Vitamin B12
Supplements · Vitamins · Supplement, Typed Supps
Your stomach has to make a helper protein before you can absorb it at all.
Stage by stage
- Intake (Tablet or sublingual): Requires intrinsic factor, a protein made in the stomach, to be absorbed at all.
- Stomach (Binds intrinsic factor): Stomach acid frees B12 from food; it binds to intrinsic factor made by stomach cells.
- Ileum (Final small intestine): The B12–intrinsic factor pair is absorbed specifically in the last stretch of the small intestine.
- Marrow & nerves (Builds cells, maintains nerves): Used to build red blood cells and maintain the protective coating around nerve cells.
What the evidence supports
- Correcting a documented deficiency — strong evidence. Clear-cut, and important — untreated deficiency causes irreversible nerve damage.
- Preventing deficiency on a vegan diet — strong evidence. B12 is not reliably available from plants. This is not a preference, it is a requirement of the diet.
- Improving energy in people with normal B12 — insufficient evidence. The single most common reason people take it, and it does nothing if you are not deficient.
- Improving cognition or preventing dementia in people with normal levels — insufficient evidence. Trials have not shown benefit where status was already adequate.
Typical dose
250–1,000 mcg daily in supplements — far above the RDA because only a small fraction of a large oral dose is absorbed
Upper limit: No upper limit set — no toxicity established from high oral intake
Timing
High-dose oral or sublingual forms use a slower back-up absorption route.
Time to effect
- 2–3 wks: Energy may improve if deficient.
- Months: Nerve symptoms, if present, resolve more slowly.
Medication interactions
- Metformin: Long-term use meaningfully lowers B12 — worth periodic testing rather than avoidance. (moderate)
- Proton pump inhibitors and H2 blockers: Less stomach acid means less B12 released from food. Supplemental B12 is unaffected. (moderate)
Common myth
“Everyone needs a B12 supplement.” In reality: Most people eating animal products make enough from diet — it matters most for vegans, older adults, and some medications.
Food sources
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