Vitamin D is a steroid derivative with anti-rickets properties, also known as the anti-rickets vitamin. The most important members of the vitamin D family are D2 and D3. vitamin D is a derivative of different vitamin D principles after ultraviolet irradiation. Plants do not contain vitamin D, but vitamin D is found in both animals and plants. Vitamin D is a fat-soluble vitamin, and there are five compounds, with vitamin D2 and vitamin D3 having the following three properties: it is found in some natural foods; cholesterol in the body is converted to vitamin D after exposure to UV light; and vitamin D works in the body by promoting calcium absorption and regulating a variety of physiological functions. Research has proved that vitamin D3 can induce the intestinal mucosa of many animals to produce a specialized calcium binding protein (CaBP), increase the permeability of the intestinal mucosa to calcium ions, and promote the absorption of calcium in the intestine.　The main function of vitamin D is to regulate calcium and phosphorus metabolism in the body, maintain the level of blood calcium and blood phosphorus, so as to maintain the normal growth of teeth and bones on the development. Vitamin D deficiency in children is prone to rickets, and overdose of vitamin D will cause acute poisoning.　1,25-(OH)2D acts on target organs through cell-specific receptors.　1,25-(OH)2D binds to the cytoplasmic receptor of intestinal mucosal cells, transported to the nucleus, and promotes the gene expression to synthesize calcium binding protein (CaBP), so that calcium ions (Ca2+) are absorbed from the bristle edge of the papillary epithelial cells of the small intestinal mucosa.　②Mobilization of bone calcium and phosphorus to the blood. Binding of bone calcium and CaBP into the blood.　(iii) Inorganic salinization of bone. Stimulate osteoblasts and induce calcium and phosphorus to be deposited in bone.　④Through distal renal tubular cell receptor, it enhances calcium reabsorption with PTH***; it inhibits PTH (PTH increases urinary phosphorus) and increases phosphorus reabsorption after the elevation of blood calcium.　⑤ Increase the content of 7-dehydrocholesterol in the hair layer of the skin through specific receptors, increase the production of insulin in pancreatic islet cells, and increase the operation of calcium in the mammary gland. Main physiological functions Vitamin D mainly has the following physiological functions: 1. Improve the absorption of calcium and phosphorus in the muscle, so that the plasma calcium and plasma phosphorus levels reach the saturation level.　2, to promote growth and bone calcification, promote sound teeth; 3, through the intestinal wall to increase the absorption of phosphorus, and through the renal tubules to increase the reabsorption of phosphorus; 4, to maintain the normal level of citrate in the blood; 5, to prevent the loss of amino acids through the kidneys.