Global efforts to support national governments in addressing vitamin A deficiency are led by the Global Alliance for Vitamin A (GAVA), which is an informal partnership between A2Z, the Canadian International Development Agency, Helen Keller International, the Micronutrient Initiative, UNICEF, USAID, and the World Bank. Therapeutic doses used for acne treatment have been shown to disrupt cephalic neural cell activity. Overall, there was no evidence of an effect of antenatal vitamin A supplementation on the risk of MTCT of HIV. Cell culture studies have linked increased bone resorption and decreased bone formation with high intakes. In foods of animal origin, the major form of vitamin A is an ester, primarily retinyl palmitate, which is converted to retinol (chemically an alcohol ) in the small intestine. None of the other carotenes have vitamin activity. Chemical structure of retinol, one of the major forms of vitamin A. Vitamin A can be found in two principal forms in foods. This article is about the family of vitamers. With relations to dentistry, a deficiency in Vitamin A leads to enamel hypoplasia. Alpha-carotene and gamma-carotene also have a single retinyl group, which give them some vitamin activity. Retinol, the form of vitamin A absorbed when eating animal food sources, is a yellow, fat-soluble substance. Authors found significant statistical heterogeneity between the three trials with information on MTCT of HIV. 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. Vitamin A is a fat-soluble vitamin and depends on micellar solubilization for dispersion into the small intestine, which results in poor use of vitamin A from low-fat diets. Since the pure alcohol form is unstable, the vitamin is found in tissues in a form of retinyl ester. First there is dryness of the conjunctiva ( xerosis ) as the normal lacrimal and mucus-secreting epithelium is replaced by a keratinized epithelium. 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 (MTCT) of HIV. For the form usually used as a supplement, see Retinol.
These toxicities only occur with preformed (retinoid) vitamin A (such as from liver). Secondary vitamin A deficiency is associated with chronic malabsorption of lipids, impaired bile production and release, and chronic exposure to oxidants, such as cigarette smoke, and chronic alcoholism. The carotenoid beta- cryptoxanthin possesses an ionone group and has vitamin activity in humans. Vitamin A deficiency can occur as either a primary or a secondary deficiency. Persistent deficiency gives rise to a series of changes, the most devastating of which occur in the eyes. Since vitamin A is fat-soluble, disposing of any excesses taken in through diet takes much longer than with water-soluble B vitamins and vitamin C. Toxic effects of vitamin A have been shown to significantly affect developing fetuses. This allows for toxic levels of vitamin A to accumulate. 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. Vitamin A is cheap and easily provided through existing health services in low-income settings. Adequate supply, but not excess vitamin A, is especially important for pregnant and breastfeeding women for normal fetal development and in breastmilk. Excessive vitamin A consumption can lead to nausea, irritability, anorexia (reduced appetite), vomiting, blurry vision, headaches, hair loss, muscle and abdominal pain and weakness, drowsiness, and altered mental status. The retinol form functions as a storage form of the vitamin, and can be converted to and from its visually active aldehyde form, retinal. Early weaning from breastmilk can also increase the risk of vitamin A deficiency. All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group. High vitamin A intake has been associated with spontaneous bone fractures in animals. Joint GAVA activity is coordinated by the Micronutrient Initiative. Hepatic (liver) injury has been found in human and animal studies where consumption of alcohol is paired with high dose vitamin A and beta-carotene supplementation. Some other ocular changes are referred to as xerophthalmia.
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