Vitamin A

Vitamin A is considered an excellent fighter against infections, dry skin and wrinkles. Therefore, this vitamin is very good for beauty and health.

Vitamin A or retinol is trans-9,13-dimethyl-7 (1,1,5-trimethylcyclohexen-5-yl-6) nonatetraene 7,9,11,13-ol. Chemically, vitamin A is a cyclic unsaturated monohydric alcohol consisting of a 6-membered β-ionone ring and a side chain consisting of two isoprene residues with a primary alcohol group. Vitamin A is fat-soluble, therefore, accumulating in the liver and other tissues with prolonged use in high doses, it can have a toxic effect. This vitamin is not soluble in water, although some of it (15 to 35%) is lost during cooking, scalding and canning of vegetables. Vitamin A can withstand heat treatment during cooking, but can be destroyed during long-term storage under the influence of light.

Vitamin A comes in two forms: ready-made vitamin A and provitamin A or plant form of vitamin A (carotene).

There are about five hundred known carotenoids. The most famous are β-carotene (it was isolated from carrots, which is why the name of the group of vitamins A carotenoids comes from the English word carrot), α-carotene, lutein, lycopene, and zeaxanthin. They are converted into vitamin A as a result of oxidative breakdown in the human body.

Vitamin A includes a number of structurally similar compounds: retinol (vitamin A - alcohol, vitamin A1, a-xerophthol); dehydroretinol (vitamin A2); retinal (retinen, vitamin A - aldehyde); retinoic acid (vitamin A - acid); esters of these substances and their spatial isomers.

Free vitamin A predominates in the blood, and retinol esters in the liver. The metabolic functions of vitamin A in the retina are provided by retinol and retinal, and in other organs by retinoic acid.

Vitamin A: Metabolism

Vitamin A is absorbed in a similar way to lipids - this process includes emulsification and hydrolysis of its esters in the lumen of the gastrointestinal tract, its adsorption and transport into the cells of the mucous membrane, re-esterification of retinol in them and the subsequent entry of vitamin A into the liver as part of chylomicrons.

Absorption of vitamin A occurs mainly in the small intestine, primarily in its upper section. Vitamin A is absorbed almost completely under normal conditions when consumed in physiological doses. However, the completeness of absorption of vitamin A largely depends on its quantity (in particular, with an increase in the dose, absorption decreases proportionally). Such a decrease is apparently associated with increased oxidation and disruption of the mechanisms of active absorption of vitamin A in the intestine, which is due to adaptive mechanisms aimed at protecting the body from vitamin intoxication.

Emulsification of retinol is a necessary stage in the process of its absorption in the gastrointestinal tract. In the presence of lipids and bile acids, free vitamin A is adsorbed by the intestinal mucosa, and its esters are adsorbed after hydrolysis by enzymes of the pancreas and the mucous membrane of the small intestine (hydrolase of carboxylic acid esters).

Up to 40% of carotene is absorbed unchanged. Complete proteins in the diet promote carotene absorption. The absorption of ß-carotene from cooked, homogenized products is improved together with an emulsion of fats (especially unsaturated fatty acids) and tocopherols. ß-carotene in the intestinal mucosa undergoes oxidation at the central double bond with the participation of a specific enzyme of the small intestine, carotene dioxygenase (carotenase), and 2 molecules of active retinal are formed. Carotenase activity is stimulated by thyroid hormones. In hypothyroidism, this process can be disrupted, which leads to the development of carotenemic pseudo-jaundice.

In children under 1 year of age, carotenase is inactive, so carotene is poorly absorbed. Inflammation of the intestinal mucosa and cholestasis lead to the fact that carotenes and vitamin A are poorly absorbed.

In the intestinal mucosa on the inner surface of the villi, vitamin A, like triglycerides, undergoes resynthesis, forming esters with fatty acids. This process is catalyzed by the enzyme retinol synthetase. The newly synthesized retinol ester enters the lymph and is transported to the liver as part of chylomicrons (80%), where it is captured by stellate reticuloendotheliocytes and then by hepatocytes. The ester form - retinyl palmitate accumulates in liver cells, and its reserves in an adult are sufficient for 23 years. Retinol esterase releases retinol, which is transported in the blood by transthyretin. The release of retinol by the liver is a zinc-dependent process. The liver is not only the main depot of vitamin A, but also the main site of synthesis of "retinol-binding protein" (RBP), with which vitamin A specifically binds in the blood. RBP belongs to the prealbumin fraction, its molecular weight is 21 kDa. The concentration of RBP in human plasma is 4 mg per 1 ml. RBP, in connection with retinol, enters into a complex with a protein of significantly higher molecular weight - thyroxine-binding prealbumin and is transported as a complex: vitamin A + retinol-binding protein + thyroxine-binding prealbumin.

The complex of vitamin A and RSB has a significant physiological significance, which consists not only in the solubilization of water-insoluble retinol and its delivery from the depot (liver) to target organs, but also in the protection of the unstable free form of the retinol molecule from chemical decay (for example, vitamin A becomes resistant to the oxidative effects of liver alcohol dehydrogenase). RSB has a protective function in cases of high doses of vitamin A entering the body, which is manifested in the protection of tissues from the toxic, in particular membranolytic, effects of the vitamin. Vitamin A intoxication develops when vitamin A in plasma and membranes is not in a complex with RSB, but in another form.

In addition to the liver, vitamin A is also deposited in the retina, somewhat less in the kidneys, heart, fat depots, lungs, in the lactating mammary gland, in the adrenal glands and other endocrine glands. Intracellularly, vitamin A is localized mainly in the microsomal fraction, mitochondria, lysosomes, in cell membranes and organelles.

In tissues, vitamin A is converted into retinyl palmitate, retinyl acetate (esters of retinol with palmitic and acetic acids) and retinyl phosphate (phosphorus ester of retinol).

Part of the retinol in the liver (vitamin A - alcohol) is converted into retinal (vitamin A-aldehyde) and retinoic acid (vitamin A - acid), that is, the alcohol group, vitamers A1 and A2, is oxidized, respectively, into aldehyde and carboxyl.

Vitamin A and its derivatives are found in the body in a trans configuration (linear form), with the exception of the retina, where cis isomers (11-cisretinol and 11-cisretinal folded form) are present.

All forms of vitamin A have biological activity: retinol, retinal, retinoic acid and their ester derivatives.

Retinal and retinoic acid are excreted by hepatocytes in bile in the form of glucuronides, retinol glucuronide is excreted in urine.

Retinol is eliminated slowly, so when used as a medicinal product, it may lead to overdose.

How does vitamin A affect the body?

Vitamin A restores the shape and strength of nails, it promotes good wound healing, thanks to it hair grows faster, it looks healthier and shinier.

Vitamin A is an antioxidant, it fights aging, strengthens the immune system, increases resistance to viruses and pathogenic bacteria.

Vitamin A is very good for the reproductive system of men and women, increases the activity of sex hormone production, and also fights such a serious disease as night blindness (hemeralopathy).

Biological functions of vitamin A

Vitamin A has a wide range of biological effects. In the body, vitamin A (its active form retinal) controls the following processes:

• Regulates normal growth and differentiation of cells of a developing organism (embryo, young organism).
• Regulates the biosynthesis of glycoproteins of the outer cytoplasmic membranes, which determine the level of cellular differentiation processes.
• Increases protein synthesis in cartilage and bone tissue, which determines the growth of bones and cartilage in length.
• Stimulates epithelialization and prevents excessive keratinization of the epithelium hyperkeratosis. Regulates the normal function of the single-layer flat epithelium, which performs a barrier role.
• Increases the number of mitoses in epithelial cells, vitamin A regulates division and differentiation in rapidly proliferating (dividing) tissues, prevents the accumulation of keratohyalin in them (cartilage, bone tissue, epithelium of the skin and mucous membranes, spermatogenic epithelium and placenta).
• Promotes the synthesis of RNA and sulfated mucopolysaccharides, which play an important role in the permeability of cellular and subcellular, especially lysosomal membranes.
• Due to its lipophilicity, it is incorporated into the lipid phase of membranes and has a modifying effect on membrane lipids, controls the rate of chain reactions in the lipid phase, and can form peroxides, which in turn increase the rate of oxidation of other compounds. It maintains the antioxidant potential of various tissues at a constant level (this explains the use of vitamin A in cosmetology, especially in preparations for aging skin).
• Having a large number of unsaturated bonds, vitamin A activates oxidation-reduction processes, stimulates the synthesis of purine and pyrimidine bases, participates in the energy supply of metabolism, creating favorable conditions for the synthesis of ATP.
• Participates in the synthesis of albumin and activates the oxidation of unsaturated fatty acids.
• Participates in the biosynthesis of glycoproteins, as a lipid carrier through the cell membrane of hydrophilic residues of mono- and oligosaccharides to the places of their connection with the protein base (to the endoplasmic reticulum). In turn, glycoproteins have broad biological functions in the body and can be enzymes and hormones, participate in antigen-antibody relationships, participate in the transport of metals and hormones, and in blood coagulation mechanisms.
• Participates in the biosynthesis of mucopolysaccharides, which are part of mucus, performing a protective effect.
• Increases the body's resistance to infection, vitamin A enhances the formation of antibodies and activates phagocytosis.
• Necessary for normal cholesterol metabolism in the body:
  • regulates the biosynthesis of cholesterol in the intestine and its absorption; with a lack of vitamin A, the absorption of cholesterol accelerates and its accumulation occurs in the liver.
  • participates in the biosynthesis of adrenal cortex hormones from cholesterol, vitamin A stimulates the synthesis of hormones, with a lack of vitamin, the nonspecific reactivity of the body decreases.
• It inhibits the formation of thyroliberins and is an antagonist of iodothyronines, suppresses the function of the thyroid gland, and thyroxine itself promotes the breakdown of the vitamin.
• Vitamin A and its synthetic analogues are capable of inhibiting the growth of some tumors. The antitumor effect is associated with stimulation of immunity, activation of humoral and cellular immune response.

Retinoic acid is involved in stimulating the growth of only bones and soft tissues:

• Regulates the permeability of cell membranes, increasing their stability, by controlling the biosynthesis of their components, in particular individual glycoproteins, and thereby affects the barrier function of the skin and mucous membranes.
• Stabilizes mitochondrial membranes, regulates their permeability and activates enzymes of oxidative phosphorylation and coenzyme Q biosynthesis.

Vitamin A has a wide range of biological effects. It promotes growth and development of the body, tissue differentiation. It also ensures normal function of the epithelium of the mucous membranes and skin, increases the body's resistance to infections, and participates in photoreception and reproduction processes.

The most widely known function of vitamin A is in the mechanism of night vision. It participates in the photochemical act of vision by forming the pigment rhodopsin, which is capable of capturing even minimal light, which is very important for night vision. Even Egyptian doctors in 1500 BC described the signs of "night blindness" and prescribed eating bull liver as a treatment. Not knowing about vitamin A, relying on the empirical knowledge of that time.

First of all, vitamin A is a structural component of cell membranes, therefore one of its functions is its participation in the processes of proliferation and differentiation of various types of cells. Vitamin A regulates the growth and differentiation of cells of the embryo and young organism, as well as the division and differentiation of rapidly proliferating tissues, primarily epithelial cells, especially the epidermis and glandular epithelium that produces mucous secretion, by controlling the synthesis of cytoskeleton proteins. Vitamin A deficiency leads to disruption of glycoprotein synthesis (more precisely, glycosylation reactions, i.e. the addition of a carbohydrate component to a protein), which is manifested by the loss of protective properties of the mucous membranes. Retinoic acid, having a hormone-like effect, regulates the expression of genes of some growth factor receptors, while it prevents metaplasia of glandular epithelium into squamous keratinizing.

If there is little vitamin A, keratinization of the glandular epithelium of various organs occurs, which disrupts their function and contributes to the occurrence of certain diseases. This is due to the fact that one of the main functions of barrier protection - the clearance mechanism does not cope with the infection, since the process of maturation and physiological desquamation is disrupted, as well as the process of secretion. All this leads to the development of cystitis and pyelitis, laryngotracheobronchitis and pneumonia, skin infections and other diseases.

Vitamin A is necessary for the synthesis of chondroitin sulfates in bone and other types of connective tissue; its deficiency disrupts bone growth.

Vitamin A is involved in the synthesis of steroid hormones (including progesterone), spermatogenesis, and is an antagonist of thyroxine, a thyroid hormone. In general, much attention in world literature is currently paid to vitamin A derivatives, retinoids. It is believed that their mechanism of action is similar to steroid hormones. Retinoids act on specific receptor proteins in cell nuclei. Then, such a ligand-receptor complex binds to specific DNA regions that control the transcription of special genes.

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Antioxidant action of vitamin A

Vitamin A and, especially, carotenoids are the most important components of the body's antioxidant defense. The presence of conjugated double bonds in the vitamin A molecule facilitates its interaction with free radicals of various types, including free oxygen radicals. This most important feature of the vitamin allows it to be considered an effective antioxidant.

The antioxidant effect of retinol is also manifested in the fact that vitamin A significantly enhances the antioxidant effect of vitamin E. Together with tocopherol and vitamin C, it activates the inclusion of selenium in glutathione peroxidase (an enzyme that neutralizes lipid peroxides). Vitamin A helps maintain SH groups in a reduced state (SH groups of a diverse class of compounds also have an antioxidant function). In particular, by preventing the oxidation of SH-containing proteins and the formation of cross-links in keratin, vitamin A thereby reduces the degree of keratinization of the epithelium (increased keratinization of the skin leads to the development of dermatitis and early aging of the skin). However, vitamin A can also act as a prooxidant, since it is easily oxidized by oxygen to form highly toxic peroxide products. It is believed that the symptoms of hypervitaminosis A are caused by its prooxidant effect on biomembranes, especially the process of lipid peroxidation in lysosomal membranes, to which vitamin A exhibits a pronounced tropism. Vitamin E, protecting unsaturated double bonds of retinol from oxidation and the formation of free radical products of retinol itself, prevents the manifestation of its prooxidant properties. It is also necessary to note the synergistic role of ascorbic acid with tocopherol in these processes.

The antioxidant effect of vitamin A and β-carotene plays an important role in preventing heart and arterial diseases, vitamin A has a protective effect in patients with angina, and also increases the content of "good" cholesterol (HDL) in the blood. They protect brain cell membranes from the destructive action of free radicals, while β-carotene neutralizes the most dangerous types of free radicals: polyunsaturated acid radicals and oxygen radicals. Being powerful antioxidants, vitamin A is a means of preventing and treating cancer, in particular, preventing the recurrence of tumors after surgery.

The most powerful antioxidant effect is possessed by the carotenoid reservatol, which is found in red wine and peanuts. Lycopene, which is rich in tomatoes, differs from all carotenoids by its pronounced tropism to adipose tissue and lipids, it has an antioxidant effect on lipoproteins, and some antithrombogenic effect.

In addition, it is the most “powerful” carotenoid in terms of protecting against cancer, especially breast, endometrial and prostate cancer.

Lutein and zeaxanthin are the main carotenoids that protect our eyes: they help prevent cataracts and reduce the risk of macular degeneration, which is the cause of blindness in every third case. With vitamin A deficiency, keratomalacia develops.

Vitamin A and immunotropic action

Vitamin A is necessary for the normal functioning of the immune system and is an integral part of the infection control process. The use of retinol increases the barrier function of the mucous membranes. Due to the accelerated proliferation of immune system cells, the phagocytic activity of leukocytes and other factors of non-specific immunity increases. β-carotene significantly increases the activity of macrophages, since they undergo specific peroxide processes that require a large number of antioxidants. In addition to phagocytosis, macrophages present antigens and stimulate lymphocyte function. There are many publications regarding the effect of β-carotene on increasing the number of T-helpers. The greatest effect is shown in individuals (people and animals) experiencing stress (improper diet, diseases, old age). In completely healthy organisms, the effect is often minimal or absent. This is due, among other things, to the elimination of peroxide radicals that inhibit the proliferation of T-cells. By a similar mechanism, vitamin A stimulates the production of antibodies by plasma cells.

The immunoactive effect of vitamin A is also associated with its influence on arachidonic acid and its metabolites. It is assumed that vitamin A suppresses the production of arachidonic acid products (refers to omega fatty acids), thereby inhibiting the production of prostaglandin E2 (a lipid physiologically active substance). Prostaglandin E2 is a suppressor of NK cells, by reducing its content, beta-carotene enhances the activity of NK cells and stimulates their proliferation.

Vitamin A is believed to protect against colds, flu, and infections of the respiratory tract, digestive tract, and urinary tract. Vitamin A is one of the main factors responsible for the fact that children in more developed countries are much more easily affected by infectious diseases such as measles and chickenpox, while in countries with a low standard of living, mortality from these "harmless" viral infections is much higher. Vitamin A prolongs life even for those with AIDS.

Vitamin A: Special Properties

Vitamin A almost does not lose its properties during heat treatment, but in combination with air during long-term storage it is destroyed. During heat treatment, 15 to 30% of vitamin A is lost.

The amount of vitamin A in these products depends on how vegetables with vitamin A are grown. For example, if the soil is too poor, then there is much less vitamin A in them. If vegetables are grown with a high content of nitrates, they tend to destroy vitamin A - both in the body and in the plants themselves.

Vegetables grown in winter have 4 times less vitamin A than those grown in summer. Greenhouse cultivation also depletes vegetables of vitamins by about 4 times. If there is no vitamin E in vegetables, vitamin A will be absorbed much worse.

Milk (natural) contains a lot of vitamin A. But only if cows are fed plants grown in fertilized soils and if their diet includes vitamin E. It protects vitamin A from destruction.

To obtain vitamin A in the form of carotene from plant foods, it is necessary to destroy the cell walls behind which carotene is contained. Therefore, these cells need to be crushed. This can be done by chewing, chopping with a knife or by boiling. Then vitamin A is well absorbed and well absorbed into the intestines.

The softer the vegetables from which we take carotene, the better vitamin A will be absorbed.

The best source of carotene, from which it is immediately absorbed, is fresh juices. However, you need to drink them immediately, because in combination with oxygen, the beneficial properties of fresh juice are destroyed. Fresh juice should not be drunk earlier than within 10 minutes.

Vitamin A: Physicochemical Properties

Vitamin A and retinol, which is part of it, are a recognized fighter against aging and for beauty. Vitamin A also contains many fat-soluble substances, retinoic acid, retinal, and retinol esters. For this property, vitamin A is also called dehydroretinol.

Vitamin A in a free state has the appearance of weakly colored yellow crystals with a melting point of 63640 C. It is soluble in fats and most organic solvents: chloroform, ether, benzene, acetone, etc., but is insoluble in water. In a chloroform solution, vitamin A has an absorption maximum at λ=320 nm, and dehydroretinol (vitamin A 2) at λ=352 nm, which is used in its determination.

Vitamin A and its derivatives are unstable compounds. Under the influence of ultraviolet rays, it quickly disintegrates to form Rionone (a substance with the smell of violets), and under the influence of atmospheric oxygen, it easily oxidizes to form epoxy derivatives. It is sensitive to heating.

How does vitamin A interact with other substances?

Once vitamin A has entered the bloodstream, it can be completely destroyed if the body does not have enough vitamin E. Vitamin A is not retained in the body if it does not have enough vitamin B4.

Vitamin A: Natural Prevalence and Needs

Vitamin A and carotenoid provitamins are widely distributed in nature. Vitamin A enters the body mainly with food of animal origin (liver of fish, especially cod, halibut, sea bass; pork and beef liver, egg yolk, sour cream, milk), it is not found in products of plant origin.

Plant products contain a precursor of vitamin A – carotene. Therefore, the body is partially provided with vitamin A due to plant products, if the process of converting food carotenoids into vitamin A is not disrupted in the body (in case of gastrointestinal tract pathology). Provitamins are found in yellow and green parts of plants: carrots are especially rich in carotene; satisfactory sources of carotene are beets, tomatoes, pumpkin; they are found in small quantities in green onions, parsley, asparagus, spinach, red pepper, black currants, blueberries, gooseberries, apricots. Carotene in asparagus and spinach has twice the activity of carotene in carrots, since carotene in green vegetables is more active than carotene in orange and red vegetables and fruits.

Where is vitamin A found?

Vitamin A can be found in animal foods, where it is in the form of an ester. Provitamins A look like orange substances, they color the vegetables that contain them orange. Plant foods also contain vitamin A. In vegetables, provitamins A are converted into lycopene and beta-carotene.

Vitamin A in combination with carotene is also found in egg yolks and butter. Vitamin A accumulates in the liver, it is a fat-soluble vitamin, so you do not need to eat food with vitamin A every day, it is enough to replenish the body with the necessary doses of vitamin A.

Vitamin A: Natural Sources

• This is liver - beef liver contains 8.2 mg of vitamin A, chicken liver contains 12 mg of vitamin A, pork liver contains 3.5 mg of vitamin A
• This is wild garlic, a green plant that contains 4.2 mg of vitamin A.
• This is viburnum - it contains 2.5 mg of vitamin A
• This is garlic - it contains 2.4 mg of vitamin A
• This is butter - it contains 0.59 mg of vitamin A
• This is sour cream - it contains 0.3 mg of vitamin A

Vitamin A requirement per day

For adults, it is up to 2 mg. Vitamin A can be obtained from pharmaceutical supplements (a third of the daily requirement), and two thirds of this vitamin - from natural products that contain carotene. For example, carrots.

The daily requirement for vitamin A for an adult is 1.0 mg (for carotene) or 3300 IU, for pregnant women – 1.25 mg (4125 IU), for breastfeeding women – 1.5 mg (5000 IU). At the same time, at least 1/3 of the daily requirement for retinol should enter the body in ready-made form; the rest can be covered by consuming yellow plant pigments – carotenes and carotenoids.

When the need for vitamin A increases

• For obesity
• During physical activity
• During heavy mental work
• In low light conditions
• When constantly working with a computer or TV
• For diseases of the gastrointestinal tract
• For liver diseases
• In case of viral and bacterial infections

How is vitamin A absorbed?

In order for vitamin A to be absorbed into the blood normally, it needs to come into contact with bile, as a fat-soluble vitamin. If you eat vitamin A but don't have any fatty foods in your diet, little bile will be released and vitamin A will be lost by up to 90%.

If a person eats plant foods with carotenoids, such as carrots, no more than a third of the beta-carotene is absorbed from it, and half of it is converted into vitamin A. That is, to get 1 mg of vitamin A from plant foods, you need 6 mg of carotene.