Vitamin B12 is found almost predominately in animal foods such as kidney, meat, liver, shellfish, fish, milk, and eggs. The source of Vitamin B12 is bacteria found in nature. Bacteria is the only creature able to produce Vitamin B12. Bacteria produce Vitamin B12 in the colon of humans and animals.
However not much vitamin B12 is absorbed across the wall of the colon so therefore it is essential that humans get B12 from animal foods. This presents a serious problem for vegetarians and vegans alike as they avoid eating animal products where vitamin B12 is found. Eggs are not a good source of B12 for vegetarians as they contain substances that block absorption of B12. Milk is a good source of B12 for vegetarians. Most vegetarians and vegans are recommended to take routine vitamin b12 supplementation. Some people can go longer on a vegatarian diet than others before they develop b12 deficiency.