Vitamin B12 (cobalamin) is a water-soluble vitamin required for DNA synthesis, erythropoiesis, and neurological function. Mainstream medicine: deficiency causes megaloblastic anemia and neuropathy; absorption requires intrinsic factor and adequate stomach acid.

Vitamin B12
Cobalamin (vitamin B12) molecular structure
Cobalamin (vitamin B12) molecular structure
Vitamin name Cobalamin
Chemical formula C₆₃H₈₈CoN₁₄O₁₄P
Solubility Water-soluble
Discovery year Late 1940s (isolated); structure determined ~10 years later via X-ray crystallography
Daily requirement (RDA) 2.4 mcg for adults (US)
Upper limit No established UL for food; high supplemental doses monitored clinically
Rich food sources Liver, beef, fish, shellfish, eggs, dairy
Ray's verdict

Ray Peat ties B12 to carotene clearance and vitamin A conversion when Thyroid is low.[1][2] He also discusses B12 with folate and B6 in homocysteine metabolism, and high serum B12 without supplementation as a possible sign of gut bacterial overgrowth.[3] [4]

B12 redirects here.

Structure / forms

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  • Hydroxocobalamin, common in food
  • Methylcobalamin, biologically active coenzyme form
  • Adenosylcobalamin, mitochondrial form in tissues
  • Cyanocobalamin, synthetic, not found in food

Carotene and vitamin A

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Hypothyroid patients often show carotenemia (yellowish skin from excess blood carotene). The liver converts carotene to vitamin A; thyroid support speeds clearance.[5]

Vitamin B12 and thyroid will lower the carotene quickly, but the calluses take a long time to wear off.

— Ray Peat[6]

Avoiding large amounts of carotene, and getting plenty of vitamin B12 to be able to convert any carotene that's in your food, helps to use vitamin A efficiently.

— Ray Peat[7]

Homocysteine and methylation

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A diet with folate, B12, B6, and good thyroid function supports safe homocysteine clearance. Milk and eggs are Peat-favored B12 sources.

In a double-blind trial, stroke survivors randomized to daily folic acid, vitamin B6, and B12 were about half as likely as those on placebo to develop new major depression over long-term follow-up. The difference in major or minor depression at the end of treatment was not significant, and the combined design cannot isolate B12.[8] In a meta-analysis of older adults, high homocysteine carried about 70% higher odds of depression. Carriers of the MTHFR 677TT genotype were about a fifth more likely than CC carriers to have current or past depression.[9] See Folic acid#Depression trials.

Deficiency and plant-based diets

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Vegetarian diets without B12 injections or reliable animal sources risk deficiency. Spirulina and plant "B12 analogs" are not adequate substitutes for egg yolks and animal foods.[10] See Vegetarianism.

Rising plasma B12 in children eating nori and spirulina did not improve their blood picture, while small amounts of fish did, so B12 from algae seems not to be bioavailable.[11]

Long-term use of acid-suppressing drugs, both proton pump inhibitors and H2 blockers, is associated with B12 deficiency, more strongly at higher doses.[12]

Undetected maternal deficiency can reach infants through breast milk. One case report documented a 6-month-old exclusively breastfed infant of a strict vegetarian mother with developmental regression and brain atrophy visible on CT; both the clinical symptoms and the brain imaging normalized after B12 treatment.[13]

High B12 without supplementation

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In the last few years, I've seen many people with twice the normal amount of B12, and it turns out that they have overgrowth of bacteria in their intestine. The B12 isn't hurting them, but when it's so high without a supplement, that means that their bacteria are going wild.

— Ray Peat[14]

Bacteria overgrowing the upper small intestine turn both dietary and newly made B12 into inactive analogues, so less true B12 reaches the host.[15] In one patient with overgrowth, serum B12 stayed normal through a B12-type anemia that cleared on small B12 doses, and a rise in inactive analogues may explain the normal reading.[16] See Gut.

Hair and aging markers

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People who gray early tend to have lower blood B12. In an Indian case-control study, patients with premature graying had lower B12, ferritin and vitamin D than matched controls.[17] In patients who grayed before age 25, low thyroid function and low B12 were both more common.[18] Schoolchildren with early gray hair had lower blood calcium and vitamin D than controls.[19] Gray hair on its own does not prove a B12 deficiency.

Dosing and food sources

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Liver: rich in B12 and fat-soluble vitamins, but Peat limits liver to ~4 oz/week due to high iron and phosphate.

Dairy and eggs: major Peat-favored B12 sources. Pasteurization reduces vitamin B2, B12 and folate very slightly; the main advantages of raw milk from a clean dairy are taste and the absence of additives.[20]

Peat generally prefers meeting B12 from food and intestinal synthesis rather than routine megadosing. If supplementing, methyl- or hydroxocobalamin are typical choices; cyanocobalamin is synthetic.

Repeated high oral doses are not free of effects: a young woman taking daily 1 mg cyanocobalamin for pernicious anemia developed acne, palpitations, anxiety, insomnia and headache, which cleared two weeks after stopping.[21]

Deficiency requires clinical workup. See Harm reduction.

References

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  1. ↑ "Thyroid: Therapies, Confusion, and Fraud," Ray Peat article
  2. ↑ "Carotenemia & Hypothyroidism"
  3. ↑ "Ray Peat, PhD on Endotoxin"
  4. ↑ Ray Peat, "Nutrition for Women", p. 89
  5. ↑ "Thyroid: Therapies, Confusion, and Fraud," Ray Peat article
  6. ↑ "Carotenemia & Hypothyroidism"
  7. ↑ "Carotenemia & Hypothyroidism"
  8. ↑ Almeida OP, Marsh K, Alfonso H, Flicker L, Davis TM, Hankey GJ. "B-vitamins reduce the long-term risk of depression after stroke: The VITATOPS-DEP trial," Ann Neurol. 2010;68(4):503-10. doi:10.1002/ana.22189. PMID 20976769. RCT, 273 stroke survivors, mean follow-up 7.1 years; new major depression 18.4% vs 23.3%, adjusted HR 0.48. End-of-treatment depression OR 0.58 (95% CI 0.31–1.09), not significant.
  9. ↑ Almeida OP, McCaul K, Hankey GJ, Norman P, Jamrozik K, Flicker L. "Homocysteine and depression in later life," Arch Gen Psychiatry. 2008;65(11):1286-94. doi:10.1001/archpsyc.65.11.1286. PMID 18981340. Meta-analysis; high homocysteine OR 1.70; MTHFR 677TT vs CC 22% more likely.
  10. ↑ "Carotenemia & Hypothyroidism"
  11. ↑ Dagnelie PC, van Staveren WA, van den Berg H. "Vitamin B-12 from algae appears not to be bioavailable," Am J Clin Nutr. 1991;53(3):695-697. doi:10.1093/ajcn/53.3.695. PMID 2000824. Plant foods supplied 0.1-2.7 µg B12/d without MCV improvement; fish with 0.15-0.5 µg/d improved MCV.
  12. ↑ Lam JR, Schneider JL, Zhao W, Corley DA. "Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency," JAMA. 2013;310(22):2435-2442. doi:10.1001/jama.2013.280490. PMID 24327038. 25,956 cases vs 184,199 controls; 2+ years of PPIs OR 1.65 (95% CI 1.58-1.73), H2 blockers OR 1.25 (1.17-1.34); over 1.5 PPI pills/d OR 1.95.
  13. ↑ Casella EB, Valente M, de Navarro JM, Kok F, "Vitamin B12 deficiency in infancy as a cause of developmental regression," Brain Dev. 2005;27(8):592-4.
  14. ↑ Ray Peat, "One Radio Network: Thyroid, PUFAs, OJ, and Sugar", One Radio Network, 2019-02-19, 1:34:43
  15. ↑ Brandt LJ, Bernstein LH, Wagle A. "Production of vitamin B12 analogues in patients with small-bowel bacterial overgrowth," Ann Intern Med. 1977;87(5):546-51. doi:10.7326/0003-4819-87-5-546. PMID 921081. Human study, jejunal aspirates from 4 patients; analogues in 3 of 4, above 25% of the oral labeled dose in 2 of 3.
  16. ↑ Murphy MF, Sourial NA, Burman JF, Doyle DV, Tabaqchali S, Mollin DL. "Megaloblastic anaemia due to vitamin B12 deficiency caused by small intestinal bacterial overgrowth: possible role of vitamin B12 analogues," Br J Haematol. 1986;62(1):7-12. doi:10.1111/j.1365-2141.1986.tb02894.x. PMID 3942698. Case report; abnormal deoxyuridine suppression test, normal serum and red cell folate.
  17. ↑ Kumar L, Gupta LK, Balai M, Mittal A, Maida R. "A Demographic and Biochemical Profile of Patients with Premature Graying of Hair (Canities) at a Tertiary Care Centre: A Case-Control Study," Indian Dermatol Online J. 2026;17(3):374-9. doi:10.4103/idoj.idoj_1049_24. PMID 41981836. Single-center case-control study; B12 P = 0.012, ferritin P = 0.020, vitamin D3 P = 0.043.
  18. ↑ Sonthalia S, Priya A, Tobin DJ. "Demographic Characteristics and Association of Serum Vitamin B12, Ferritin and Thyroid Function with Premature Canities in Indian Patients from an Urban Skin Clinic of North India: A Retrospective Analysis of 71 Cases," Indian J Dermatol. 2017;62(3):304-8. doi:10.4103/ijd.ijd_221_17. PMID 28584374. Retrospective chart review, 71 cases; anemia, ferritin and fasting glucose showed no link.
  19. ↑ Bhat RM, Sharma R, Pinto AC, Dandekeri S, Martis J. "Epidemiological and investigative study of premature graying of hair in higher secondary and pre-university school children," Int J Trichol. 2013;5(1):17-21. doi:10.4103/0974-7753.114706. PMID 23960391. Case-control study, 35 cases; serum ferritin also low.
  20. ↑ Ray Peat, Q&A, Dietary Guidelines, forum-hosted compilation.
  21. ↑ Morales-Gutierrez J, Díaz-Cortés S, Montoya-Giraldo MA, Zuluaga AF. "Toxicity induced by multiple high doses of vitamin B12 during pernicious anemia treatment: a case report," Clin Toxicol. 2020;58(2):129-131. doi:10.1080/15563650.2019.1606432. PMID 31018715. Symptoms after a total of 12 mg.