Computer generated model of the chemical structure of Vitamin D3. Vitamin D3 (cholecalciferol, activated 7-dehydrocholesterol) Vitamin D is important for the absorption of calcium and phosphorus and for the normal depositing of these minerals into bone. It is found naturally in fish liver oils and artificially supplementation. Sunlight is the primary source of this vitamin by converting 7-dehydrocholesterol (in the skin) to vitamin D3.