Vitamin D3 is a natural form of vitamin D. It is essential for the regulation of calcium and phosphorus within the body, and plays an important role in maintaining the structure of bone and teeth, and normal immune function. It also plays a vital role in building and maintaining muscle. This is because the exposure of skeletal muscles to vitamin D influences the genetic processes that enhance muscle cell type, function and growth. At the same time, vitamin D suppresses the expression of myostatin, a protein that regulates muscle size by inhibiting growth. In addition, vitamin D increases the number of type II or fast twitch muscle fibres and in particular that of type IIA cells, resulting in an increase in muscular power output, while its deficiency causes type IIA cell atrophy and a loss of strength.