Vitamin A


Vitamin A
Vitamin A, also called retinol, is a fat-soluble vitamin.

In the body, it exists in the form of retinol, retinal, retinoic acid and retinyl phosphate. These molecules are altered by the oxygen in the air, accelerated deterioration in sunlight and heat.

The foods of animal origin (meat, dairy products and especially liver) contain retinol and retinol esters while plants contain mainly carotenes which are precursors of retinol. A molecule beta-carotene, a hydrolysis liaison 15-15 'under the influence of a carotenoid monooxygénase (ββ-carotene 15.15' monooxygénase), gives two molecules of vitamin A. By against the other two carotenes (alpha and gamma) will give birth to only one molecule of vitamin A.

The needs for vitamin A are estimated at approximately 5 000 IU per day.

An international unit of vitamin A is 0.3 retinol is regarded as the basic unit, retinol equivalent, which compares the activity of various vitamin derivative of vitamin A.

The most useful form of vitamin A is the retinol, although one can also find in the form of retinal or retinoic acid. Retinol is a hydrophobic molecule synthesized from isoprene. Vitamin A is involved in bone growth and synthesis of pigments of the eye. Retinol is sometimes used to treat severe acnes.

Vitamin A and vision
A severe shortage of vitamin A can cause blindness, it manifests itself in a pale skin and hair. If it is prolonged, vitamin A can be fatal, like an overdose: livers of some animals can indeed contain a dose of vitamin A potentially toxic to a human being.

Vitamin A is a vitamin important for the vision, through its various forms (retinol / retinal all-trans/9-cis/11-cis; corresponding to a isomerism of the chain isoprénique) also called vitamères it comes to form a photopile in nerve cells of the eye: rods. In direct link with opsine, retinal 11-cis form rhodospin which, in contact with a photon, dissociates itself in opsine retinal and all-trans. After action of an enzyme called dehydrogenase, retinal all trans is transformed into all trans retinol, is undergoing the action of a isomerase and finally a second dehydrogenase, to restore the 11-cis retinal and complete the cycle. The nerve impulses has been transmitted.

There are large quantities of vitamin A in milk, eggs and liver and provitamin A in carrots, apricots, khaki or spinach.

Vitamin A and skin
Consuming foods rich in vitamin A, such as carrots, melon, apricot, yellow peach or mango provides an hâlé tone (brown) while promoting hydration of the skin. It would stimulate the production of melanin pigment, which is tan. In addition, vitamin A deficiency causes juvenile acne. It is important for young people to consume adequate amounts of vitamin A for the health of their skin.

Pharmacodynamics of vitamin A
1. Absorption.

The absorption is through the intestine and requires the presence of fat and bile salts.
The thyroxine supportive.

2. Storage.

It occurs almost exclusively in the liver, much incidentally in the adrenal capsules, the testicles, ovaries, kidneys.

3. Rates blood.

The rétinolémie is normally range between 100 and 120 IU per litre in the serum. In reality, this rate may suffer transient elevations in relation to dietary intakes.
At below 70 IU, we can admit a shortcoming. However, the blood is not a true reflection of the visceral level, liver in particular.

4. Excretion.

Nothing in healthy individuals, it may appear in the urine during certain diseases (pneumonia, nephritis, diabetes, anemia), always with albuminuria.

5. Metabolism of carotenes.

The hydrolysis of carotenes with production of retinol would traditionally under the influence of a liver enzyme origin, caroténase. In fact, the transformation of carotenes was primarily an intestinal headquarters and the role of the liver, in any case secondary, is not established.

Vitamin A

1. Needs daily.

For healthy adults, daily needs are about 5 000 IU These needs can be increased under certain circumstances physiological.
For the child, they are generally 2 000 to 4 500 IU 1 to 12 years and, for the teenager, 5 000 to 6 000 IU 13 to 20 years.
During pregnancy and lactation, the daily intake should reach 6 000 to 8 000 IU

2. Causes of failure.

* The lack of contribution is most often responsible for Hypovitaminosis A, especially in infants subjected to a regime without milk. Adults may also suffer from an insufficient supply when they submit to strict regimes, eliminating their diet: dairy products, eggs, fat, green vegetables.

* In addition to this lack of supply, there is also a lack of use, because the metabolism of vitamin A requires the integrity of liver and intestinal functions. In addition, intestinal disorders inhibit the transformation of beta-carotene into vitamin A.

3. Symptoms of deficiency.

The first signs of deficiency are photophobia and night blindness. He added quickly various troubles on growth, leading to the dedifferentiation of épithéliums with decreased resistance to infection and affecting certain endocrine glands and various metabolisme.

Therapeutic indications of vitamin A
The administration of vitamin A is indicated not only during deficiency symptoms but also against certain metabolic disorders and in case of susceptibility to infection.
Vitamin A is also useful for prophylaxis in circumstances that enhance physiological needs (pregnancy, lactation, premenstrual syndrome) but also in other circumstances such as stunted growth, decreased resistance to infections, photophobia, night blindness , Xerophthalmia, kératomalacie, ichthyosis, keratosis piliaire, hyperkeratosis, acne, hair dry, dull and brittle.

Equivalent retinol
Few chemical reactions where the rate of progress is equal to 1. Thus, for the β-carotene, one speaks in terms of retinol equivalents. Indeed, to get retinol from β-carotene, there must be a chemical reaction catalyzed by a dioxygenase, and the return is "only" 1 / 6. This allows nevertheless get 1 gram of retinol from 6 grams of β-carotene.

Read also Vitamin D