Vitamin D biology: from the discovery to its significance in chronic kidney disease.
Vitamin D was discovered and had its chemical structure described in the early years of the last century. Although classified as a nutrient because it was found in small quantities in butter, it soon became clear that exposure of skin to sunlight, supplies most of the vitamin D necessary for good health in human beings. Vitamin D (D3 or cholecalciferol) synthesis in the skin is extremely rapid and remarkably robust despite the complexity of the mechanisms involved. However, a number of factors related to latitude location, season, and skin characteristics can interfere with the photoproduction of vitamin D. The 2 forms of vitamin D (D3 or D2-ergocalciferol) are biologically inactive and require activation in the liver and kidney. The product of the first hydroxylation of vitamin D in the liver, 25-hydroxyvitamin D (25(OH)D), is the marker of vitamin D status. Hypovitaminosis D (serum 25(OH)D, <30 ng/mL) is highly prevalent in the general population, and patients with chronic kidney disease seem to be at higher risk for the development of hypovitaminosis D. It is believed that, besides the traditional factors, protein losses, gastrointestinal malabsorption, and defective skin synthesis of vitamin D might contribute to the elevated number of patients with suboptimal level of vitamin D status.