Vitamin D

Vitamin D (vitamin D) is a derivative of steroids and can be divided into vitamin D2 and vitamin D3. Vitamin D2 is mostly contained in plant foods. It is synthesized by plant ergosterol through sunlight. Vitamin D3 can be synthesized from human skin and adipose tissue at 7-dehydrocholesterol by sunlight. Vitamin D is a fat-soluble vitamin. Vitamin D from food is absorbed by the small intestine together with fat. With the assistance of bile, chylomicrons are formed, which enter the blood through the lymphatic vessels and are transported into the liver together with the vitamin D3 synthesized by itself. In the liver, 25 (OH) D3 is formed by the monooxidase system (25-hydroxylase) in hepatocyte microsomes. 25 (OH) D3 is in the mitochondria & alpha; hydroxylase of renal proximal tubular epithelial cells Under the action of the system, it is transformed into 1,25 (OH) 2D3, which is the largest biological action form of vitamin D, which can promote the synthesis of intestinal calcium-binding protein, and increase the serum calcium and phosphorus content through the following three aspects: ① promote the intestine The absorption and operation of calcium and phosphorus in the tract increase the retention in the body; ② improve the reabsorption of calcium and phosphorus ions by the renal tubules, and reduce the excretion of calcium and phosphorus in the urine; Under the action of parathyroid hormone (PTH), it promotes the absorption of calcium and phosphorus and mobilizes the release of calcium and phosphorus in bone. Therefore, in the early stage of vitamin D deficiency, blood calcium is normal and blood phosphorus is slightly lower. When the vitamin D is further reduced, even if the secretion of PTH is increased, the amount of calcium absorbed is not much due to the decrease of the calcium content in the original urine, and the blood calcium is also reduced at this time.