Vitamin B12
Essential for red blood cell formation, nerve health, and methylation. Deficiency is common in vegetarians, older adults, and anyone on long-term acid blockers or metformin.
Replacing deficiency is dramatic and well-documented — fatigue, mood, and cognitive symptoms reverse within weeks if caught before nerve damage. Supplementing when already replete provides little benefit. Effects appear within 1–4 weeks for energy and mood; neurological recovery can take months.
B12 (cobalamin) is required for DNA synthesis, red blood cell production, and the maintenance of the myelin sheath around nerves. It's also a key methylation cofactor — converting homocysteine to methionine, which then feeds the methylation cycle that handles neurotransmitter synthesis, gene regulation, and detox. Found almost exclusively in animal foods, it requires intrinsic factor (made in the stomach) for absorption. Deficiency develops quietly over years and shows up as fatigue, neuropathy (tingling, numbness), brain fog, mood changes, and macrocytic anemia. By the time the blood test catches it, neurological damage may already be partly irreversible.
- Form
- Methylcobalamin — the active methylated form. Best general choice. Hydroxocobalamin — long-lasting depot form (used in injections). Adenosylcobalamin — the mitochondrial form, useful in combination. Avoid cyanocobalamin (synthetic, requires conversion, leaves a small cyanide residue your body has to clear). Sublingual lozenges or sprays bypass the intrinsic-factor issue.
- Dose
- 500–1,000 mcg/day sublingual for maintenance. 1,000–5,000 mcg/day for documented deficiency. IM injection (1,000 mcg weekly to monthly) for severe deficiency, pernicious anemia, or post-bariatric.
- Timing
- AM with food. Many people report B12 is mildly energising — don't take at night.
- Schedule
- Daily for maintenance. Less frequent (1–2x weekly) acceptable if dose is high — B12 stores in the liver for years.
- Side effects
- Very safe. Rarely, acne (with high-dose methyl forms in sensitive individuals). The 'B12 makes you energetic' effect is real if you were deficient — neutral if you weren't.
- Interactions
- Metformin — long-term use depletes B12; anyone on metformin should supplement. PPIs and H2 blockers (omeprazole, famotidine, etc.) — reduce stomach acid needed to liberate B12 from food. Nitrous oxide — inactivates B12 (relevant for dental and recreational exposure).
- Contraindications
- Leber's hereditary optic neuropathy (LHON) — cyanocobalamin specifically is contraindicated; methyl or hydroxy forms are fine. Otherwise essentially none.
- Lab markers
- Serum B12 — the standard test, but limited. Below 200 pg/mL is deficient; 200–400 is grey zone. Methylmalonic acid (MMA) — far more sensitive; elevated MMA = functional B12 deficiency even with normal serum B12. Homocysteine — elevates with B12, folate, or B6 deficiency.
- Food sources
- Animal foods only: shellfish (clams, mussels — extremely high), liver and organ meats, fish (salmon, sardines, trout, tuna), beef, lamb, poultry, eggs, dairy. Fortified nutritional yeast and plant milks for vegans. Vegans and most vegetarians need to supplement — plant-form analogs in algae and seaweed don't reliably substitute.
Pairs with
Folate / methylfolate — partners in the methylation cycle. Always co-supplement (high folate alone can mask B12 deficiency on a blood test while neurological damage continues). Vitamin B6 (P5P). TMG and choline (methylation cofactors).
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