What is Vitamin D3? Vitamin D3 (also known as cholecalciferol) is a fat soluble vitamin which plays an important role in maintaining blood levels of calcium. The majority of vitamin D in the body is created during skin exposure to sunlight, but can also be found in egg yolk, fish oils and oily fish such as salmon, herring and mackerel. Cholecalciferol is an animal source of vitamin D3. What does Vitamin D3 do? Vitamin D3 increases the absorption of calcium from food and reduces the excretion of calcium from the body. These effects ensure that calcium levels are maintained and allow adequate levels to be stored in bones and teeth. Vitamin D promotes healthy blood calcium levels and in turn may help bone strength. Vitamin D is also responsible for maintaining healthy muscles and nerves, and for maintaining a healthy immune system.