Vitamin B12

Your intake

What each level of vitamin b12 does

Approximate dose-response bands. Individual response varies — these are starting points, not prescriptions.

1. Severely lowYOU ARE HERE

   0 mcg – 0.79 mcg

   Pernicious anemia, neurological symptoms, fatigue. Risk is high in vegans, older adults, and metformin users.

2. Insufficient

   0.79 mcg – 2.4 mcg

   Energy and cognitive function may be affected before frank deficiency shows on labs.

3. Adequate

   2.4 mcg – 3.6 mcg

   DV (2.4 mcg) met. Methylcobalamin and methylfolate together support methylation cycles.

4. Therapeutic

   3.6 mcg – 4.8 mcg

   Higher oral doses (500–1000 mcg) commonly used for sub-optimal status — B12 has no UL.

5. Diminishing returns

   4.8 mcg – +

   No established harm at high oral doses; excess is renally cleared.

Overview

Cobalamin is the largest, most structurally complex vitamin, containing a cobalt ion at the centre of a corrin ring. It cycles between methylcobalamin (methionine synthase) and adenosylcobalamin (methylmalonyl-CoA mutase) and is the cofactor that distinguishes folate's clinical presentation in deficiency. Strict vegans, older adults on PPIs, and gastric bypass patients are the highest-risk groups.

Functions

• ●Cofactor for methionine synthase — methylates homocysteine to methionine
• ●Cofactor for methylmalonyl-CoA mutase — branched-chain amino acid and odd-chain fatty acid catabolism
• ●Required for myelin sheath maintenance
• ●Required for normal hematopoiesis and DNA synthesis (indirectly via folate)

Mechanism

Methionine synthase transfers a methyl group from 5-MTHF to homocysteine, regenerating tetrahydrofolate and producing methionine (precursor to SAMe, the universal methyl donor). When B12 is low, folate is trapped as 5-MTHF and DNA synthesis stalls (megaloblastic anemia). Methylmalonyl-CoA accumulates when adenosylcobalamin is deficient, disrupting myelin lipid synthesis — explaining the neurological signs.

Benefits

• ●Reverses pernicious anemia and dietary B12 deficiency
• ●Lowers homocysteine when paired with folate and B6
• ●Prevents subacute combined degeneration of the spinal cord (irreversible if untreated)
• ●Modestly improves cognition in deficient older adults; not in replete adults

Deficiency

Common in older adults (atrophic gastritis impairs absorption), PPI/metformin users, vegans, and post-bariatric patients. Frequently missed because folate fortification corrects the haematological picture while neurological damage progresses.

Excess

No UL; B12 is excreted readily and toxicity has not been reported. Some observational data suggest very high serum B12 in non-supplementing adults may flag underlying liver or hematologic disease.

Forms

• Methylcobalamin
  Active methyl-donor form; bypasses methylation conversion step
• Adenosylcobalamin
  Active mitochondrial form for methylmalonyl-CoA mutase
• Hydroxocobalamin
  Longest serum half-life; preferred IM injection form
• Cyanocobalamin
  Synthetic, stable, cheap; requires conversion; releases cyanide (clinically trivial dose)

Food sources

• Clams (cooked) · 3 oz85 mcg
• Beef liver (cooked) · 3 oz70 mcg
• Cooked salmon · 3 oz4.8 mcg
• Beef (cooked) · 3 oz2.4 mcg
• Greek yogurt · 1 cup1.3 mcg
• Egg (whole) · 1 large0.5 mcg

Supplement forms

Methylcobalamin and adenosylcobalamin are the two coenzyme forms. Hydroxocobalamin has the longest serum half-life. Cyanocobalamin works for most healthy people but requires conversion and is usually penalized by Formulate's scoring. Sublingual or oral high-dose (1,000+ mcg) supplements work for most absorption deficits because passive uptake bypasses intrinsic factor.

Bioavailability

Two absorption routes: a saturable intrinsic-factor-dependent route in the terminal ileum (absorbs ~1–2 mcg per meal) and an unregulated passive route that absorbs ~1% of any dose. High-dose oral B12 (1,000+ mcg) exploits the passive route and matches injections for most deficiencies — except true pernicious anemia, where IM is more reliable.

Longevity relevance

Adequacy is non-negotiable for cognitive and neurological healthspan, especially after age 50. The neurological damage from prolonged deficiency is partially or fully irreversible — early correction matters. No evidence that megadosing past adequacy adds benefit.

Relationships

References

• NIH ODS — Vitamin B12guideline
• Linus Pauling Institute — Vitamin B12review
• Stabler — B12 deficiency review (NEJM 2013)review

About Vitamin B12

Myelin synthesis, red blood cell production, methylation cycle.

Role

Nerves · red blood cells

Daily target

2.4 mcg (DV)

Also called

vitamin b12, vitamin b-12, cobalamin, cyanocobalamin, methylcobalamin, hydroxocobalamin

Forms with lower absorption: cyanocobalamin. Prefer better-absorbed forms when supplementing.