Vitamin D3 is the most important compound of the Vitamin D group, which is also referred as cholecalciferol. Vitamin D3 can be taken up by the body through diet and supplements. Vitamin D from the diet and the sunlight synthesized dermal vitamin D is biologically inactive. The activation vitamin D is facilitated by its enzymatic conversion or hydroxylation in the kidney and liver. A negative feedback loop regulation is known for synthesis of vitamin D which is formed by exposure of sunlight. This regulates the process and avoids toxicity. It is known to have vital role in homeostasis and metabolism of calcium. Deficiency of vitamin D leads to various disorders like rickets and osteomalacia. Vitamin D3 is converted to calcidiol in the liver. This is also referred as calcifediol. Vitamin D3 and vitamin D2 are measured in the serum for determination of vitamin D status of an individual. Some portion of calcidiol is converted to calcitriol by the kidneys which is the biologically active form of vitamin D.

Calcium and phosphate concentrations in the bloodstream are regulated by calcitriol. It is also known to affect the immune and neuromuscular function. Hypercalcemia is a condition which indicates a strong vitamin D toxicity. Untreated cases of hypercalcemia lead to calcium deposits in the organs like liver, heart and kidney. Supplementation of vitamin D3 in elder patients has shown reduced risk of death. Studies have also shown positive effect and anti-cancer properties of Vitamin D supplements. Further studies in this area are required to understand the effect of vitamin D3 on cancer. Various sources of vitamin D3 include Fish liver oils, Tuna, Sardines, Lichens, Beef liver etc. Vitamin D3 is also produced commercially by exposing 7-dehydrocholesterol to UVB light. 7-dehydrocholesterol is a naturally found in the liver of fish, or from sheep lanolin.