Vitamin A (also known as retinol) is an unsaturated monohydric alcohol with ester ring, including vitamin A 1 and A2. Vitamin A 1 and A2 have similar structures)
Retinol can be synthesized from β -carotene from plants. Under the catalysis of β -carotene-15, 15'- dioxygenase (dioxygenase) in vivo, β -carotene can be converted into two molecules of retinol, which can be reduced to retinol under the action of retinol reductase. Therefore, beta-carotene is also called provitamin A.
It was extracted from cod liver by American chemist Davis in 19 13. It is yellow powder, insoluble in water, soluble in organic solvents such as fat and oil. The chemical properties are relatively stable, but it is easily destroyed by ultraviolet rays, so it should be kept in a brown bottle.
Vitamin A is a component of rhodopsin that feels weak light in visual cells. Rhodopsin is composed of opsin and 0- cis retinoic acid, which is related to dark vision.
Vitamin A exists only in animal food, A 1 exists in the liver of mammals and saltwater fish, and A2 exists in the liver of freshwater fish. No vitamin A was found in plant tissues. The lack of vitamin A in human body affects the dark adaptation ability, such as children's developmental retardation, dry skin, dry eye, night blindness and so on.
The minimum daily vitamin A requirement of normal adults is about 3,500 international units (0.3 micrograms of vitamin A is equivalent to 1 international unit), and that of children is about 2,000-2,500 international units.
CASNo.:11103-57-4
The role of vitamin a:
Maintain a normal visual response. Maintain the normal morphology and function of epithelial tissue. Maintain normal bone development. It can maintain the function of skin cells, make skin soft and tender, and has the effect of anti-wrinkle and wrinkle removal. Lack of vitamin A will reduce the function of epithelial cells, resulting in decreased skin elasticity, dryness, roughness and loss of luster.
Physiological functions of vitamin a:
Vitamin A is an essential nutrient for complex organisms, which affects almost all tissues and cells in different ways. Although it is one of the earliest discovered vitamins, its physiological function has not been fully revealed so far. As far as we know, the most important physiological functions of vitamin A (including carotene) include:
(1) Keep your vision.
⑵ Promote growth
(3) Enhance fertility.
(4) scavenging free radicals, etc.
Absorption and metabolism of vitamin A;
Vitamin A is emulsified together with fatty decomposition products of bile acids in small intestine and absorbed by intestinal mucosa. The storage of vitamin A in human body increases with the increase of age, which is obviously lower than that of young people in old age, and the storage of different sexes is also different. The average half-life of vitamin A in vivo is 128~ 154 days. When vitamin A is not taken, the daily loss (catabolism) rate in the liver is about 0.5%.
There are two kinds of foods rich in vitamin A: first, provitamin A, that is, various carotene, exists in plant foods, such as green leafy vegetables, yellow vegetables and fruits, such as spinach, alfalfa, pea seedlings, red sweet potatoes, carrots, green peppers, pumpkins and so on; The other is vitamin A from animal food, which can be directly used by human body and mainly exists in animal liver, milk and dairy products (non-skimmed milk) and eggs.
Side effects of vitamin A Adults taking more than 50000IU a day for several months can cause poisoning. If children consume more than 18500IU a day, it will cause poisoning. The main manifestations are: due to the enhanced activity of osteoclasts, bone decalcification, increased bone brittleness, dark growth, thickening of long bones and joint pain; Dry skin, itching, scaling, rash, peeling, hair loss and fragile nails; Irritability, fatigue, headache, nausea, vomiting, muscle weakness, restlessness. Loss of appetite, abdominal pain, diarrhea, hepatosplenomegaly, jaundice; Hemoglobin and potassium in the blood are reduced, coagulation time is prolonged, and bleeding is easy.