La Vitamin A (vit A) is a fat-soluble vitamin - which is diluted in fatty acids - of both vegetable and animal origin, widely present in food, with numerous biological functions and rarely lacking in the diet; the excess can be harmful and have serious complications, especially for the fetus of the pregnant woman.
E' essential for: epithelial cells, bone and tooth growth, normal sexual maturation in adolescents and fertility in adults; increases resistance to infections by supporting the immune system, ensures good vision and allows for low-light vision, protects the skin from damage caused by sun exposure and has a powerful antioxidant effect - fights free radicals, counteracts the harmful effects caused pollution and smoking - and appears to play a protective role on prostate cancer.
Retinol is regularly taken with food and comes mainly from nutritional sources of animal origin; however, it can be synthesized in the intestine starting from β carotene (beta-carotene), a precursor / provitamin of predominantly vegetable origin (yellow-orange dye), belonging to the vast group of carotenoids (over 600 types). This process takes place thanks to an enzyme which, for each molecule of β carotene taken into processing, it obtains the two unit of vitamin A.
Vitamin A comes stored in the organism inside the liver, ensuring the proper functioning of the body for up to one or two years - in a healthy and well-nourished subject.
I rich foods of beta-carotene are mainly yellow, orange and red vegetables, carrots, pumpkin, apricots, sweet potatoes, melons, but also dark green leafy vegetables (spinach, broccoli, chicory, chard, etc.). Retinol, on the other hand, is found mainly in the liver, egg yolk (where carotenoids are also present), crustaceans, whole milk, butter and cheese.
L'excess of retinol is more harmful than that of β carotene - of which overdosing has minimal effects. Mega doses of vitamin A in pregnancy can have an effect teratogeno.
We recommend taking at least 1.000 IU (international units) per day (day) of retinol ed equivalent for the sedentary individual, while for the sportsman, during gestation and breastfeeding it is appropriate increase these doses.
In your country, considering the quality of food albeit with its territorial variability, the daily requirement of vitamin A is always guaranteed and currently DON'T there are cases of shortage.
Let's go into more detail.
Introduction to the chemistry of vitamin A
Retinol is a fat-soluble vitamin. It is a higher alcohol, found in nature mainly in esterified form, the structure of which was discovered by Karrer in 1931. It consists of a β-iononic ring and a side chain containing a series of conjugated double bonds. The biologically active forms of vitamin A are:
- Retinoic acid.
Retinol as such is found in foods of animal origin, while its precursors carotenoids are found in those of plant origin.
The carotenoids currently identified are about 600, those with provitamin activity are: α-, β-, γ-carotene and β-cryptoxanthin.
Other carotenoids present in the diet, but without provitaminic activity, are: lycopene, zeaxanthin, lutein and canthaxanthin.
At the level of the intestinal mucosa, most of the carotenes are transformed, by the action of a 15,15'-carotene-dioxygenase, into retinaldehyde, which can then be reduced to retinol. Theoretically, any β-carotene molecule can be formed the two of retinol, in practice DON'T it is absorbed more di 1/3 and less than half is used, so from one μg of β-carotene we will have 0,167 μg of retinol (1/6).
Absorption and Metabolism
Absorption and metabolism of vitamin A
Retinol esters are hydrolyzed by pancreatic lipases and carboxyl ester lipases and by enteric retinyl ester hydrolases.
Not anymore of 75% of ingested retinol comes absorbed by facilitated diffusion (at physiological concentrations) and by passive diffusion (at high concentrations). The process is influenced by both the quantity and quality of dietary lipids. In addition to the presence of bile acids.
In the enterocytes (cells of the intestine), retinol is esterified and becomes part of the chylomicrons, which through the lymphatic circulation reach the blood circulation and go to the liver, which contains 50% to 80% of the body's retinol.
Hepatic retinol can be released into the bloodstream, where it is transported as retinol-BP associated with transthyretin (prealbumin) to the tissues; the blood concentration of retinol is equal to 40 ÷ 80 μg / 100ml.
Functions of Vitamin A
Vitamin A is essential for:
- Transport (retinol)
- Storage (retinyl ester)
- Cell differentiation (retinoic acid)
- Reproduction (retinol)
- Vision (retinaldehyde).
Vitamin A also performs other functions that are not fully understood, probably in the form of retinoic acid:
- Deficient animals exhibit adrenal insufficiency and reduced glycogen formation
- It is needed in the mucous tissue for the synthesis of mucopolysaccharides
- It plays an important role in the preservation of cell membranes (antioxidant function: it fights free radicals, and for this reason it is an ingredient in the most effective anti-wrinkle creams, counteracts the harmful effects of smoking and pollution as well as providing a valid help in fighting the skin aging), in protein synthesis and in the formation of bones and skeleton.
Vitamin A deficiency
Retinol deficiency causes:
- Squamous epithelial cell metaplasia, with initial onset of follicular hyperkeratosis (cornification in the hair follicles), followed by frinoderma (toad skin) with loss of skin in the form of large scales;
- Corneification of the bronchiolar mucous membranes (xerosis), which facilitates the implantation of bronchiolitis;
- Corneification of the epithelium of the renal pelvis leading to pyelitis and / or cystopyelitis;
- Xerophthalmia, conjunctival and corneal xerosis, characterized by dryness, thickening, pigmentation and loss of luster, with the formation of whitish-gray spots (Bitot's spots);
- Keratomalacia, colliquative necrosis of the cornea, with the appearance of an ulcer that can evolve to destroy the cornea itself with protrusion and prolapse of the iris and lens;
- Hemeralopia or twilight blindness and nocthalopia or the ability to see better at night than during the day, due to the decrease in the concentration of rhodopsin in the rods.
Vitamin A toxicity
High doses of vitamin A (over 300 mg) cause acute intoxication characterized by: nausea, vomiting, migraine, visual disturbances and loss of movement coordination, symptoms that disappear in a short time with an adequate intake of retinol.
High doses of retinol (6÷12 mg) For years cause the onset of a chronic syndrome with: hair loss, loss of appetite, anemia, muscle pain and neurological symptoms.
High doses of carotenoids cause an alteration of the skin pigmentation (yellow-orange color).
Foods rich in vitamin A
Vitamin A is contained in foods of animal origin and in particular: meat, offal, some fish, eggs, milk, cheese, butter.
Carotenoids are found in foods of plant origin and in particular:
- In orange-yellow vegetables such as carrots, pumpkins, peppers;
- In green leafy vegetables such as spinach and broccoli;
- In some fruits, such as apricots, melons, yellow peaches, pink grapefruit and papaya.
However, carotenoids are also present in foods of animal origin such as eggs and milk and derivatives.For further information: Foods with Vitamin A
Vitamin A requirement
The recommended intake levels of vitamin A are referred to as retinol equivalent (RE).
- 1 RE = 1 μg retinol = 6 μg β-carotene = 12 μg other carotenes = 3,33 IU;
- 1 IU = 0,3 μg retinol = 1,8 μg β-carotene = 3,6 other carotenes.
The recommended intake levels are:
- 700 RE for man;
- 600 KING for women;
- 700 RE for the pregnant woman;
- 950 RE for the nurse.
In recent years, a new conversion index has taken hold, the RAE (retinol equivalent activity), preferred because it correlates better with the human capacity for absorption and conversion of carotenoids into vitamin A.
1 µg RAE = 1 µg retinol = 2 µg all-trans-β-carotene from supplements = 12 µg of all-trans-β-carotene from food = 24 µg α-carotene or β-cryptoxanthin from food.