Use: To Rule out vitamin D deficiency. The majority of 25-OH vitamin D (25-D) in the circulation is derived from the conversion of 7-dehydrocholesterol in the skin that is irradiated with ultraviolet radiation in the UVB range (wavelength 290 nm to 315 nm).1-5 The extent of vitamin D formation is not tightly controlled and depends primarily on the duration and intensity of the UV irradiation. Levels produced typically reach a plateau within 30 minutes of exposure. Unfortunately, use of a sunscreen with SPF as low as 15 reduces the rate of vitamin D production by 99.9%. Overproduction of vitamin D in the skin is prevented by the photosensitive conversion of vitamin D to tachysterol or lumisterol. Vitamin D is not very water-soluble, so it must be delivered to and carried in the blood as a complex with vitamin D-binding protein. Once in the circulation, vitamin D is metabolized to 25-hydroxy vitamin D (25-D) by the liver. The 25-D form of the hormone is the principle circulating reservoir in plasma and is generally the best indicator of overall vitamin D status. 25-D is further metabolized by the kidney to produce the biologically active form of vitamin D, 1,25-dihydroxy vitamin D (1,25-D). Renal production of 1,25-D is tightly controlled by parathyroid hormone and is important in the regulation of serum calcium homeostasis.The hormonally active form of vitamin D,1,25-D plays an integral role in calcium homeostasis and the maintenance of healthy bone.1-4 1,25-D stimulates the absorption of calcium at the level of the intestine and may also serve to increase calcium and phosphate resorption at the kidney level. Vitamin D deficiency leads to the mobilization of calcium from bone. Individuals with more severe vitamin D deficiency can develop osteomalacia and/or osteoporosis. Osteomalacia in children, also referred to as rickets, results in well-described skeletal malformations, since children's bones are actively growing. Recent clinical and epidemiological studies suggest that vitamin D deficiency may play a role in several conditions unrelated to bone, including prostate cancer, breast cancer, colon cancer, heart disease, hypertension, multiple sclerosis, and type 1 diabetes.A number of studies have shown that vitamin D deficiency is very common, especially in certain high-risk populations.1,2 This situation has occurred, in part, because the foods in the typical American diet are very low in vitamin D. Fatty fish, such as mackerel and salmon, and fish liver oils are some of the few natural dietary sources of vitamin D. Most people do not eat enough of these foods to maintain adequate vitamin D levels. In the United States, vitamin D is added to milk in order to prevent the occurrence of rickets in the pediatric population. Unfortunately, too many children do not drink enough milk to raise their vitamin D levels to the optimum range. Also, recent studies have shown that the level of vitamin D in fortified milk is frequently much lower than that recommended by the FDA. Human milk contains very little vitamin D because many mothers are deficient, so children of mothers who choose to breast-feed are at risk of developing rickets if they are not given supplemental vitamin D. The American Academy of Pediatrics recommends that infants who are exclusively breast-feeding should be given a supplement of vitamin D.