physiological function
1, maintain normal visual function.
The photoreceptors of the eye are rod cells and cone cells in the retina. Both of these cells have photosensitive pigments, namely rhodopsin sensitive to weak light and rhodopsin sensitive to strong light. Both rhodopsin and rhodopsin are composed of retinol and retinaldehyde. After rhodopsin is irradiated by light, 1 1- cis-retinoic acid forms trans-retinoic acid, which is separated from opsin and loses its color. This process is called bleaching. If you enter the darkness, you can't see because the rhodopsin sensitive to low light disappears.
The separated retinoic acid is reduced to all-trans retinoic acid, which is further converted into trans retinoic acid ester (or cis isomer) and stored in pigment epithelium. Retinyl ester is converted into trans-retinol by retinyl ester hydrolase, and oxidative isomerization forms 1 1- cis retinoic acid. It recombines with protein to form rhodopsin, which restores its sensitivity to weak light, so that it can see clearly in the dark under certain illumination. This process is called dark adaptation. Retinol released from the liver binds to retinol binding protein (RBP), and then binds to prealbumin in plasma, which is transported to the retina to participate in the photochemical reaction of the retina. If vitamin A is sufficient, rhodopsin can regenerate quickly and completely, so the recovery time of dark adaptation is short. If vitamin A is insufficient, the regeneration of rhodopsin is slow and incomplete, so the recovery time of dark adaptation is prolonged, and in severe cases, night blindness may occur.
2. Maintain the health of epithelial cells and promote the synthesis of immunoglobulin.
Vitamin A can participate in the synthesis of glycoprotein, which is very important for the normal formation, development and maintenance of epithelium. When vitamin A is insufficient or deficient, it can lead to abnormal glycoprotein synthesis intermediates, accumulation of low molecular weight polysaccharides and lipids, proliferation and thickening of epithelial basal layer, accelerated cell division, increased myofibril synthesis, and flat, irregular and dry surface cells. Endometrial keratinization of respiratory tract, gastrointestinal tract and urogenital system, such as nose, pharynx and larynx, weakens the natural barrier (structure) to prevent bacterial invasion and is easy to be infected. In children, it is easy to be complicated with respiratory tract infection and diarrhea. Some kidney calculi is also related to keratinization of urinary tract. Excessive intake of vitamin A does not increase the resistance to epithelial infection with dose.
Immunoglobulin is a glycoprotein, so vitamin A can promote the synthesis of this protein, which has an important impact on the immune function of the body. When it is deficient, cellular immunity will decline.
3. Maintain the normal growth and development of bones.
Vitamin A promotes the biosynthesis and differentiation of bone cells in protein. When it is deficient, the balance between osteoblasts and osteoclasts is destroyed, or due to the enhancement of osteogenic activity, the bone proliferates excessively, or the formed bone is not absorbed. If pregnant women lack vitamin A, it will directly affect fetal development and even stillbirth.
4. Promote growth and reproduction
Vitamin A contributes to cell proliferation and growth. When animals are deficient in vitamin A, growth stagnation appears obviously, which may be related to the decrease of animal appetite and protein utilization rate. Vitamin A deficiency affects spermatocyte production of sperm epithelium in male animals, periodic changes of vaginal epithelium in female animals, placental epithelium and embryo formation. Vitamin A deficiency will also reduce the activity of enzymes needed to catalyze the formation of progesterone precursors and reduce the production of steroids in adrenal gland, gonad and placenta, which may be the reason for affecting reproductive function.
5, inhibit tumor growth
Clinical trials show that retinoic acid can delay or prevent precancerous lesions and prevent chemical carcinogens, especially for epithelial tumors, and has achieved good results as an adjuvant treatment in clinic. β -carotene has been reported as an antioxidant in recent years. It is an effective antioxidant to capture active oxygen, which is of great significance to prevent lipid peroxidation, cardiovascular diseases and tumors and delay aging.
6, nutritional supplements
It can be used as a nutritional additive in cosmetics to prevent rough skin and promote normal growth and development, and can be used in cream emulsion.
Although vitamin A has the above points, it has more beneficial effects on the human body, but like other nutrients, the extremes meet. Excessive intake can cause headache, nausea and diarrhea, hepatosplenomegaly and so on. So you need to take it in moderation.