This vitamin is also known as retinol (also called retinal or retinoic acid), a word much favoured by manufacturers of skin preparations that aim to retard the ageing process.
It also has the chemical names retinol palmitate and retinol acetate. "Vitamin A" is actually the collective name for compounds with the biological activity of retinol, which was originally isolated from the retina in the eye. Vitamin A is absorbed by the body in the form of retinol.
Plant pigments called carotenoids, converted to retinol by the body through the process of metabolism, are found in various vegetables and fruits. These compounds are also referred to as "provitamins A" as they are precursors of the useful form of the vitamin. Not all of the carotenoid varieties have significant provitamin A activity.
The most active form of these provitamins is beta-carotene. This precursor has been the subject of many a research study during the last couple of years, as it is believed to have strong anti-cancer properties.
What does it do for you?
Your body needs vitamin A to produce rhodopsin, the pigment that helps you to see in the dark.
It also helps the mucosal linings of the mouth, digestive, urogenital and respiratory systems to stay moist, to prevent intrusion of foreign organisms and toxins, and to heal after infection or inflammation, as well as ensuring the development of strong bones, a functional reproductive system and healthy skin.
The vitamin plays an important role in reproduction and also helps your immune system, enabling you to ward off bacterial, parasitic or viral infections. For this reason, vitamin A supplements help children to recover more quickly from measles, and lower the risk of developing serious complications from measles.
Vitamin A is also a potent antioxidant. The word "antioxidant" refers to a substance that can inhibit the activity of free radicals - unusually reactive, strongly oxidising atoms capable of causing a wide range of biological damage in the body, of which the most pronounced is cancer.
How much vitamin A do you need?
The recommended dietary allowance (RDA) for vitamin A is usually expressed in microgram, but can also be expressed in International Units (IU) or milligram.
The RDA for infants is based on the amount of retinol present in mother's milk. Breastfeeding women need a higher daily dose of the vitamin to compensate for the amount of the vitamin lost via breast milk.
The current RDA/adequate intake for vitamin A is 900 microgram per day for male adults, 700 microgram per day for female adults, and 1 300 microgram for breastfeeding women.
Which foods have vitamin A?
Preformed vitamin A doesn't occur naturally in fruit and vegetables, but can be found in food of animal origin. Because this vitamin is fat-soluble, it is associated with the fatty components of animal products, like the fat in milk and eggs.
Liver is a very rich source of vitamin A. Vitamin A can also be found in generous amounts in oysters and fatty fish such as herring, anchovy and mackerel. Cod-liver oil is an excellent source of vitamin A.
Dark-green, leafy, and yellow-orange fruits and vegetables contain high amounts of provitamin A, like beta-carotene. Fruits with a deeper colour are said to have higher beta-carotene content (kids are usually taught that they should eat fruit and veggies with colours that resemble the colours on traffic lights).
Excellent sources of provitamin A are:
- Vegetable soup
- Green salad
- Orange juice
- Sweet potatoes
- Beef stew
- Mixed vegetables
In certain vegetables, like carrots, the provitamins are not readily available for absorption by the body as these are binded to proteins. Cooking the vegetables can break these bonds.
Unlike the water-soluble vitamins (like vitamin C), vitamin A is relatively stable when exposed to heat and light. Although cooking generally doesn't destroy the vitamin, it can be rendered inactive when it is exposed to so-called "transition" elements, like iron and copper.
The vitamin is, however, protected in the presence of vitamin E. For this reason, vitamin E or other synthetic antioxidants is usually added to supplements that contain vitamin A.
How much vitamin A is too much?
Exceeding 25 000 IUs a day for three months can cause liver and skin damage. In children, taking more than 5 000 IUs or 1,5 mg a day can also cause hair loss, bone damage, headache, vomiting and double vision.
Dry lips are usually the first sign of vitamin A overdose, followed by dryness of the nasal mucosa and eyes.
It is important to note that certain supplements on the South African market contain as much as 5 000 IUs of vitamin A per tablet. If these supplements were to be given to children it could be detrimental to their health – especially if the child already gets vitamin A through his or her diet.
It's also important to note that a major review of 68 studies found that vitamin A supplementation, in combination with other supplements, increased mortality (death) significantly.
Check the content of any multivitamin, cod (or fish) liver oil or vitamin A supplement before you buy it and make sure that the tablets' RDA doesn't exceed 100%. Don't give adult vitamin supplements to kids.
NOTE: High vitamin A intake during pregnancy has been
linked to an increased rate of birth defects. Therefore, pregnant women
should not take supplements with a high vitamin A content.
Signs of vitamin A deficiency
Vitamin A deficiency is the leading cause of blindness in the developing world. One of the first signs of vitamin A deficiency is impaired vision that manifests itself as night blindness.
Other signs of deficiency include poor growth in children, failure of the reproductive system in adults, gingivitis, dry skin and susceptibility to infection.
Severe deficiency can also lead to spontaneous abortion, anaemia and death.
On a global level, it is estimated that 250 million children are at risk of vitamin A deficiency. The United Nations Children's Fund (UNICEF) estimates that as many as one million child deaths can be ascribed to this global problem.
It is also a significant problem in South Africa, especially among lower income groups.
Although vitamin A deficiency is usually the result of
insufficient dietary intake, it can also result from malabsorption due
to pancreatic insufficiency, liver disease, protein-energy malnutrition
or zinc deficiency.
Research on vitamin A
Cystic fibrosis sufferers seem to benefit from taking vitamin A. It’s also been shown to help reduce the risk of measles in children. It appears that vitamin A can help reduce the risk of developing cancer, respiratory problems in children and glaucoma.
It was also found that antioxidants, like beta-carotene, can improve learning and memory, and that they minimise the effect of ageing on the brain.However, one study also indicated that people living in Western nations might increase their risk for bone fractures by getting too much vitamin A.