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.
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 convert to and from its visually active aldehyde form, retinal.
All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group.
Both structural features are essential for vitamin activity. The orange pigment of carrots beta-carotene can be represented as two connected retinyl groups, which are used in the body to contribute to vitamin A levels. Alpha-carotene and gamma-carotene also have a single retinyl group, which gives them some vitamin activity.
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 deficiency is “the leading cause of preventable childhood blindness”, according to UNICEF. It also increases the risk of death from common childhood conditions such as diarrhea. UNICEF regards addressing vitamin A deficiency as critical to reducing child mortality, the fourth of the United Nations’ Millennium Development Goals.
Vitamin A deficiency can occur as either a primary or a secondary deficiency. A primary vitamin A deficiency occurs among children and adults who do not consume an adequate intake of provitamin A carotenoids from fruits and vegetables or preformed vitamin A from animal and dairy products. Early weaning from breastmilk can also increase the risk of vitamin A deficiency.
Zinc deficiency can also impair absorption, transport, and metabolism of vitamin A because it is essential for the synthesis of the vitamin A transport proteins and as the cofactor in conversion of retinol to retinal. In malnourished populations, common low intakes of vitamin A and zinc increase the severity of vitamin A deficiency and lead to physiological signs and symptoms of deficiency. A study in Burkina Faso showed major reduction of malaria morbidity by use of combined vitamin A and zinc supplementation in young children.
Due to the unique function of the retinal as a visual chromophore, one of the earliest and specific manifestations of vitamin A deficiency is impaired vision, particularly in reduced light–night blindness. Persistent deficiency gives rise to a series of changes, the most devastating of which occur in the eyes. Some other ocular changes are referred to as xerophthalmia.
Other changes include impaired immunity increased risk of ear infections, urinary tract infections, meningococcal disease, hyperkeratosis white lumps at hair follicles, keratosis pilaris and squamous metaplasia of the epithelium lining the upper respiratory passages and urinary bladder to a keratinized epithelium. In relation to dentistry, a deficiency in vitamin A may lead to enamel hypoplasia.
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.
While strategies include intake of vitamin A through a combination of breast feeding and dietary intake, delivery of oral high-dose supplements remain the principal strategy for minimizing deficiency.
Observational studies of pregnant women in sub-Saharan Africa have shown that low serum vitamin A levels are associated with an increased risk of mother-to-child transmission of HIV.
Low blood vitamin A levels have been associated with rapid HIV infection and deaths. Reviews on the possible mechanisms of HIV transmission found no relationship between blood vitamin A levels in the mother and infant, with conventional intervention established by treatment with anti-HIV drugs.
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.
This syndrome includes headache, blurring of vision, and confusion, associated with increased intracerebral pressure. Symptoms begin to resolve when intake of the offending substance is stopped. Vitamin A is found in many foods, including the following list. Conversion of carotene to retinol varies from person to person, and the bioavailability of carotene in food varies.
|Source||Retinol activity equivalences|
|cod liver oil||30000|
|liver beef, pork, fish||6500|
|sweet potatofood 1||961|
|collard greens frozen then boiled||575|
|bell pepper/capsicum, red||157|
|bell pepper/capsicum, green||18|
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.