Vitamin A - Retinol
Vitamin A is actually a family of fat-soluble vitamins. Retinol is one of the most active, or usable, forms of vitamin A, and is found in animal foods such as liver and eggs. It can be converted to retinal and retinoic acid, other active forms of the vitamin A family. Some plant foods contain orange pigments called provitamin A carotenoids that the liver can convert to retinol. Beta-carotene is a provitamin A carotenoid found in many foods. Lycopene, lutein, and zeaxanthin are also carotenoids commonly found in food, but your body cannot convert them to vitamin A.
Vitamin A plays an important role in vision, bone growth, reproduction, cell division and differentiation. It maintains the surface linings of your eye and your respiratory, urinary, and intestinal tracts. When those linings break down, bacteria can enter your body and cause infection. The immune system helps prevent or fight off infections by making white blood cells that destroy harmful bacteria and viruses. Vitamin A may help lymphocytes, a type of white blood cell that fights infections, function more effectively. Vitamin A also may help prevent bacteria and viruses from entering your body by maintaining the integrity of skin and mucous membranes. Vitamin A is required for night vision, and for a healthy skin. It assists the immune system, and because of its antioxidant properties is great to protect against pollution and cancer formation and other diseases. It also assists your sense of taste as well as helping the digestive and urinary tract and many believe that it helps slow aging. It is required for development and maintenance of the epithelial cells, in the mucus membranes, and your skin, and is important in the formation of bone and teeth, storage of fat and the synthesis of protein and glycogen
Although vitamin A is probably best known for promoting and maintaining healthy eyesight, it has other important functions as well. One of its major contributions is to improve the body's resistance to infection. It does this in part by maintaining the health of the skin, mucous membranes, and other surface linings (intestinal tract, urinary tract, respiratory tract) so that harmful bacteria and viruses can't get into your body. Another way that vitamin A boosts immunity is by enhancing the infection-fighting actions of the white blood cells called lymphocytes. Vitamin A is also vital to the growth of bones, the division of cells in your body, and to human reproduction.
Retinol, the most useful form of vitamin A, (along with retinal and retinoic acid) is a fat-soluble, antioxidant vitamin important in vision and bone growth. It is sometimes used in the treatment of severe acne. This is a compound synthesized from isoprene. Another form of retinol is retinyl palmitate. 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. Retinol is the immediate precursor to two important active metabolites: retinal, which plays a critical role in vision, and retinoic acid, which serves as an intracellular messenger that affects transcription of a number of genes. Retinol is one of the most active, or usable, forms of vitamin A, and is found in animal foods such as liver and eggs. Retinol is often called preformed vitamin A and can be toxic. This condition, called hypervitaminosis A can cause birth defects, liver abnormalities, and reduced bone mineral density that may result in osteoporosis. When toxic symptoms arise suddenly, which can happen after consuming very large amounts of preformed vitamin A over a short period of time, signs of toxicity include nausea and vomiting, headache, dizziness, blurred vision, and muscular uncoordination. For this reason, it is important to check your multivitamins to make sure that the source of vitamin A is not retinol or palmitate. All vitamin supplement products that contain any form of vitamin A must list vitamin A as the main ingredient on the label, even if it is all in the form of beta carotene, so do not be alarmed. Some multivitamins like Cooper Complete have labels that list the ingredient as "Vitamin A (as natural mixed carotenoids)". In such cases, you will have no need to worry. However, in some cases you may have to look at the "other ingredient" section of the label to determine what the product includes.
Beta-carotene is the molecule that gives carrots their orange colour. It is part of a family of chemicals called the carotenoids, which are found in many fruit and vegetables, as well as some animal products such as egg yolks. Beta carotene, as well as the other members of the carotenoid family, are precursors to vitamin A, meaning that they are transformed into the vitamin after entering the body. The metabolism of beta carotene is much different than that of retinol because the body can convert it into as much or as little of vitamin A as it needs. For this reason, you cannot ingest toxic levels of beta carotene. However, eating high levels of it may actually turn your skin an orange color! This provitamin A has been linked to a lower risk of cataracts, heart disease, and cancers, such as rectal cancer, melanoma, and bladder cancer. As a potent immune-system booster and a powerful antioxidant--it counters the effects of cell-damaging molecules called free-radicals--beta-carotene has an important role to play in human health. In addition to the numerous studies on beta-carotene's effectiveness for heart disease and cancer, researchers have been exploring the nutrient's potential for treating chronic fatigue syndrome, Alzheimer's disease, fibromyalgia, male infertility, and psoriasis. Interestingly, low levels of beta-carotene and other antioxidants have been linked to the development of cataracts, a clouding of the eye's lens that impairs vision.