Vitamin D, 25-Hydroxy – 081950

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.