25]

Reconstructive Practice​

SituationAction
Cystectomy with bowel diversionThe EAU bladder cancer guideline text states that B12 should be measured annually, because low levels have been reported in 17% of patients with bowel diversion. This is guideline text rather than a graded recommendation, and the panel's follow-up table records no consensus on B12 testing through year 10.[27] NCCN post-cystectomy follow-up specifies annual B12 testing after the first year, based on clinical judgment (category 2A).[31] The AUA/ASCO/SUO guideline discussion (Statement 31, Expert Opinion) also advises assessing B12 in follow-up, because the risk of deficiency and neurologic damage is increased after resection of more than 60 cm of ileum or use of the terminal ileum.[30]
NLUTD with bowel reconstructionAUA/SUFU recommends annual clinical assessment, basic metabolic panel and urinary-tract imaging (Statement 58, Expert Opinion); the guideline text adds that B12 should be monitored over time and supplemented if appropriate when terminal ileum is used.[28]
Augmentation, ileal ureter or continent reservoirDocument segment and length, preserved terminal ileum, baseline nutrition and subsequent levels. Ileocecal reservoirs such as Indiana and Mainz I can disrupt terminal-ileal absorption; in a 70-patient reconstruction series (median follow-up 65 months; malabsorption measured by Schilling test), ileocecal use was associated with B12 malabsorption, while ileal length alone was not.[33]
New gait change, paresthesias or macrocytosisTest promptly regardless of the routine surveillance date; consider coexisting copper deficiency and other neurologic causes.[9][10]
Bariatric historyFollow the bariatric nutritional protocol; baseline/annual assessment and more frequent early or risk-based testing may be required.[18]
Restricted diet or long-term metformin/acid suppressionCheck intake, supplement use and symptoms; investigate when the overall risk warrants it.[4][7]
Vegan diet or reduced intake (including with GLP-1 receptor agonists)Confirm supplementation and intake before major reconstruction, because reduced intake adds a mechanism independent of absorption.

Hepatic stores can delay deficiency for several years, so symptoms can appear long after the index operation. The typical presentation is a long-term cystectomy survivor with progressive paresthesias, gait instability or cognitive change. A normal early postoperative level does not remove the need for later surveillance, and the absence of anemia does not exclude a clinically important deficit. For partial ileal use or other malabsorption outside NICE’s lifelong-IM groups, oral replacement at 1–2 mg/day can be considered when adherence and response can be assessed; pharmacologic doses can be absorbed passively without intrinsic factor.[17][26]

See Also​

References​

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2. Calderón-Ospina CA, Nava-Mesa MO. "B Vitamins in the nervous system: Current knowledge of the biochemical modes of action and synergies of thiamine, pyridoxine, and cobalamin." CNS Neuroscience & Therapeutics. 2020;26(1):5–13. doi:10.1111/cns.13207

3. Moravcová M, Siatka T, Krčmová LK, Matoušová K, Mladěnka P. "Biological Properties of Vitamin B12." Nutrition Research Reviews. 2025;38(1):338–370. doi:10.1017/S0954422424000210

4. Patel H, McGuirk R. "Vitamin B12 Deficiency: Common Questions and Answers." American Family Physician. 2025;112(3):294–300. PMID 40961307

5. Coelho D, Suormala T, Stucki M, et al. "Gene Identification for the cblD Defect of Vitamin B12 Metabolism." The New England Journal of Medicine. 2008;358(14):1454–1464. doi:10.1056/NEJMoa072200

6. Allen LH. "Micronutrients — Assessment, Requirements, Deficiencies, and Interventions." The New England Journal of Medicine. 2025;392(10):1006–1016. doi:10.1056/NEJMra2314150

7. NIH Office of Dietary Supplements. Vitamin B12: Health Professional Fact Sheet. Accessed September 12, 2026.

8. Huang HH, Cohen AA, Gaudreau P, et al. "Vitamin B-12 Intake From Dairy but Not Meat Is Associated With Decreased Risk of Low Vitamin B-12 Status and Deficiency in Older Adults From Quebec, Canada." The Journal of Nutrition. 2022;152(11):2483–2492. doi:10.1093/jn/nxac143

9. Gwathmey KG, Grogan J. "Nutritional neuropathies." Muscle & Nerve. 2020;62(1):13–29. doi:10.1002/mus.26783

10. Stabler SP. "Vitamin B12 Deficiency." The New England Journal of Medicine. 2013;368(2):149–160. doi:10.1056/NEJMcp1113996

11. Reynolds E. "Vitamin B12, Folic Acid, and the Nervous System." The Lancet Neurology. 2006;5(11):949–960. doi:10.1016/S1474-4422(06)70598-1

12. Toh BH, van Driel IR, Gleeson PA. "Pernicious Anemia." The New England Journal of Medicine. 1997;337(20):1441–1448. doi:10.1056/NEJM199711133372007

13. Hashash JG, Elkins J, Lewis JD, Binion DG. "AGA Clinical Practice Update on Diet and Nutritional Therapies in Patients With Inflammatory Bowel Disease: Expert Review." Gastroenterology. 2024;166(3):521–532. doi:10.1053/j.gastro.2023.11.303

14. Wang H, Li L, Qin LL, et al. "Oral Vitamin B12 Versus Intramuscular Vitamin B12 for Vitamin B12 Deficiency." Cochrane Database of Systematic Reviews. 2018;3:CD004655. doi:10.1002/14651858.CD004655.pub3

15. Lacombe V, Vinatier E, Roquin G, et al. "Oral Vitamin B12 Supplementation in Pernicious Anemia: A Prospective Cohort Study." The American Journal of Clinical Nutrition. 2024;120(1):217–224. doi:10.1016/j.ajcnut.2024.05.019

16. Mauermann ML, Staff NP. "Peripheral Neuropathy." JAMA. 2026;335(3):255–266. doi:10.1001/jama.2025.19400

17. Wolffenbuttel BHR, McCaddon A, Ahmadi KR, Green R. "A Brief Overview of the Diagnosis and Treatment of Cobalamin (B12) Deficiency." Food and Nutrition Bulletin. 2024;45(1_suppl):S40–S49. doi:10.1177/03795721241229500

18. Mechanick JI, Apovian C, Brethauer S, et al. "Clinical Practice Guidelines for the Perioperative Nutrition, Metabolic, and Nonsurgical Support of Patients Undergoing Bariatric Procedures — 2019 Update." Obesity. 2020;28(4):O1–O58. doi:10.1002/oby.22719

19. Obeid R, Andrès E, Češka R, et al. "Diagnosis, Treatment and Long-Term Management of Vitamin B12 Deficiency in Adults: A Delphi Expert Consensus." Journal of Clinical Medicine. 2024;13(8):2176. doi:10.3390/jcm13082176

20. Lacombe V, Chabrun F, Lacout C, et al. "Persistent Elevation of Plasma Vitamin B12 Is Strongly Associated With Solid Cancer." Scientific Reports. 2021;11(1):13361. doi:10.1038/s41598-021-92945-y

21. Arendt JFH, Sørensen HT, Horsfall LJ, Petersen I. "Elevated Vitamin B12 Levels and Cancer Risk in UK Primary Care: A THIN Database Cohort Study." Cancer Epidemiology, Biomarkers & Prevention. 2019;28(4):814–821. doi:10.1158/1055-9965.EPI-17-1136

22. Ermens AA, Vlasveld LT, Lindemans J. "Significance of Elevated Cobalamin (Vitamin B12) Levels in Blood." Clinical Biochemistry. 2003;36(8):585–590. doi:10.1016/j.clinbiochem.2003.08.004

23. Arendt JF, Nexo E. "Cobalamin Related Parameters and Disease Patterns in Patients With Increased Serum Cobalamin Levels." PLoS One. 2012;7(9):e45979. doi:10.1371/journal.pone.0045979

24. Liu K, Yang Z, Lu X, et al. "The Origin of Vitamin B12 Levels and Risk of All-Cause, Cardiovascular and Cancer Specific Mortality: A Systematic Review and Dose-Response Meta-Analysis." Archives of Gerontology and Geriatrics. 2024;117:105230. doi:10.1016/j.archger.2023.105230

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26. NICE. Vitamin B12 deficiency in over 16s: diagnosis and management (NG239). Published March 6, 2024; minor update December 2024.

27. European Association of Urology. Muscle-invasive and Metastatic Bladder Cancer: Follow-up. 2026 guideline.

28. AUA/SUFU. Adult Neurogenic Lower Urinary Tract Dysfunction guideline. 2021; Statement 58 and discussion.

29. Sanz-Cuesta T, Escortell-Mayor E, Cura-Gonzalez I, et al. "Oral versus intramuscular administration of vitamin B12 for vitamin B12 deficiency in primary care: a pragmatic, randomised, non-inferiority clinical trial (OB12)." BMJ Open. 2020;10:e033687. doi:10.1136/bmjopen-2019-033687.

30. Holzbeierlein J, Bixler BR, Buckley DI, et al. Treatment of Non-Metastatic Muscle-Invasive Bladder Cancer: AUA/ASCO/SUO Guideline (2017; Amended 2020, 2024). J Urol. 2024;212(1):3-10. doi:10.1097/JU.0000000000003981

31. National Comprehensive Cancer Network. NCCN Clinical Practice Guidelines in Oncology: Bladder Cancer. Version 3.2026. August 6, 2026. BL-E 3–4 of 6; Discussion MS-17, MS-28. Guideline.

32. Patel KK, Mejia Munne JC, Gunness VRN, et al. "Subacute combined degeneration of the spinal cord following nitrous oxide anesthesia: A systematic review of cases." Clin Neurol Neurosurg. 2018;173:163–168. doi:10.1016/j.clineuro.2018.08.016

33. Ganesan T, Khadra MH, Wallis J, Neal DE. "Vitamin B12 malabsorption following bladder reconstruction or diversion with bowel segments." ANZ J Surg. 2002;72(7):479–482. doi:10.1046/j.1445-2197.2002.02460.x

34. American Diabetes Association Professional Practice Committee for Diabetes. "3. Prevention or Delay of Diabetes and Associated Comorbidities: Standards of Care in Diabetes—2026." Diabetes Care. 2026;49(Suppl 1):S50–S60. doi:10.2337/dc26-S003

35. Miller JW, Smith A, Troen AM, Mason JB, Jacques PF, Selhub J. "Excess Folic Acid and Vitamin B12 Deficiency: Clinical Implications?" Food Nutr Bull. 2024;45(1_suppl):S67–S72. doi:10.1177/03795721241229503