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Nutrient Details For : Vitamin A

A nutrient is any element or compound necessary for or contributing to an organism's metabolism, growth, or other functioning. Click name for details Arsenic Biotin Boron Calcium Carbohydrates Carnitine Carotenoids Chloride Choline Chromium Copper Energy Fiber Fluoride Folate Iodine Iron Lipids Magnesium Manganese Molybdenum Niacin Nickel Pantothenic acid Phosphorus Phyto-chemicals Potassium Protein Riboflavin Selenium Silicon Sodium Thiamin Vanadium Vitamin A Vitamin B 12 Vitamin B 6 Vitamin C Vitamin D Vitamin E Vitamin K Zinc Further Trace Elements

Vitamin A, a fat-soluble vitamin, plays essential roles in vision, growth, and development; the development and maintenance of healthy skin, hair, and mucous membranes; immune functions; and reproduction.

The chemical name of vitamin A is retinol Retinol is also found as retynyl (vitamin A) esters such as retinyl acetate and retinyl palmitate.

The major storage site of vitamin A in the body is in the liver, primarily in the form of retinyl esters.

The best known function of vitamin A is in vision, where it participates (as the metabolite retinal) in the visual cycle. However, in the chemical form retinoic acid, vitamin A plays an important role in control of gene expression. This function maintains differentiation of epithelial cells such as skin, lung, and intestinal tissue.

Retinoic acid can be formed from retinol in the body, and shows strong biological activity in some functions but not in vision.

Deficiencies:

Night blindness is one of the early signs of vitamin A deficiency, because of the role of vitamin A in vision. Bacterial invasion and permanent scarring of the cornea of the eye (xerophthalmia) is a symptom of more profound deficiency, but this is due to a different mechanism, the lack of vitamin A for control of gene expression. Profound vitamin A deficiency also results in altered appearance and function of skin, lung, and intestinal tissues. Children are most at risk of vitamin A deficiency because they have not yet developed adequate vitamin A stores.

It has been estimated that 0.5 million children in the world become blind each year, 70% of these due to vitamin A deficiency. Over half of these blind children die from malnutrition and associated illnesses.

Recommendations:

For adult human males, the Recommended Dietary Allowance (RDA) is 1000 ug Retinyl Equivalents (RE)/d; for adult females, 800 ug RE. Food sources: The RDA can be met by consuming dietary preformed vitamin A (retinyl esters) from liver, eggs and fortified foods, and provitamin A carotenoids such as beta-carotene, which are found in green leafy vegetables as well as in orange and red fruits and vegetables.

No one should consume vitamin A in quantities exceeding the RDA without a doctor's advice because of the dangers of toxicity.

Toxicity:

Acute intake of extremely high doses of vitamin A (>200 mg RE in adult humans) can cause nausea, vomiting, headache, and increased cerebrospinal pressure. Symptoms are generally transient. Chronic high intakes (e.g., >10x RDA) can cause hair loss, bone and muscle pain, headache, liver damage, and increased blood lipid concentrations.

A particular danger in pregnant women is teratogenesis (birth defects). On the other hand, carotenoids as a source of vitamin A are not toxic, even with very high intakes.

Recent research news:

Studies focusing on the role of retinoic acid isomers in the control of gene expression are explaining effects of vitamin A in hitherto unexpected metabolic pathways as well as in established functions. This role of vitamin A in gene expression undoubtedly explains the anti-cancer and anti-acne effects of vitamin A, for example.

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