What is the role of vitamin A

Vitamin A is an important nutrient that is indispensable to the human body. It has the functions of protecting eyesight, promoting bone growth and development, preventing endometrial keratinization, and promoting human reproductive development. Especially those who use their eyes for a long time should supplement vitamin A appropriately. So what are the functions of vitamin A? How to supplement vitamin A?

1. Maintain normal visual function.

The eye's photoreceptors are the rods and cones in the retina. Both types of cells contain photosensitive pigments, namely rhodopsin, which is sensitive to weak light, and rhodopsin, which is sensitive to strong light. Both rhodopsin and rhodopsin are composed of opsin and retinal. When rhodopsin is exposed to light, 11-cis-retinal is isomerizes into trans-retinal and separates from the visual protein, causing it to lose its color. This process is called "bleaching". If you enter a dark place, you will not be able to see things because the rhodopsin, which is insensitive to weak light, disappears.

The separated retinal is reduced to all-trans retinal, which is further converted to trans retinyl ester (or isomerized to cis) and stored in the pigment epithelium. Retinyl ester hydrolases in the retina convert retinyl esters into trans-retinol, which is then oxidized and isomerized to form 11-cis-retinal. It then recombine with protein to form rhodopsin, restoring sensitivity to weak light, allowing one to see objects in a dark place at a certain illumination intensity. This process is called Dark Adaptation. Retinol released by the liver binds to retinol binding protein (RBP), and then to prealbumin in the plasma, and is transported to the retina to participate in the photochemical reaction of the retina. If vitamin A is sufficient, the regeneration of rhodopsin is fast and complete, so the dark adaptation recovery time is short; if vitamin A is insufficient, the regeneration of rhodopsin is slow and incomplete, so the dark adaptation recovery time is prolonged, and in severe cases, night blindness may occur.

2. Maintain the health of epithelial tissue cells and promote the synthesis of immunoglobulins.

Vitamin A can participate in the synthesis of glycoproteins, which is very important for the normal formation, development and maintenance of the epithelium. When vitamin A is insufficient or deficient, it can lead to abnormalities in glycoprotein synthesis intermediates and the accumulation of low molecular weight polysaccharides-lipids, causing hyperplasia and thickening of the epithelial basal layer, accelerated cell division, increased synthesis of tension fibrils, and changes in the surface layer such as flattening, irregularity, and drying of cells. Nose, pharynx, larynx and other respiratory, gastrointestinal and genitourinary systems

The keratinization of the endometrium weakens the natural barrier (structure) that prevents bacterial invasion, making it prone to infection. In children, respiratory infections and diarrhea are very likely to occur simultaneously. Some kidney stones are also related to urinary tract keratinization. Excessive intake of vitamin A does not increase resistance to epithelial infections with dose. Immunoglobulin is a glycoprotein, so vitamin A can promote the synthesis of this protein and has an important impact on the body's immune function. When it is deficient, cellular immunity decreases.

3. Maintain normal bone growth and development.

Vitamin A promotes protein biosynthesis and bone cell differentiation. When it is deficient, the balance between osteoblasts and osteoclasts is disrupted, resulting in either excessive bone proliferation due to enhanced osteogenic activity or non-absorption of already formed bone. If pregnant women lack vitamin A, it will directly affect the development of the fetus and even cause death.

4. Promote growth and reproduction.

Vitamin A helps in cell proliferation and growth. When animals lack vitamin A, they show obvious growth stagnation, which may be related to the animals' reduced appetite and decreased protein utilization. When vitamin A is deficient, it affects the production of spermatocytes in the spermatic epithelium of male animals, causes changes in the cycle of the vaginal epithelium in females, and also affects the placental epithelium, thereby hindering embryo formation. Vitamin A deficiency also causes a decrease in the activity of enzymes required to catalyze the formation of progesterone precursors, thereby reducing the production of steroids in the adrenal glands, gonads, and placenta, which may be the reason for affecting reproductive function.