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1) General features.

The vitamin A, or retinol, is a superior alcohol formed by a tri-methyl cyclohexene ring bound to a long side-chain with the alcoholic group (-OH) at its extremity and 4 double bonds (-CH=CH-), all in "trans" position, in its most active form, typical of animals.

- It's frequently present in an ester of the palmitic acid and it belongs to the category of liposoluble vitamins.

- It is a yellow coloured substance, stable to heat, but sensible to oxygen, acids and light. So, it's better to protect fruits and vegetables that contain it from light, keeping them in the refrigerator or covering them, if you want to keep it out, on the table.

- Its compounds with biologic activity are the retinal, or retinaldehyde, with an aldehyde group (-CH=O) at the place of the -OH group and the corresponding retinoic acid, with a -COOH (carboxylic group) at its place.

2) Natural sources.

As retinol, this vitamin is present only in the products of animal origin, expressed in mg/100g of edible food:

- Veal liver: 13

- Fish liver: 15

- Eggs: 0.34

- Milk: 0.03-0.2

- Butter: 1.0

- Cheese: 0.2-0.4

In the coloured vegetables and fruits, it's present in the form of precursor compounds (Alpha, Beta and Gamma-carotene and crypto-xantine):

- Carrots: 12 mg/100g

- French beans: 0.5

- Beets: 2.0

- Fennel: 1.0

- Spinach: 2.4

3) Biologic action.

The absorption of this vitamin is favoured by the presence of lipids in food like olive oil or milk derivatives and by the action of the bile. Its daily needs varies from 250-300 micro-g for 1-3 years old children to 750 micro-g/day for pregnant women and 950 during breast-feeding.

- Its most important function is in the sensibility of retina to crepuscular light, when this is scarce. The retinal cells contain a photo-sensible pigment, the rodopsine, formed by a protein (opsine) bound to the 11-cis isomer of the retinal. The energy of light transforms, in a reversible way, this 11-cis double bond in 11-trans (with the two parts of the molecule on different sides respect to the axis of the >C=C< ).

This compound separates itself from the opsine, starting a series of reactions of reactive groups. The consequent chemical energy variations create potential differences that are responsible of the sight impulse sent to the brain. Then, the trans-retinal returns in the cis-form, after its carriage to the liver that reduces it to retinol and regenerates the 11-cis retinal.

- Vitamin A is also important in the growth and protection of skin (from precocious ageing), mucosa and teeth, thanks to its anti-oxidant action against the free radicals.

- It's protective for the integrity of chromosomes and hereditary genes, in the form of retinoic acid.

- It's involved in the synthesis of sterols and steroid hormones, in its -OH (alcoholic) form.

4) Vitamin A deficiency.

The lack of this vitamin in diet can cause serious problems to eyes, reducing their sensibility to light, with the retina less and less adapted to night sight (emeralopia).
If the lack of this vitamin continues, the cornea can suffer of a progressive degradation (keratomalacia), featured by softening, ulceration and perforation of the cornea, also leading to blindness.

Also the skin suffers of the scarcity of this vitamin, becoming too dry, rough and easily chapped. This is the so-called hyperkeratosis, with atrophy of sweat and sebum glands and of the hair and fur bulbs.

Moreover, the mucosa of the organism is weakened, causing a reduced resistance of our organism to infections.

References:

http://medical-dictionary.thef reedictionary.com/

"Lineamenti di Scienza della Nutrizione", Ciangherotti/Avanzino/Lantieri , ECIG Editor (1986) (university text of nutrition science)

http://www.cibo360.it

Learn more about this author, Aldo Bonincontro.
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