Vitamin B12

Water-soluble · cobalamin

What does Vitamin B12 do in your body?

Needed for nerve function, DNA synthesis and making red blood cells. Found naturally almost only in animal foods.

How it works, step by step

1. Stomach acid frees B12 from food protein.
2. It binds intrinsic factor, made in the stomach.
3. The complex is absorbed at the end of the small intestine (ileum).
4. B12 is carried in blood and stored in the liver.

What does Vitamin B12 do?

Coenzyme for two enzymes: one recycles homocysteine (with folate), the other processes certain fats and amino acids — important for myelin.

Where does Vitamin B12 act in the body?

Bone marrow, nerves (myelin sheath), brain. The liver stores several years' supply.

Which foods contain Vitamin B12?

Meat, fish, eggs, dairy; fortified plant drinks, nutritional yeast and cereals.

How much Vitamin B12 do adults need?

RDA: 2.4 mcg for adults. No upper limit set.

What happens with too little Vitamin B12?

Can cause anemia, fatigue, numbness and memory problems; nerve damage can become permanent if untreated. Most at risk: vegans without supplements or fortified foods, older adults, and people taking metformin or long-term acid reducers.

Can you get too much Vitamin B12?

No toxicity known.

What does Vitamin B12 interact with?

Needs stomach acid and intrinsic factor to be absorbed. Metformin and proton-pump inhibitors can reduce absorption.

What is known and unknown about Vitamin B12?

Roles and deficiency risks are established.

Evidence & sources

• People eating fully plant-based diets need B12 from fortified foods or supplements. Evidence: established. Known: B12 is made by microbes and isn't reliably present in plant foods. Unknown: — Source: NIH Office of Dietary Supplements, Vitamin B12 — Health Professional Fact Sheet (Current revision).

Connected topics

• Stomach
• Nerve Signaling
• Blood
• Folate (B9)
• Salmon
• Liver

Try it in your browser

General education only, not medical advice. Drafted Oct 2026 · pending qualified expert review.