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Vitamin A

Vitamin A-A vitamin. Vitamin A is a collective (umbrella) term for three different molecules. The three molecules are named retinol, retinal, and retinoic acid. By examining the diagram below, we can see that these three different forms of vitamin A affect different tissues in the body.





Retinal is the aldehyde obtained from the oxidation of retinol. Basically, retinal and retinol are similar in structure and can be interconverted to each other, depending on need.

Retinol (also called vitamin A1) is what we call a primary alcohol. Retinol can be found in animal tissues and exists in a structure combined with a fatty acid. We can see how this can contribute to retinol's fat-soluble nature, as discussed in our introduction to vitamins.

Retinoic acid is the acid product of oxidized retinal. Above we mentioned that retinol and retinal can be interconverted depending on need. Retinoic acid, in comparison, cannot be interconverted. Once retinoic acid is formed it remains in this form and is used up in maintenance of epithelial cells (skin cells lining cavities in body and on skin surface) Read about retinoic acid and its application to anti-wrinkle creams and it's risks.

Beta-carotene-One of four similar pigments, found in plants and having colours ranging from violet to yellow. Each of the four carotenes is preceded by a Greek letter (alpha, beta, gamma, or delta) and merely indicates the slight difference in structure. Carotenes are found in many dark green, leafy and yellow vegetables such as carrots, sweet potatoes, squash, and turnips. Carotenes are also found in yellow fruits such as oranges peaches, apricots, cantaloupes, etc.). They are fat-soluble, and can be split into 2 molecules of retinal by enzymes in the intestinal wall and the liver. In humans, B-carotene is the major provitamin (precursor) of vitamin A. B-carotene is not absorbed as well as retinol, but is in greater availability in the diet.

Beta-carotene can be used as a provitamin (precursor) of vitamin A since it can be split into 2 molecules of retinal. Oral Beta-carotene preparations have also been used to reduce the severity of photosensitivity in patients with certain types of porphyria. Beta-carotene has also been used as a method of artificial tanning.

Absorption and transport of vitamin A

As the food we've consumed is digested, retinyl esters are released. With the help of bile (which helps aid in the digestion of fats), these esters are absorbed into the intestinal mucosa and are hydrolyzed (broken apart), releasing retinol and free fatty acids (fats or lipids). Retinol from these esters and from carotenes as well, are re-esterified (bonded to a fat again) in the intestinal mucosa and secreted into the lymphatic system as chylomicrons. The lymphatic system is a network of vessels which travel parallel to blood vessels and carry proteins and fat. When retinyl esters in chylomicrons reach the liver, they are taken up and stored, until needed. When vitamin A is needed, retinol is released from the liver, and transported to the tissue in need. Retinol is transported by a protein called retinol-binding protein (RBP). There are specific receptors on RBP which are attracted to receptors on tissues that are in need of vitamin A. When receptors from the tissue and RBP interact, retinol is released from RBP, where it enters the tissue.

Functions of vitamin A

-Growth

Retinoic acid (a derivative of retinol) is thought to be the form of vitamin A that plays a role in bone growth and development.

-Vision

Vitamin A plays a vital role in the visual cycle. Vitamin A is part of the rhodopsin, the visual pigment of the rod cells in the retina. Rhodopsin consists of Delta11-cis-retinal (simply a chemical description of the molecule) bonded to a protein called opsin. When a photon (an amount of light) of light penetrates the eye and hits the retina (back of the eye), the rhodopsin undergoes a photochemical (light + chemical) reaction and dissociation of all-trans-retinal and opsin. When this dissociation occurs, it stimulates a nervous impulse. This nervous impulse travels through the optic nerve to the brain, and we interpret this as vision. The by-products of these chemicals are sent back to the liver for processing and return to the retina. Vision is a very interesting chemical event.

-Reproduction

Vitamin A derivatives retinal and retinol are needed for reproductive processes, such as spermatogenesis (sperm formation), and preventing fetal resorption in the female. Retinoic acid is not chemically able to act in this capacity. Animals given vitamin A in the form of retinoic acid from birth become blind and sterile.

-Epithelial cells

The retinoic acid form of Vitamin A is needed for the functional integrity of epithelial cells. This integrity includes maintenance and mucous secretion.

-Sources of vitamin A

Good sources of vitamin A are liver, kidney, egg yolk, butter, and cream. Yellow vegetables and fruits (discussed above) are good sources of the carotenes. Red palm oil is also a good source of Beta-carotene, which functions as provitamin A.

-Requirement of vitamin A

The recommended daily allowance (RDA) for an adult male is 1000 retinol equivalents (RE) and 800 RE for the female. A retinol equivalent is simply a quantity of retinol or another form of the vitamin. Remember, that vitamin A exists in several forms and we can obtain it from breaking down carotene. Therefore, 1 RE= 1 microgram of retinol. 1 RE also equals 6 micrograms of carotene, or 12 micrograms of other carotenoids. Remember what we discussed above about the absorption of retinol. Retinol is absorbed better than carotenes. However, we get more carotenes in the diet. Therefore we have to consume more carotenes to get the same effect as less retinol. Thus the change in retinol equivalents. Another measurement of vitamin A that is used is called the international unit (IU). 1 microgram of retinol=3.33 IU. Don't let this confuse you. The more you use it, the easier it gets. Just copy the chart below for reference and stick it on your refrigerator :o)


-Deficiency of vitamin A

One of the earliest signs of vitamin A deficiency is night blindness, also called nyctalopia. Short term deficiency is usually reversible, but chronic vitamin A deficiency usually leads to irreversible loss of visual cells. Sever deficiency of vitamin A can lead to xerophthalmia, a dryness of the conjunctiva and cornea. This leads to ulceration and blindness if untreated. Xerophthalmia is usually only seen in third world countries. However, night blindness has a worldwide distribution. Deficiencies of vitamin A can result in less food intake. This may be due to loss of taste through keratinization of the taste buds. Decreased food intake will additionally affect growth. With decrease in bone growth, the skeletal system fails to keep parallel growth rate with the central nervous system. This results in central nervous system damage. As we can see, vitamin A is important all along the developmental phase of the child, and throughout adulthood.

Do not make the mistake of taking vitamin A during pregnancy ! Administration of vitamin A during pregnancy can result in malformation of the fetal brain. This is irreversible.

-Toxicity of vitamin A

Signs and symptoms of toxicity of vitamin A have been recorded at daily doses of 25,000 IU. Symptoms of acute hypervitaminosis A include nausea, vomiting, headache, diarrhoea, stupor, and papilledoema, all suggestive of a brain tumour, interestingly enough. Chronic toxicity is associated with weight loss, nausea and vomiting, dryness of lips, bone and joint pain. Synthetic retinoids used in the treatment of acne do not give the same symptoms, but as with vitamin A, should be avoided in pregnancy, because of the risk of congenital malformations.




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