Vitamin B12 is now considered by nutritionists as the generic name for a group of compounds having vitamin B12 activity.
Vitamin B12 is an essential part of several enzyme systems that carry out a number of basic metabolic functions. Most reactions involve transfer or synthesis of one-carbon units, such as methyl groups. Vitamin B12 is metabolically related to other essential nutrients, such as choline, methionine and folic acid. Although the most important tasks of vitamin B12 concern metabolism of nucleic acids and proteins, it also functions in metabolism of fats and carbohydrates.
Feedstuffs of animal origin are reasonably good sources of vitamin B12—meat, liver, kidney, milk, eggs and fish. Kidney and liver are excellent sources, and these organs are richer in vitamin B12 from ruminants than from most nonruminants. Vitamin B12 presence in tissues of animals is due to the ingestion of vitamin B12 in animal feeds or from intestinal or ruminal synthesis. Among the richest sources are fermentation residues, activated sewage sludge, and manure.
The origin of vitamin B12 in nature appears to be microbial synthesis. It is synthesized by many bacteria but apparently not by yeasts or by most fungi. There is no convincing evidence that the vitamin is produced in tissues of higher plants or animals. Microbial synthesis of this vitamin in the alimentary tract is of considerable importance for animals.
Plant products are practically devoid of B12. The vitamin B12 reported in higher plants in small amounts may result from synthesis by soil microorganisms, excretion of the vitamin onto soil, with subsequent absorption by the plant. Root nodules of certain legumes contain small quantities of vitamin B12.