Vitamin A (Retinol).
Vitamin A is an essential fat-soluble coenzyme that contributes to  maintenance of good health, since it triggers numerous biochemical reactions. The vitamin itself is not contained in foods but it is found as retinyl esters or as carotenoids. The latest are compounds contained in plants and give the characteristic yellow, orange or red color to leaves, roots and fruits. There are about six hundred carotenoids but alpha-carotene and beta-carotene are the most common of them.
Retinyl esters are contained in animal dietary sources. Both retinyl esters and carotenoids are considered pre-vitamin A substances. Retinyl esters and carotenoids, after been ingested, break to retinol which is the final precursor to vitamin A. A synthetic form of the latest is contained in relevant food supplements. Vitamin A as a prominent antioxidant can prevent skin ageing, cancer, stroke and coronary heart disease.
Vitamin A assists in formation and maintenance of healthy teeth, bones, red blood cells, soft tissue, hair and retina which is part of the eye. It also can improve vision in low light and the functions of the immune system and the epithelial cells, including skin and mouth mucosa. Vitamin A contributes to the synthesis of glycoproteins and growth hormone.
A lot of active forms of vitamin A are used in skin care topical preparations. For instance retinol, retinoic acid, isotretinoin, retinal and retinyl palmitate can be contained in skin care creams for the treatment of acne. Retinoids, such as tretinoin or retin-A, are vitamin A origin drugs. Tretinoin can protect collagen in the skin against any damage due to exposure to the sun. It can also boost collagen synthesis and delay extrinsic skin ageing.
Sources of vitamin A.
Retinyl esters are contained in animal sources such as turkey, chicken, beef, pork, eggs, diary products, kidney, liver, halibut fish, sardine and cod oil. The majority of such foods have to be eaten in moderation, since the cholesterol and saturated fat content of them is a serious concern. However, skim milk is low-fat and it is a reliable source of vitamin A, since it is fortified with retinyl esters.
Carotenoids, especially beta-carotene are found in plant sources, such as carrots, sweet potatoes, mangos, pumpkin, asparagus, winter squashes, cantaloupe, pink grapefruit, tangerines, plums, apricots, papayas, peas, broccoli, Brussels sprouts, cabbage and in most of the dark green, leafy vegetables, like spinach, dandelion greens and lettuce. In fact, there is a general rule which says that the more intense the color of a vegetable or fruit, the higher the carotenoids content is. The good thing with the plant sources of beta-carotene is that they do not contain cholesterol or saturated fats. On the other hand the drawback is that pro-vitamin A included in such foods is not as well absorbed as the one found in animal sources.
If a balanced diet is not possible on a long term basis, food supplements can be used as a vitamin A or a beta-carotene source. The recommended dose should never be exceeded, though. Try the  button, if you want to find vitamin A supplements online.
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Vitamin A deficiency and toxicity.
A person, whose diet does not include adequate vitamin A, is far more prone to infectious conditions and sight problems. Vitamin A deficiency is the aftermath of malnutrition and it is common in developing countries, whereas it is quite rare in developed ones. It is estimated that each year, nearly 500,000 children in the third world lose their sight due to a vitamin A deficiency.
Occasionally, a mild type of vitamin A deficiency may occur and it is referred to as sub-clinical deficiency. The condition affects children and it is characterized by low storages of retinol and does not present the typical symptoms. The signs may include unexplained diarrhea, predisposition to respiratory infections, inadequate or slow bone development and low body growth rate. Children at preschool age and children with pancreas or liver disease and gastrointestinal disorders, which prevent  proper fat absorption, may be prone to sub-clinical vitamin A deficiency. Children of very poor nutritional background may present the condition as well.
Regular excessive alcohol use, inadequate intake of protein and low levels of zinc or iron and conditions, such as celiac disease, Crohn's disease, and pancreatic disorders, may be risk factors for vitamin A deficiency.
Large dosages of Vitamin A can build up and get toxic, since the no longer needed amounts of retinol can accumulate in body fat and not be excreted in the urine, as the water-soluble vitamins do.
A clear sign of hypervitaminosis, which is a vitamin A overdose situation, is the color of the skin which can chance to yellowish or orange. The skin color recovers, as soon as the excessive intake of carotenoids is reduced to normal rates.
For more information about vitamin A toxicity visit the relevant page on this website.
Recommendations for vitamin A.
When it comes to dietary allowances, retinol is usually measured in international units. One IU of vitamin A is equivalent to approximately 0.3µg (300 nanograms). 3.3 international units count for one Retinol Activity Equivalent (RAE). The latest is the most commonly used element for measurement of vitamin intake dose. In the U.S. the recommended dietary allowance (RDA) for vitamin A is 700 RAE or 2,300 IU per day for women and 900 RAE or 3,000 IU per day for men. These represent the minimum requirements, though.
For an optimum health the recommended daily dosages are 1,950 RAE or 5,000 IU for adults, although some experts recommend 3,900-9800 RAE or 10,000-25,000 IU.
Our belief is that supplements with total carotene content equivalent to 10,000 IU of vitamin A, like the one recommended above, are the best option. Unless you have been advised by your doctor for a different dosage, take one capsule daily. If you are a smoker though, you may have to limit the intake to one capsule every two or three days. In any case, vitamin A supplementation must be combined with daily intake of at least 250mg of vitamin C for non smokers and 500mg for smokers.
For children or pregnant females a physician's advice should be sought. The toxic level for vitamin A is 19,500 RAE or 50,000 IU per day.
In general, a balanced diet that includes reasonable amounts of animal derived foods, dark green and leafy vegetables and deep colored fruits should provide sufficient retinyl esters, beta-carotene and other types of carotenoids.
Studies have given evidence that in heavy smokers, high uptake of vitamin A can increase the risk for cancer, especially lung cancer, if it is not accompanied with relevant vitamin C and vitamin E supplementation. Thus, it is strongly recommended for smokers to take at least the minimum daily dosage of C and E vitamins, while taking vitamin A supplements.
Excess alcohol consumption can deplete vitamin A. On the other hand, an overburdened liver with alcohol may be more prone to vitamin A toxicity. Thus, people having the habit to consume large amounts of alcohol on a regular basis, should seek medical advice, before taking vitamin A supplements.
Vitamin A is quite delicate and can be destroyed during cooking or storage. Therefore, if possible, fruits and vegetables must be consumed fresh and raw. Apart from winter squashes and sweet potatoes, any other vegetables should be kept in the fridge. When it comes to food preparation, baking, broiling and braising must be preferred and not frying instead, because vitamin A can be lost in frying fat.
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