Views: 2 Author: Site Editor Publish Time: 2022-07-11 Origin: Site
Vitamin A is a fat soluble vitamin, an organic compound with the chemical formula of c20h30o. It is stable to heat, acid and alkali and is easy to be oxidized. Ultraviolet light can promote its oxidative damage. Vitamin A includes A1 and A2, which is retinol. Vitamin A2, namely 3-dehydroaretinol, has a physiological activity of 40% of that of vitamin A1.
Vitamin A can promote growth and reproduction, maintain normal secretion of bone, epithelial tissue, vision and mucosal epithelium, and other physiological functions. Vitamin A and its analogues can prevent precancerous lesions. Lack of performance for growth retardation, dark adaptation ability and the formation of night blindness. Dry eye disease occurs due to the drying, scaling, excessive keratosis of epidermal and mucosal epithelial cells and the reduction of lacrimal gland secretion. In severe cases, cornea softens, perforations and blindness. Respiratory epithelial cells keratinize and lose cilia, reducing resistance and susceptibility to infection. The recommended intake of vitamin A (RNI) for adults in China is 800 per day for men μ G retinol activity equivalent, 700 per day for women μ G retinol activity equivalent. foods containing more vitamin A include the liver, egg yolk and milk powder of birds and animals. Carotene can be changed into vitamin A in the intestinal mucosa, red yellow and dark green vegetables, and fruits contain more carotene.
Vitamin A is a fat soluble vitamin that is extremely important and easily deficient, and is necessary for human body to maintain normal metabolism and function. It was discovered by American scientists Elmer MC colum and Margaret Davis between 1912 and 1914. In fact, as early as more than 1000 years ago, Sun Simiao, a Chinese medical scientist in the Tang Dynasty (581-682 A.D.), recorded in Qianjin Fang that animal liver can be used to treat night blindness, and there are also documents about Brazilian natives treating dry eye disease with cod liver oil and Danes treating dry eye disease with olive oil. Before Margaret Davis and others extracted a yellow viscous liquid vitamin A from cod liver, people did not know the existence of vitamin A, so he first named it "fat soluble a" (a is the first letter of German dry eye disease "augendarre"). With the discovery of new fat soluble substances necessary for human body, by 1920, "fat soluble a" was officially named vitamin A by British scientists.
Vitamin A is not a single compound, but a series of retinol derivatives including retinol, retinone, retinoic acid, retinol acetate and retinol palmitate. Their molecular structures are shown in the figure "molecular structure of vitamin a". Vitamin A only exists in animals, and is abundant in fish, especially cod liver oil. Plants do not contain vitamin A, but many vegetables and fruits contain Pro Vitamin A - carotene, which can be broken down into vitamin A in the small intestine, including 1 molecule β- Carotene can be decomposed into 2 molecules of vitamin A, while 1 molecule of a-carotene or γ- Radish can only produce 1 molecule of vitamin A.
Vitamin A is yellow flake crystal or crystalline powder, insoluble in water and glycerol, and soluble in most organic solvents such as alcohols, ethers, hydrocarbons and halogenated hydrocarbons. Its chemical properties are relatively stable, but it will be easily damaged when exposed to heat, light or air. It should be kept away from light. The chemical name of retinol, the main body of vitamin A, is all trans 3,7-dimethyl-9 - (2,6,6-trimethyl-1-cyclohexenyl-1) - 2,4,6,8-non-tetraene-1-ol. There are 4 conjugated double bonds on its side chain and 16 geometric isomers in theory. Due to the steric hindrance effect, the geometric isomers existing in nature are only all trans, 9-cis, 13 CIS, 9,13-bis CIS and 11 CIS with steric hindrance, Among them, all trans has the highest biological activity. Because vitamin A acetate (retinol acetate) is more stable than vitamin A alcohol (retinol), the commodities called "vitamin a" on the market are actually vitamin A acetate. It is a light yellow oily liquid, which can be solidified after freezing, almost odorless or slightly fishy, but no rancidity, very soluble in chloroform or esters, also soluble in absolute ethanol and vegetable oil, but insoluble in glycerol and water, Unstable in air and light.