|Vitamin B12 (cyanocobalamin, cobalamin) is found naturally in food sources (principally animal products) in protein-bound forms. Animal products are the principal food sources of vitamin B12. B12 cannot be made by plants or by animals. It is thought that only bacteria (eubacteria, archaebacteria) manufacture the vitamin. The B12 in animal products is derived from bacterial B12 sources. Cyanocobalamin is the principal form of vitamin B12 used in nutritional supplements and for fortification of foods. Methylcobalamin is also available
for nutritional supplementation and hydroxocobalamin is available for parenteral administration. Good sources of vitamin B12 include liver, tuna, cottage cheese, yogurt and eggs. Most standard multivitamin supplements also provide the recommended daily allowance of vitamin B12.
In nature, B12 is solely produced by bacteria found in animals (including humans), so that dirt could actually be considered a natural source of B12. Vitamin B12 is made by bacteria and fungi, but not by yeasts or higher plants. Friendly bacteria reside in large quantities in the gastrointestinal tract of animals and humans. The richest dietary sources of vitamin B12 are liver, especially lamb's liver, and kidneys. Eggs, cheese and some species of fish also supply small amounts, but vegetables and fruits are very poor sources. The richest dietary sources of cobalamin are the liver, brain and kidney. Other sources, include egg yolk, clams, oysters, crabs, sardines, salmon and heart. Lower amounts of cobalamin are found in fish, beef, lamb, pork, chicken, cheese and milk. Plant foods are generally devoid of B12. Some fermented plant products, e.g., tempeh, may have some vitamin B12. Pseudovitamin B12 refers to B12-like substances which are found in certain organisms, such as Spirulina spp. (blue-green algae, cyanobacteria). However, these substances do not have B12 biological activity for humans. Food-form B12 is comprised of protein-bound methylcobalamin and adenosylcobalamin. The only reliable unfortified sources of vitamin B12 are meat, dairy products and eggs. Another alternative source of vitamin B12 is fortified cereal. Other sources of vitamin B12 are fortified soy milk, vitamin B12 fortified meat analogues (food made from wheat gluten or soybeans to resemble meat, poultry or fish)], and vitamin B12 supplements. There are vitamin supplements which do not contain animal products.
Vegans are recommended to ensure their diet includes foods fortified with vitamin B12. A range of B12 fortified foods are available. These include yeast extracts, Vecon vegetable stock, veggieburger mixes, textured vegetable protein, soya milks, vegetable and sunflower margarines, and breakfast cereals. For the lacto-ovo-vegetarian, reliable sources would include dairy products and eggs which can supply substantial amounts of B12. A slice of vegetarian cheddar cheese (40g) contains 0.5 µg. A boiled egg contains 0.7 µg. Fermentation in the manufacture of yoghurt destroys much of the B12 present. Boiling milk can also destroy much of the B12. Mushrooms cultivated on manure enriched compost will contain vitamin B12. If the mushrooms are not over washed before use they will contain some B12. There is 0.26ug of vitamin B12 in 100g of mushrooms. A serving of 4-6 mushrooms weighs 75g. Fermented soy products, such as miso and tempeh, shiitake (dried mushrooms) and algae such as spirulina and nori contain practically no vitamin B12.