Vitamin A
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The word vitamin was originally derived from Funk’s term "vital amine." In 1912, he was referring to Christian Eijkman’s discovery of an amine extracted from rice polishings that could prevent beriberi. Funk’s recognition of the antiberiberi factor as vital for life was indeed accurate. Researchers have since found that vitamins are essential organic compounds that the human body cannot synthesize. Vitamins A, D, K, and E are classified as fat-soluble vitamins, whereas others are classified as water-soluble vitamins. 10
The U.S. RDA for vitamin A is the amount of the vitamin used as a standard in nutrition labeling of foods. This allowance is based on the 1968 Recommended Dietary Allowances (RDA) for 24 sex-age categories set by the Food and Nutrition Board of the National Academy of Sciences. The 1989 RDA has been set at 800 retinol equivalents per day for women 19 to 50 years of age and 1,000 retinol equivalents for men 19 to 50 years of age. 7
Vitamin A occurs in several different forms. Pre-formed vitamin A is a family of substances called retinol or retinyl esters (Teratology Society; 1987). Retinyl palmitate and retinyl acetate are examples of retinyl esters that are commonly used in vitamins (FDA Talk Paper; October 6, 1995). Pre-formed vitamin A is also found in animal products like liver and in fortified cereals and vitamin tablets (FDA Talk Paper; October 6, 1995). Beta-carotene, found in fruits and vegetables, is a vitamin A precursor, which means that it is converted to vitamin A in the body. 22
Carotenoids are dark colored dyes found in plant foods that can turn into a form of vitamin A. One such carotenoid is beta-carotene. Beta-carotene is an antioxidant. Antioxidants protect cells from damage caused by unstable substances called free radicals. Free radicals are believed to contribute to certain chronic diseases and play a role in the degenerative processes seen in aging. 12
The Upper Limit of vitamin A set by the Food and Nutrition Board of the National Academy of Sciences is 10,000 IU per day. Vitamin A can be found in such dietary supplements as palmitate, acetate or fish liver oil, all of which are derived from animal sources. Although vitamin A is an essential nutrient, an excessive chronic intake of these forms of vitamin A can be toxic, leading to hair loss, confusion, liver damage, and - more recently discovered - bone loss. 19
Retinol is sometimes called "true" vitamin A because it is nearly ready for the body to use. Retinol is found in such animal foods as liver, eggs, and fatty fish. It can also be found in many fortified foods, such as breakfast cereals, and in dietary supplements. 5
Beta-carotene is a precursor for vitamin A. The body needs to convert it to retinol or vitamin A for use. Beta-carotene is found naturally in plant foods, mostly orange and dark green ones such as carrots, sweet potatoes, mangos, and kale. 5
Eating a variety of foods that contain vitamin A (and carotenes) is the best way to get an adequate amount. Healthy individuals who eat a balanced diet rarely need supplements. In fact, too much vitamin A can be toxic. The list of foods will help you select those that are good sources of vitamin A as you follow the Dietary Guidelines. The list of good sources was derived from the same nutritive value of foods tables used to analyze information for recent food consumption surveys of the U.S. Department of Agriculture, Human Nutrition Information Service. 7
Vitamin A can be found in both animal and plant foods. It is found as retinol in animal foods and as carotenoids in plant foods. These are compounds the body can convert into vitamin A. The single best source of vitamin A is liver. 47
Vitamin A is also found in a variety of dark green and deep orange fruits and vegetables, such as carrots, sweet potatoes, pumpkin, spinach, butternut squash, turnip greens, bok choy, mustard greens, and romaine lettuce. Beta carotene is the most active carotenoid (the red, orange, and yellow pigments) form of vitamin A, but it is inefficiently absorbed and converted to retinol in comparison to vitamin A from animal sources. FYI, cooking (but not overcooking) increases the bioavailability of carotenoids in plant foods; and, absorption of vitamin A from the diet is improved when consumed along with some fat in the same meal. 35
Vitamin A found in foods that come from animals is called preformed vitamin A. It is absorbed in the form of retinol, one of the most usable (active) forms of vitamin A. Sources include liver, whole milk, and some fortified food products. Retinol can be made into retinal and retinoic acid (other active forms of vitamin A) in the body [1]. 23
Vitamin A that is found in colorful fruits and vegetables is called provitamin A carotenoid. They can be made into retinol in the body. In the United States, approximately 26% of vitamin A consumed by men and 34% of vitamin A consumed by women is in the form of provitamin A carotenoids [1]. Common provitamin A carotenoids found in foods that come from plants are beta-carotene, alpha-carotene, and beta-cryptoxanthin [11]. Among these, beta-carotene is most efficiently made into retinol [1,13-15]. Alpha-carotene and beta-cryptoxanthin are also converted to vitamin A, but only half as efficiently as beta-carotene [1]. 23
retinyl palmitate) and vitamin A acetate (retinyl acetate) are the principal forms used as nutritional supplements. Retinyl palmitate is a more stable version of retinol, however, because the skin has to further break down retinyl palmitate, much higher concentrations are required to provide the similar benefits. When choosing between the two, it is better to go with the formula containing retinol rather than retinyl palmitate. The precursors of vitamin A (retinol) are the carotenoids (most commonly beta-carotene). Retinol, retinal, retinoic acid, and related compounds are known as retinoids. Retinal can be converted by the body to retinoic acid, the form of vitamin A known to affect gene transcription. Beta-carotene and other carotenoids that can be converted by the body into retinol are referred to as provitamin A carotenoids. 18
Due to the outstanding scientific work of these and many other researchers, the administration of cod liver oil to growing children�a tradition found among Arctic peoples such as the Scandanivians and Eskimos�became standard practice until after the Second World War. Ironically, as Americans have stopped giving cod liver oil to their children, programs to administer vitamin A to children in Africa and Asia have had astonishing success in preventing blindness and infectious disease. This vitamin-A-treatment program was the brainchild of yet another brave researcher, Alfred Sommer, an ophthalmologist at Johns Hopkins University, who patiently lobbied for an international program after observing the wonderful effects of vitamin-A supplementation in Indonesia and Nepal. 24
Vitamins, though they are available from a variety of sources, are unevenly distributed in natural sources. For example, some vitamins, such as vitamin D, are produced only by animals, whereas other vitamins are found only in plants. 40
References
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