In plant foods, vitamin A is found as a provitamin carotenoid, primarily beta-carotene. Beta-carotene and a number of other carotenoids can be converted into vitamin A in the human small intestine.
Vitamin A I*** is found in five forms in the human body: retinol (retinol), retinal (retinal), retinoic acid (retinoic acid), retinyl esters (retinyl esters), and carotenoids. carotenoids).
The family of vitamins carried within fatty acids is called preformed vitamins, and includes retinyl esters, retinol, retinaldehyde, and retinoic acid.
Retinol is the main form of vitamin A in the body. Because retinol is unstable, it exists in human tissues primarily as retinyl esters, which combine with palmitic acid in fatty acids to form retinyl palmitate.
Retinal is the oxidized form of retinol, an aldehyde, which can also be reoxidized to form retinoic acid.
Beta-carotene is an important plant source of vitamins among the more than 600 carotenoids that occur in nature. Carotenoids are widely distributed in plants in nature in yellow, orange and red colors. About 50 carotenoids produce vitamin A and can therefore be called provitamin A carotenoids, with betacarotene being the most abundant and efficiently converted pro-vitamin A in our food. betacarotene can be split by the enzyme converting enzyme in the small intestine into the forms of retinaldehyde, then retinol, and finally retinyl esters, which are mixed with retinol in the form of retinyl esters. Finally, it is stored in the body as retinyl esters along with the rest of the vitamin A family from animal sources.
When people consume fats containing vitamin A, they are first hydrolyzed in the small intestine by lipolytic enzymes from the pancreas to retinol, which is absorbed by the cells of the small intestine and then turned into chylomicrons with fatty acids, which are systemically circulated through the body via the lymphatic system. Most of the esterified alcohols are finally absorbed by the liver and stored with the fat in the stellate cells of the liver.
When people use plant carotenoids, they are also absorbed in the small intestine. The carotenoids are broken down by the enzyme split deoxygenase into retinaldehyde, and it is the beta carotene that is split in two and then into two retinol molecules, which join with the retinol molecules from the animal source and turn into chyme, which is ultimately stored in the liver.
Because vitamin A is insoluble in the blood, it is also synthesized in the liver when the liver releases retinol, via the plasma retinol binding protein (BRP, plasma retinol bingding protein). Because the molecule is small and easily precipitated in the renal filtration, the retinol-binding protein forms a complex (BRP-TTP-retinol complex) via thyroid transport proteins to prevent loss as it passes through the kidneys. This complex breaks down when the body needs it, releasing retinol for the body to absorb.
It is through this mechanism that the body adjusts to control the body's overall vitamin A levels.
Vitamin A has several functions. The main functions are: firstly, to promote vision; secondly, to participate in protein synthesis and cell division, to maintain the health of the body's epithelial tissues and skin; and secondly, to play a major role in human fertility, human growth and the functioning of the immune system.
Each of the five forms of vitamin A has its own task. Retinol is important for fertility and is the main form of transportation in the body; retinaldehyde activates vision and is an intermediate form between retinol and retinoic acid. Retinoic acid acts like a hormone, regulating body cell differentiation, growth and embryonic development. Some of the animals used in experiments where they were given only retinoic acid from the vitamin A family were shown to grow normally, but were blinded. This is because retinoic acid, as shown above, cannot be reversed to retinaldehyde, which is essential for vision.
The vitamin A family, mainly retinoids, has an irreplaceable role in the animal eye: in the cornea of the eye, thanks to its role in making the outer window of the crystalline lens of the eye bright; and, secondly, in the conversion of light energy into nerve impulses on the retina, which is the process of conversion to vision.
Vitamin A, mainly retinoic acid, can control cell development by influencing gene expression like a hormone. Gene regulation of retinoic acid occurs in many tissues and cells of the body. In particular, this gene regulation affects epithelial cells such as the lungs, trachea, skin, gastrointestinal tract, blood vessel walls, and cornea. Vitamin A maintains the structure and function of these epithelial cells.
For men, retinol is involved in sperm development; for women, vitamin A plays an important role in the normal development of the fetus during pregnancy.
Children who are deficient in Vitamin A do not develop normally, and when they are given Vitamin A supplements, they generally gain weight and grow taller.
Vitamin A deficiency can lead to inorganic salt deposits in the connective tissue of the body's bones. There are also several immune system functions that are closely related to vitamin A. Vitamin A is also associated with the body's white blood cells, such as lymphocytes and phagocytes, and thus plays an important role in the body's defense against pathogens.
The United Nations Children's Fund (UNICEF) cites vitamin A deficiency due to malnutrition as the leading cause of preventable blindness in developing countries.
The earliest symptom of vitamin A deficiency is an inability to adjust to darkness, or night blindness, which is primarily due to insufficient retinol in the retina. Severe deficiencies can lead to dry eye disease and eventually corneal ulcers, scarring and blindness.
Skin damage, stunted growth in children, and greater susceptibility to infectious diseases such as respiratory illnesses and diarrhea are also symptoms of vitamin deficiency.
Some people who suffer from liver disease or gastrointestinal disorders are also deficient in vitamin A as these disorders interfere with the body's ability to absorb and utilize vitamin A.
Vitamin A deficiency in women during pregnancy can cause malformations in the developmental stages of the fetus.
Vitamin A has acute and chronic toxicity.
Overdose of vitamin A (more than 300 mg milligrams per dose in adults) can cause nausea, vomiting, and headaches, with symptoms disappearing only after a few days. Overdose can lead to drowsiness and malaise, accompanied by itchy skin and peeling of the epidermis. Overdosing can sometimes be fatal.
Vitamin A chronic toxicity is a growing concern. Long-term use of 7.5-9 mg per day by adults can cause symptoms of chronic toxicity, affecting the skin and nervous system. It can lead to hepatic hypertrophy and loss of liver function, decalcification of the bones which in turn leads to brittleness of the bones.
Overdose of vitamin A in women during pregnancy can lead to abnormal cell death in the spinal cord, resulting in birth defects in newborns. Taking large doses of vitamin A during the first seven weeks of pregnancy is the most harmful. This is why those women who are already pregnant and those who are planning to become pregnant must stay away from liver foods and liver preparations. Vitamin A supplement foods, medications for acne such as isotretinoin, also known as an acne medication, and other retinol-containing medications, including retinoids used to care for the skin, are all things that pregnant women must stay away from.
Animal livers, which store very high concentrations of vitamin A, were avoided by Arctic explorers in the old days, and the Inuit do not eat the livers of polar bears and seals.
Carotenoid foods do not lead to vitamin A overload because of limited absorption in the body and lower retinol conversion. Prolonged overdose of beta carotene can lead to carotenemia, a harmless condition that turns the skin a yellow-orange color. Once you stop taking beta carotene, your skin will also return to normal. Some people who eat large quantities of carrots over a long period of time also experience yellowing of the skin.
From the five forms of vitamin A, there are four forms can come from animals, animal sources of vitamin A rich, high conversion rate; plant sources of beta carotene body absorption is limited, the conversion rate is low; and drugs to take vitamin A if you do not control the dosage, easy to cause overdose after the symptoms of acute and chronic poisoning.
It can be seen that vitamin A more toxic, the lack of which affects vision, male sperm quality, female fertility embryonic development, children's growth and the body's immune ability.
In reality, vitamin A deficiency occurs mostly in poor populations in developing countries, where malnutrition is caused by a lack of abundant food sources.
Therefore, under most normal circumstances, dietary supplements should also be used, and adequate amounts of vitamin A can be obtained from normal food sources. Dietary supplements can also generally prevent acute and chronic toxicity symptoms caused by excessive amounts of vitamin A, in addition to taking care not to eat too much animal liver, too much fish oil, and too much egg yolk, etc. too often.