In brief
A vitamin that keeps your blood and nerve cells healthy and helps make DNA. It is found naturally in animal foods.
- Active B12 (holotranscobalamin) is considered the best measure of B12 in the body, according to Pathology Tests Explained.
- People who eat little or no animal food might not get enough B12. Some breakfast cereals and nutritional yeasts are fortified with it.
- Large doses of folic acid can correct the anaemia of B12 deficiency but not the nerve damage, so they can hide a B12 deficiency.
How it works
A shortage can cause megaloblastic anaemia, which can make you feel tired or weak, and nerve problems such as numb or tingling hands and feet. Other symptoms include problems with balance, low mood, confusion and poor memory. A deficiency should be treated as soon as possible, because it can damage the nervous system.
Stomach acid releases B12 from food, and it then joins intrinsic factor, a protein made by the stomach, so it can be absorbed. People at higher risk of low B12 include strict vegetarians and vegans after many years, older people, people with pernicious anaemia, and long-term users of metformin or proton pump inhibitors.
Food sources
- Fish and seafood
- Meat and poultry
- Eggs
- Milk, yoghurt and cheese
- Fortified breakfast cereals and nutritional yeast
From the AskBioMap food guide:
Connections
6 connections on the map. Select a topic to read its page.
Vitamin B12 is needed for
Vitamin B12 helps keep nerve cells healthy, and a shortage can cause numb or tingling hands and feet.
Source: NIH Office of Dietary Supplements, Vitamin B12 fact sheet for consumers
Vitamin B12 and folate are needed to make red blood cells.
Vitamin B12 helps the enzyme methionine synthase turn homocysteine into methionine.
Source: NIH Office of Dietary Supplements, Vitamin B12: fact sheet for health professionals
Vitamin B12 is linked with
- Brain fog Supported
A shortage of vitamin B12 can cause confusion and poor memory.
Sources: NIH Office of Dietary Supplements, Vitamin B12 fact sheet for consumers; Pathology Tests Explained, Vitamin B12 and folate
B12 deficiency can cause megaloblastic anaemia, which can make you feel tired or weak.
Source: NIH Office of Dietary Supplements, Vitamin B12 fact sheet for consumers
Vitamin B12 is measured by
Active B12 is considered the best measure of B12 in the body.
Lab tests
- Full blood count (FBC)One of the most common blood tests. It counts and measures red blood cells, white blood cells and platelets.
- Vitamin B12 and folate testsBlood tests for vitamin B12 and folate, usually ordered after a full blood count shows a problem, or for nerve symptoms.
- Homocysteine testMeasures homocysteine, an amino acid. It is most often used to assess vitamin B12, folate and B6 levels.
Medicines and supplements
General education only. Never start, stop or change a medicine without your prescriber.
Sources
Each connection above names its own source. See every source AskBioMap uses.
Summarised from the NIH Office of Dietary Supplements and Pathology Tests Explained. Sources read on 13 and 19 October 2026. General education only, not medical advice. Spotted a mistake? Please tell us through the HuBu-Intel contact page.