Castle's extrinsic factor
Animal protein factor
Vitamin B12 is one of eight water-soluble B vitamins; all of the B vitamins work together synergistically, supporting each other in their various roles in the body. Vitamin B12 is the largest and most complex of all the vitamins.
Vitamin B12 is necessary for the normal formation of blood cells, nervous and immune systems. It also contributes to normal energy- yielding metabolism and normal functioning of the nervous system.
Vitamin B12 is produced exclusively by bacteria in the digestive tract of animals. Therefore, animal protein products are the main source of vitamin B12, in particular organ meats such as liver and kidney. Other good sources are fish, shellfish, eggs and dairy products.