Vitamin D

The collective term for a group of substances that help to regulate the balance of phosphate and calcium in the body, aid calcium absorption in the intestine, and promote strong bones and teeth. Good sources include oily fish, liver, and egg yolk; vitamin D is also added to margarines. In the body, vitamin D is synthesized by the action of ultraviolet light on a particular chemical in the skin. Deficiency may occur in people with a poor diet, in premature infants, and in those deprived of sunlight. It can also result from malabsorption. Other causes include liver or kidney disorders and some genetic defects. Prolonged use of certain drugs, such as phenytoin, may also lead to deficiency. Deficiency in young children causes rickets; long-term deficiency in adults leads to osteomalacia. Excessive intake of vitamin D may lead to hypercalcaemia and abnormal calcium deposits in the soft tissues, kidneys, and blood vessel walls. In children, it may cause growth retardation.