Vitamin B12 (cobalamin) is one of eight B vitamins. It's water-soluble, meaning excess is excreted rather than stored in fat - but your liver does store enough to last 3-5 years, which is why deficiency develops slowly and often goes unnoticed for a long time.
What B12 Does in the Body
Red blood cell formation. B12 is essential for the production of healthy red blood cells. Without it, cells grow abnormally large and can't carry oxygen effectively - a condition called megaloblastic anaemia.
Nerve function. B12 maintains the myelin sheath - the protective coating around nerve fibres. Damage to this sheath causes progressive neurological symptoms that can become permanent if deficiency continues untreated.
DNA synthesis. Every time a cell divides, it needs to replicate its DNA. B12 is a required cofactor in this process - making it particularly critical during pregnancy when cell division is rapid.
Energy metabolism. B12 helps convert food into usable energy by supporting the metabolism of fatty acids and amino acids. This is partly why B12 deficiency causes fatigue.
Food Sources of Vitamin B12
B12 is produced by bacteria. Animals accumulate it by eating bacteria-contaminated food or water. Plants don't accumulate it - which is why B12 is found almost exclusively in animal products.
| Food | Serving | B12 Content |
|---|---|---|
| Beef liver | 75g | 60mcg (2,500% RDA) |
| Clams | 85g | 84mcg |
| Salmon | 150g | 3.2mcg |
| Tuna (canned) | 85g | 2.5mcg |
| Beef | 85g | 1.4mcg |
| Milk | 240ml | 1.2mcg |
| Eggs | 2 large | 1.0mcg |
The daily recommended intake for adults is 2.4mcg. Despite B12 being only in animal products, most omnivores meet their needs easily. Vegans and strict vegetarians need fortified foods (plant milks, nutritional yeast, fortified cereals) or a B12 supplement to avoid deficiency.
Signs of B12 Deficiency
Because the liver stores B12 for years, deficiency typically develops slowly - often over years in vegans or people with absorption problems.
Early signs include fatigue, weakness, and numbness or tingling in the hands and feet. More advanced deficiency causes cognitive changes (memory problems, difficulty concentrating), depression, and in severe cases, irreversible nerve damage and dementia-like symptoms.
A blood test measures serum B12 levels. Below 148 pmol/L is generally considered deficient; 148-220 pmol/L is borderline. If neurological symptoms are present, treatment shouldn't wait on borderline results.
Who's at Risk
Vegans and vegetarians - dietary intake is limited to fortified foods. Supplementation is strongly recommended.
Adults over 50 - the stomach produces less hydrochloric acid with age, which impairs B12 absorption from food. Even people eating adequate B12 may not absorb it properly.
People taking metformin (for type 2 diabetes) - metformin reduces B12 absorption in roughly 10-30% of users.
People with pernicious anaemia - an autoimmune condition where the stomach doesn't produce intrinsic factor, the protein needed to absorb B12. Requires B12 injections or very high-dose oral supplements to bypass absorption.
Key Takeaway
B12 is essential for nerve function, red blood cell production, and DNA synthesis. Most people eating animal products are fine. Vegans, older adults, and anyone with absorption issues need to actively manage their B12 intake through fortified foods or supplementation.