What is vitamin d for

Vitamins are a type of trace elements that are indispensable to our human body. There are many types, such as vitamin A, vitamin B, vitamin C, vitamin D, etc. The vitamin D that is closely related to our health is vitamin D2 and vitamin D3. Many people know that vitamins are important, but when it comes to the specific functions of vitamin D, not many people can explain it clearly. So, what does vitamin D do? Let’s take a closer look at the effects.

(1) Maintaining stable serum calcium and phosphorus concentrations. When the blood calcium concentration is low, it induces the secretion of parathyroid hormone and releases it into the kidneys and bone cells. In the kidney, PTH not only stimulates carboxylase at position 1 and inhibits carboxylase at position 24, but also promotes the excretion of phosphorus from the urine and the reabsorption of calcium in the renal tubules. In the bone, PTH works synergistically with 1,25-(OH)2-VD3 to mobilize calcium from the bone. In the small intestine, 1,25-(OH)2-VD3 promotes calcium absorption. These three pathways restore blood calcium to normal levels and feedback control the secretion of PTH and the synthesis of 1,25-(OH)2-VD3. When the blood calcium is high, it stimulates the thyroid c cells to produce calcitonin, which prevents calcium from being mobilized from the bones and promotes the excretion of calcium and phosphorus from the urine. The absorption of phosphorus in the small intestine is active and requires energy. When sodium, glucose, 1,25-(OH)2-VD3 and serum phosphorus are low (below 8mg%), the synthesis of 1,25-(OH)2-VD3 is stimulated, promoting the absorption of calcium and phosphorus in the small intestine. Since PTH does not participate in the reaction, calcium is excreted from the urine but phosphorus is not, causing a slight increase in blood calcium and a greater increase in phosphorus, restoring blood phosphorus to normal values.

(2) Promotes the delivery of calcium to the fetus during pregnancy and lactation. In addition to being affected by serum calcium and phosphorus concentrations and dietary calcium and phosphorus supply, 1-carboxylase is also affected by hormones. Postmenopausal women have reduced 1,25-(OH)2-VD3 concentrations, which makes them more susceptible to symptoms such as osteomalacia.

The plasma concentration of 1,25-(OH)2-VD3 increases during pregnancy, continues to rise during lactation, and gradually returns to normal levels after weaning. The levels of 1,24,25(OH)2-VD3, on the contrary, decrease during pregnancy and return to normal after weaning. The placenta also has a carboxylase, and nephronophthisis can also synthesize 1,25-(OH)2-VD3 during pregnancy. The mammary gland is also the target tissue of 1,25-(OH)2-VD3, which is directly related to the level of calcium in milk. During pregnancy and lactation, the mother can export calcium from her own bones to maintain the normal growth of the fetus and baby. Those with adequate supply of vitamin D can regain calcium after weaning. Those who are deficient in vitamin D have a poor recovery ability.

(3) Mechanism of action of 1,25-(OH)2-VD3: The effect of 1,25(OH)2D3 on the small intestine is to induce the synthesis of CABP. 1,25-(OH)2-VD3 forms a complex with the receptors of small intestinal cells and enters the cell nucleus and chromosomes, promoting the synthesis of CABP messenger RNA (mRNA), which is transcribed into CABP in the cytoplasm. This protein causes calcium ions to cross the microvillus brush border and accumulate in the mitochondria or other parts of intestinal cells. Ca2+ is squeezed out of the basal-lataaral membrane by Na+. The effect of 1,25-(OH)2-VD3 on reabsorption of Ca2+ in the renal tubules is the same as that in the small intestine. 1,25-(OH)2-VD3 can also mobilize calcium from bones when plasma calcium is low and dietary calcium is deficient, but cabp is not found in bones. 1,25-(OH)2-VD3 can promote calcium absorption in the small intestine, but it cannot mobilize calcium from bones. Therefore, the mechanism of action of 1,25-(OH)2-VD3 on bones is different from that on the small intestine, but it is not clear yet.

The mechanism of bone mineralization has not yet been elucidated. Supplementation of 1,25-(OH)2-VD3 to animals and humans deficient in vitamin D does not help the deposition of minerals in bones. Although many vitamin D metabolites have been isolated from animals, none has been found to have a significant effect on bone mineralization. At this stage, we only know that vitamin D promotes the absorption of calcium and phosphorus, and can mobilize calcium and phosphorus from the bones, so that plasma calcium and phosphorus reach normal values, promote bone mineralization, and continuous renewal.