Vitamin A is a little complicated. There are two main forms of vitamin A: provitamin A—carotenoids, beta-carotene and others—and preformed vitamin A, or retinoids. Provitamin A, carotenoids, are found in plants.
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Vitamin A is fat soluble and antioxidant
Carotenoids are yellow, orange and red pigments that give carrots their orange colour and make tomatoes red. There are many forms of provitamin A, but beta-carotene is the only one that is metabolised by mammals into vitamin A as needed. Other important carotenoids include lutein, zeaxanthin and lycopene.
Preformed vitamin A, which comes in three sub-forms of retinol, retinal and retinoic acid, is the most active form of vitamin A and is mostly found in animal sources of food. This is also the form supplied by most supplements. Some supplements provide a combination of provitamin A (beta-carotene) and preformed vitamin A. Vitamin A is fat soluble and stored in the body, and can build up to toxic levels if consumed in too large quantities. About 50 to 85 percent of the body’s total retinol is stored in the liver. It is also found in many other tissues in smaller concentrations. From the liver, vitamin A reaches its target cells/tissues with a carrier molecule. The eyes also require adequate vitamin A intake and store it.
There are two sources from which we can get vitamin A. If we get it from an animal source, we get the vitamin A in the active form called retinol. The liver has the highest amounts of vitamin A. It is also found in fish oil, dairy products, and fortified cereals.
However, if we eat leafy green vegetables we get a provitamin A called beta-carotene. This is in all the brightly coloured fruits and vegetables. The advantage of the plant-based beta-carotene is that it is not toxic at high levels because it is not stored in the liver in the same way that active vitamin A is.
The structure of beta-carotene is interesting. It is actually two different vitamin A molecules connected. Our bodies break apart beta-carotene into two pieces and convert it into active vitamin A (retinol). Retinol then can be transformed into other forms of active vitamin A.
Beta-carotene is found in deep orange and dark green vegetables, like carrots and spinach, and those eating five servings of fruits and vegetables a day will get this in their regular diet.
Vitamin A is arguably the most multifunctional vitamin in the human body and is essential for human survival at every stage of life. The list of major functions of vitamin A is quite long, but vision and visual acuity is a major one. It also helps genes turn on and off, helps cells develop normally, and helps with bone growth. It keeps tissues, body lining and skin healthy, and protects against infections. Vitamin A plays an important role in cellular differentiation, in the immune function and reproduction. Vitamin A escorts excess calcium out of the body. Vitamin A is also an antioxidant. Foods rich in the carotenoids such as lutein and zeaxanthin (spinach and kale) may protect against cataracts, while lycopene may lower prostate cancer risk. Dietary carotenoids seem to lessen the likelihood of age-related macular degeneration. An amazing number of activities …
How does vitamin A function?
Retinol and retinyl esters are pre-formed active vitamin A. Retinol can be converted by the body to retinal, which can, in turn, be converted to retinoic acid. Retinoic acid regulates gene transcription. Retinol, retinal, retinoic acid and related compounds are known as retinoids. Beta-carotene and other food carotenoids can be converted into retinol by the body.
Fat soluble vitamin A compounds, retinoids, are predominantly stored in the liver in the retinol form. When appropriate, they are released into the bloodstream to be delivered to other tissues, for example to the retina at the back of the eye for storage. From here, retinol, where it converted to retinal when needed and is shuttled to the cells which are specialised for vision in low-light conditions and for the detection of motion.
Vitamin A is also essential for mammalian eye development. Retinoic acids act as hormones to affect gene expression and thereby influence numerous physiological processes.
Sources of vitamin A:
Animal sources of vitamin A:
Vitamin A is packed into the oily/fatty parts of food from animal sources. When we eat animal products, like liver, kidney, eggs, shrimp, fish, fish oil, fortified milk and other dairy products we get vitamin A already in the active form.
When ingested, vitamin A is broken free from the food with the aid of pancreatic enzymes and bile, and is converted into retinol in the small intestine, is absorbed into the plasma and lymphatics from blood and taken up into the liver and stored within lipid globules. From there vitamin A reaches its target cells and tissues with a carrier molecule.
Plant sources of beta-carotene:
When we eat dark green, leafy vegetables, especially spinach, turnip greens, deep yellow and orange fruits and vegetable, such as carrots, sweet potatoes, pumpkins, yellow squash, tomatoes, apricots and mangoes, we get a provitamin A called beta-carotene.
Vitamin A deficiency:
Vitamin A deficiency usually results from insufficient intake of vitamin A from animal products and fruits and vegetables. If you have less vitamin A than is optimal, you will have decreased cell division, which has profound effects on the visual system and is the major cause of preventable blindness worldwide, mainly in developing countries. It is most prevalent among children and women of child-bearing age. Worldwide, vitamin A deficiency is particularly devastating for children and usually they develop a kind of night blindness and can go on to full blindness and to childhood mortality, due to decreased cell division and deficient development. In countries where people and children are given extra vitamin A as a supplement, a significant reduction in childhood blindness and mortality has been achieved.
Other individuals at risk of deficiency are those with poor absorption of lipids and those with inflammatory bowel disease.
Vitamin A helps with infections, and therefore deficiency is associated with an increased susceptibility to infections, as well as thyroid and skin disorders.
Conversely, if you have too much vitamin A you will then have somewhat over stimulated cell division which may lead to organ dysfunction and problems resulting from that. For example, over-consumption of pre-formed vitamin A can be highly toxic and is especially contraindicated prior to and during pregnancy as it can result in severe birth defects. The tolerable upper intake level (UL) for vitamin A in adults is set at 3mg per day. The UL does not apply to vitamin A derived from carotenoids, i.e. from plants.
Vitamin A prophylaxis appears to significantly reduce childhood mortality in regions at high risk of vitamin A deficiency. Further, high-dose vitamin A supplementation is widely recommended for children over six months of age when they are infected with measles while malnourished, immunodeficient or are at risk of measles complications.
Fortunately, in developed countries, we have very little vitamin A deficiency.
There is no indication that the vitamin A requirement of healthy elderly individuals differs from those of other adults. It should be remembered, however, that diseases impede vitamin A absorption, storage and transport, and therefore deficiency might be more common in the elderly than in other age groups.
Since vitamin A is fat soluble, it is important to ensure the necessary fat intake in our diets.
Many people get too much preformed vitamin A from food and supplements. Large amounts of supplemental vitamin A, but not beta carotene, can be harmful to bones and may give rise to birth defects.
Over-stimulated cell division and organ dysfunction (abdominal pain, blurred vision, headache). The majority of vitamin A toxicity cases are due to the chronic ingestion of large amounts of synthetic (or pre-formed) vitamin A. Too much vitamin A harms bones and may give rise to birth defects. Water-miscible, emulsified, and solid forms of retinol supplements, which include those in candy-like supplements marketed for children, are more toxic than oil-based preparations.
By contrast, the metabolism of provitamin A, beta-carotene from plant sources, is highly regulated, so excessive ingestion of this form of vitamin A is unlikely to cause toxicity, only skin yellowing.
Many supplements provide a combination of retinol and beta-carotene. Because retinol intakes of 1.5mg RAE may be associated with an increased risk of osteoporosis in older adults, some companies have reduced the retinol content in their multivitamin supplements to 0.75mg RAE.
Retinoic acid (or derivatives) is used at pharmacological doses in the treatment of various skin disorders such as acne.
Note: use of retinoids by pregnant women causes birth defects and is therefore contraindicated prior to and during pregnancy.