The active form of Vitamin D3,Calcitriol, exerts its effect by binding to the Vitamin D receptors (VDRs) which are widely distributed through many body tissues. Vitamin D3 has a half life of about 50 days as it is fat soluble. Vitamin D3 is absorbed in the small intestine and bound to specific a-globulins and transported to the liver where it is metabolised to 25-hydroxy Vitamin D3 (Calcidiol). A second hydroxylation to 1,25-dihydroxy Vitamin D3 (Calcitriol) occurs in the kidney. This metabolite is responsible for the vitamin's ability to increase the absorption of calcium. Non-metabolised Vitamin D3 is stored in tissues such as fat and muscle.Vitamin D3 is eliminated via faeces and urine.