Vitamin D3. Molecular graphic of vitamin D3. Red spheres represent carbon atoms, yellow spheres represent hydrogen. Solid lines represent bonds. Vitamin D3 is one of the D-group vitamins. It is present in oily fish (such as sardines, herring and tuna), liver and egg yolk, and is made in the skin after exposure to sunlight. It plays several important roles in the body, helping to regulate calcium and phosphate balance, and is essential for strong bones and teeth. Deficiency leads to rickets in children and osteomalacia in the elderly.