Vitamin-A

Vitamin-A is a group of unsaturated nutritional organic compounds that includes retinol, retinal, and several pro vitamin-A carotenoids most notably beta-carotene. Vitamin A has multiple functions: it is important for growth and development, for the maintenance of the immune system, and for good vision. Vitamin A is needed by the retina of the eye in the form of retinal, which combines with protein opsin to form rhodopsin, the light-absorbing molecule necessary for both low-light scotopic vision and color vision.

In foods of animal origin, the major form of vitamin-A is an ester, primarily retinyl palmitate, which is convert to retinol chemically an alcohol in the small intestine. The retinol form functions as a storage form of the vitamin, and can be converted to and from its visually active aldehyde form, retinal.

Vitamin A can be found in two principal forms in foods:

Retinol, the form of vitamin A absorbed when eating animal food sources, is a yellow, fat-soluble substance. Since the pure alcohol form is unstable, the vitamin is found in tissues in a form of retinyl ester. It is also commercially produced and administered as esters such as retinyl acetate or palmitate.

The carotenes alpha-carotene, beta-carotene, gamma-carotene; and the xanthophyll beta-cryptoxanthin all of which contain beta-ionone rings, but no other carotenoids, function as provitamin A in herbivores and omnivore animals, which possess the enzyme beta-carotene, dioxygenase in the intestinal mucosa to cleave and convert provitamin A to retinol.

it is important for growth and development, for the maintenance of the immune system, and for good vision. Vitamin A is needed by the retina of the eye in the form of retinal, which combines with protein opsin to form rhodopsin, the light-absorbing molecule necessary for both low-light scotopic vision and color vision.

Vitamin A can be found in two principal forms in foods:

Retinol, the form of vitamin A absorbed when eating animal food sources, is a yellow, fat-soluble substance. Since the pure alcohol form is unstable, the vitamin is found in tissues in a form of retinyl ester. It is also commercially produce and administered as esters such as retinyl acetate or palmitate.

The carotenes alpha-carotene, beta-carotene, gamma-carotene; and the xanthophyll beta-cryptoxanthin all of which contain beta-ionone rings, but no other carotenoids, function as provitamin A in herbivores and omnivore animals, which possess the enzyme beta-carotene ,’-dioxygenase in the intestinal mucosa to cleave and convert provitamin A to retinol.

 Vitamin A deficiency is estimate to affect approximately one third of children under the age of five around the world. It is estimate to claim the lives of , children under five annually. Between , and , children in developing countries become blind each year owing to vitamin A deficiency, with the highest prevalence in Africa and southeast Asia.

Vitamin A supplementation

A 2012 review found no evidence that beta-carotene or vitamin A supplements increase longevity in healthy people or in people with various diseases. A 2011 review found that vitamin A supplementation of children at risk of deficiency aged under five reduced mortality by up to 24%.

However, a 2016 and 2017 Cochrane review concluded there was not evidence to recommend blanket vitamin A supplementation for all infants less than a year of age, as it did not reduce infant mortality or morbidity in low- and middle-income countries. The World Health Organization estimated that vitamin A supplementation averted 1.25 million deaths due to vitamin A deficiency in 40 countries since 1998.

Side effects

Given that vitamin A is fat-soluble, disposing of any excess taken in through diet takes much longer than with water-soluble B vitamins and vitamin C. This allows for toxic levels of vitamin A to accumulate. These toxicities only occur with preformed vitamin A (retinoid). The carotenoid forms (for example, beta-carotene as found in carrots) give no such symptoms, but excessive dietary intake of beta-carotene can lead to carotenodermia, a harmless but cosmetically displeasing orange-yellow discoloration of the skin.

 In general, acute toxicity occurs at doses of 25,000 IU/kg of body weight, with chronic toxicity occurring at 4,000 IU/kg of body weight daily for 6–15 months. However, liver toxicities can occur at levels as low as 15,000 IU (4500 micrograms) per day to 1.4 million IU per day, with an average daily toxic dose of 120,000 IU, particularly with excessive consumption of alcohol. In people with kidney failure, 4000 IU can cause substantial damage. Signs of toxicity may occur with long-term consumption of vitamin A at doses of 25,000–33,000 IU per day.

Vitamin A and derivatives in medical use

Retinyl palmitate has been used in skin creams, where it is broken down to retinol and ostensibly metabolised to retinoic acid, which has potent biological activity, as described above. The retinoids for example, 13-cis-retinoic acid constitute a class of chemical compounds chemically related to retinoic acid, and are used in medicine to modulate gene functions in place of this compound. Like retinoic acid, the related compounds do not have full vitamin A activity, but do have powerful effects on gene expression and epithelial cell differentiation. Pharmaceutics utilizing megadoses of naturally occurring retinoic acid derivatives are currently in use for cancer, HIV, and dermatological purposes. At high doses, side-effects are similar to vitamin A toxicity.

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