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Vitamin A is vital to human health, but most of us don't make any particular effort to get enough of it. Answers to the question of what does vitamin A do and whether it's best to take vitamin A capsules are answered here.
The substance we call vitamin A is a chemical known as retinyl palmitate. It's a special kind of fatty acid that gets transformed into retinol as food is digested in the small intestine. The human body can also manufacture vitamin A from beta-carotene, the plant chemical found in carrots, and make a less potent form of vitamin A from the alpha-carotene and gamma-carotene found in other orange and yellow fruits and vegetables.
Vitamin A plays a number of vital roles throughout the body, including:
The best known of these applications of vitamin A are skin health and vision. The most common medications for severe, cystic acne, for instance, are all concentrated chemical derivatives of vitamin A. In skin conditions caused by overactivity of the skin, such as acne, eczema, psoriasis, and even measles, vitamin A helps skin cells find an orderly life cycle so that overactive skin cells retire.
These aging skin cells are recycled, leaving the remaining skin cells to function normally. Vitamin A has the same function in membranes all over the body, even helping the membranes in the sex organs resist viral infections.
Vitamin-A rich carrots are folk remedy that really works in helping to restore night vision, but the vitamin is also essential for daytime vision. The retina senses vision when light falls on rods and cones, light causing a chemical reaction in a vitamin A derivative that creates the electrical charge that sends the signal to the brain.
Children, in particular, have to have vitamin A to develop healthy eyes, and severe vitamin A deficiency is one of the world's leading causes of blindness. Just a single dose of vitamin A before the age of three, however, can prevent blindness in children. Even though the treatment costs only pennies, every year between 250,000 and 500,000 children permanently lose their site, mostly in Southeast Asia and Africa.
Severe vitamin A deficiencies can also cause eye problems in adults. Usually, the first change in eyes that are deficient in vitamin A is a condition called xerophthalmia, or dry eye. As the cornea dries out, it becomes more susceptible to infection, and debris builds up in tiny placques that eventually completely cover the lens of the eye. Just a tiny amount of vitamin A, however, prevents this condition.
Vitamin A, in the form of retinyl palmitate, is something you should never take in megadoses. Because the vitamin is fat soluble instead of water soluble, any overdose will stay in your fat cells and continue to cause toxic effects over a very long period, especially if you lose weight.
It is particularly important for women who are pregnant or who could become pregnant to avoid overdoses of vitamin A, since a very large dose can cause birth defects. Birth defects usually occur after exposure to 300,000 IU or more in a single dose, but it's always better to take much less than any potentially toxic dose. Most adults should not take more than 5,000 IU per day. People who have kidney disease also have difficulty metabolizing more than about 4,000 IU of vitamin A a day. Children should not be given more than 1,500 IU a day, except as a catch-up treatment to prevent blindness.
Anyone who takes too much vitamin A on an ongoing basis is likely to experience dry hair and hair loss, dry skin, nosebleeds, fever, insomnia, weight loss, anemia, diarrhea, and headaches.
The safest way to get your vitamin A is to take mixed carotenoids, naturally sourced alpha-carotene, beta-carotene, and gamma-carotene. Since your body only converts these safe plant compounds into vitamin A when vitamin A is needed, they do not build up to toxic levels in your body. Doses of up to 25,000 IU a day are safe for beta-carotene and mixed carotenoids.
Black RE et al., Maternal and child undernutrition: global and regional exposures and health consequences, The Lancet, 2008, 371(9608), p. 253.