Reflex Nutrition Vitamin D3 is very important on maintaining proper functioning muscles and for your immune system. This can help by protecting you from diseases and maintaining health bones and teeth. This is called the sunshine vitamin because you gain vitamin D from the ultra violet beta rays from the sun. Keeping optimum levels of Vitamin D within the body, can be hard to achieve on a daily basis. Therefore supplementation if often required to ensure a sufficient Vitamin D3 intake.