Vitamin D has several important functions including absorption of calcium and phosphorous, and facilitating normal immune system function. Sufficient amount of the vitamin is required for normal growth and development of bones and teeth, as well as improved resistance against certain diseases. There is growing evidence that there are huge benefits of vitamin D in promoting the human health, not only in infants for prevention of rickets but also effects on the immune system, blood pressure, reducing the risk of some cancers, prevention of diabetes mellitus type 1 trough stimulation of the pancreatic beta cells to secrete insulin. In contrast to these benefits certain patients genetically predisposed are at risk to develop a serious even fatal disease such as idiopathic infantile hypercalcemia. Withdrawal of vitamin D and reduction of calcium intake are lifesaving interventions for these babies. Recently it was found that recessive mutations in CYP24A1 gene are responsible for this disease. This gene encodes the enzyme 24 vitamin D hydroxylase which is important in the degradation metabolic pathway of the vitamin D. Although it was generally believed that idiopathic infantile hypercalcemia is the disease limited to infancy a number of studies yields that adults may have serious morbidity including nephrolithiasis, nephrocalcinosis, intermittent episodes of hypercalcemia leading to chronic kidney disease and in few cases to end stage renal disease. Therefore one should be very cautious in liberal prescribing vitamin D supplements and excessive exposure to sunlight, particularly in individuals with genetic predisposition
