Good sources of vitamin B12 include foods such as fish, meat, eggs, poultry, milk and milk products. Vitamin B12 is also added to foods such as cereals and can be taken in supplement form, according to the National Institutes of Health.
Food sources such as clams and beef liver have the highest amounts of naturally occurring vitamin B12, according to the National Institutes of Health. Clams contain 84.1 micrograms per serving, while beef liver has 70.7 micrograms per serving. Fortified cereals are next with 6 micrograms per serving, and chicken has the least amount at 0.3 micrograms per serving. In dietary supplements, vitamin B12 is present as cyanocobalamin, which is converted by the body to the active forms of vitamin B12 called methylcobalamin and 5-deoxyadenosylcobalamin. Some forms of vitamin B12 are also sold as tablets or lozenges.
Vitamin B12 is important because it is needed for the production of red blood cells, which carry oxygen to all the organs in the body, as WebMD explains. Without enough red blood cells, the body becomes anemic, resulting in fatigue, weakness, numbness, paleness, depression and even dementia. When there is a serious lack of vitamin B12, food sources are usually not enough to bring levels back to normal and injections of the vitamin may be needed.
Vitamin B12 is also an essential part of DNA synthesis because it binds to the protein in food, according to Mayo Clinic. It is absorbed into the bloodstream when stomach acid releases B12 from the protein during digestion and combines with a substance called intrinsic factor.