Vitamin B12 is a member of a family of related molecules called corroids that contain a corrin nucleus made up of tetrapyrolic ring structure.It functions as co enzyme for transmethylation reaction involving the synthesis of purineDeficiency of B12 can lead to megaloblastic anemia and atheroma.It is synthesized by bacteria, as the single source. It is present in all forms of animal tissues which achieves B12 from bacteria. It doesn't occur in plants and vegetables. The highest level occurs in animal liver, beef and mutton. Dietary meat and fish are also good source of this vitamin.

Compounds with Vitamin D activity are sterol derivatives. The two most important forms are ergocalciferol (vitamin D2) of plant origin and chick peas or garbanzo beans calciferol (Vitamin D3) of animal origin. Vitamin D2 was the vitamin used for fortification of milk. During the exposure of sunlight provitamin D3 is converted to vitamin D3. Most foods have negligible amounts of Vitamin D. Certain marine fishes are good sources of vitamin D. Egg yolk, butter and milk have some Vitamin D.Vitamin D3 can be made in the human skin with the help of sunlight. Casual exposure to sunlight can provide vitamin D.Vitamin D is essential for the metabolism of Ca in the body. Deficiency of vitamin D can lead to rickets and osteomalacia. Fortification of the food products like milk is done with vitamin D to ensure the intake of Vitamin D through foods.