Sources and metabolism of vitamin D

Another source is the exogenous energy obtained from ingested foods. For example, animal liver, eggs, and milk all contain vitamin D, while ergosterol contained in plant foods such as vegetable oil and mushrooms must be irradiated with ultraviolet light. Change into calciferol, which can be absorbed by the human body, or vitamin D2. Neither endogenous vitamin D3 nor exogenous vitamins D2 and D3 have biological activity and require further metabolism in the body to obtain a strong anti-rickets effect.

Vitamin D, which is first converted into 25-hydroxycholecalciferol [25-OHD3] in the endoplasmic reticulum and mitochondria of liver cells through the action of the 25-hydroxylase system, has a weak anti-rickets effect. Then in the mitochondria of the epithelial cells of the proximal convoluted tubule of the kidney, it is further hydroxylated to 1,25-dihydroxycholecalciferol [1,25-OH2D3] through the action of the 1-hydroxylase system. Its biological activity is greatly enhanced and it can act on distant target organs through blood circulation, mainly intestines, kidneys and bones. The 25-hydroxycholecalciferol produced by the liver and the 1,25-dihydroxycholecalciferol produced by the kidney can both self-regulate through feedback mechanisms. The metabolism of vitamin D2 is the same as that of vitamin D3. Vitamin D3 is not only regulated by its own blood concentration, but also directly regulated by blood phosphorus concentration, parathyroid hormone and calcitonin, and indirectly regulated by blood calcium concentration. Hypocalcemia promotes increased secretion of parathyroid hormone, 1,25-OH. D. Increased synthesis increases blood calcium; high blood calcium promotes calcitonin secretion and inhibits 1,25-OH2D3 synthesis; low blood phosphorus directly promotes the increase of 1,25-OH2D3 synthesis, while high blood phosphorus plays an inhibitory role. (Editor in charge: Tian Tian)