B12 is a water soluble vitamin involved in various chemical reactions. We all need to get this from our food intake as we can not make it ourselves. But it is also a vitamin that is rarely seen to be deficient in the general healthy population as it’s widely available in all animal based foods. It is recommended as a supplement to the elderly and strict vegans.
In the human body it converts another B vitamin, folate (folic acid) into the form needed to make DNA, so it's part of the formative processes in the body. If either B12 or folic acid are low there may not be enough DNA made for the cells that make our blood cells, leading to abnormally large, oval shaped red and white blood cells, a condition known as megaloblastic anaemia.
B12 is part of the many components of energy production that goes on in every cell. It is also part of the structure of nerve cells, forming the outer coating of these cells, the myelin, which can crudely be described as the plastic insulating coating around an electrical wire, required for proper nerve conduction. If there is deficiency for long periods this can lead to irreversible neurological damage.
Signs of low B12 include symptoms of aneamia such as tiredness, breathlessness, palpitations, pale skin and anorexia, diarrhoea, red smooth tongue, possibly with ulcers, numbness, tingling in hands and feet, unsteadiness, poor memory.
B12 is stored mainly in the liver and is broken down very slowly and used very efficiently, so that stores last a long time, many years in fact, even if intake is stopped altogether. The daily guideline amount is therefore the very tiny amount of 1.5ug/day to cover for periods of zero intake and to increase stores.
B12 is first made by bacteria, the original source. All animal products contain B12 and these are the main sources for humans. Some algae and seaweeds contain the vitamin, though the absorption of these is not as good. Green plants do not contain B12 at all, unless they are contaminated by bacteria from soil. Legumes and root vegetables such as beans, potatoes and carrots may therefore be more likely to contain some contaminated sources of B12.
Because plant foods do not contain B12, strict vegans (who avoid dairy and eggs and all animal products) may be at risk of deficiency over many years and a supplement is recommended as a precautionary measure. But despite this most vegans and vegetarians show no evidence of deficiency. Possible reasons may be because there is some B12 contamination from soil, the body is very efficient at keeping hold of it’s B12 stores and because there may be some manufacture and absorption of it by the bacteria in the small intestine, though this has not been substantiated.
Non animal food source of B12 include cereals that have been fortified with B12. One serving of these often contains the full daily guideline amount. A serving of yeast extract such as marmite contains about half of what you need a day. Other fortified foods include some products made to resemble meat made from wheat or soya beans and fortified soy milks.
Deficiency in man is most commonly not from vegan eating but from problems with absorption due to a break down of the cells lining the stomach or intestines that take part in the uptake into the body.
Folic acid can mask a deficiency in B12, since these two vitamins work together and the lack of B12 may not be noticed until more specific and advanced deficiencies in B12 are seen. If you are in one of the groups at risk of deficiency if may be useful to take these two in combination, as part of a general multi vitamin/mineral capsule.