Vitamin A is a term used to refer to a group of compounds, including retinol, retinal and retonic acid, which possess the biological activity of all trans retinol. The major usable active form of vitamin A is retinol. In addition vitamin A can be converted from its precursors, which include carotenoids. There is over 600 carotenoids, but only around 10% exhibit vitamin A activity, meaning can be converted to retinol. Common provitamin A carotenoids are beta-carptene, alpha-carotene, and beta-cryptoxanthin. Beta-carotene is most efficiently made into retinol, while alpha-carotene and beta-cryptoxanthin are converted half as efficiently. Some carotenoids like lycopene, lutein, and zeaxanthin are not converted to retinol, however are thought to be of physiological importance to the body.
Vitamin A is essential in many important functions, including a role in vision, bone growth, reproduction, cell division and differentiation as well as immune system actions. When vision in the dark doesn’t occur or the process is delayed, vitamin A deficiency may be the cause. Besides night blindness, other signs and symptoms of vitamin A deficiency include, anorexia, retarded growth, increased susceptibility to infections, obstruction and enlargement of hair follicles, and keratinization of epithelial cells of the skin with accompanying failure of normal differentiation. In addition vitamin A deficiency diminishes the mobilization of iron from stores, what can lead to microcytic anemia. Protein and zinc are related to vitamin A metabolism. Animal studies show that carotenoids function as antioxidants, however results from human studies are mixed. Antioxidants protect cells from potentially damaging free radicals, which may contribute to the development of chronic diseases. In addition to antioxidant roles, there are suggested roles in cell proliferation, growth, and differentiation, as well as enhancement of cell-mediated immune functions.
Large doses of vitamin A taken even for a short period of time may result in acute hypervitaminosis A. In adults, chronic oral intakes of retinol in amounts exceeding RDA only 3 – 4 grams can also result in hypervitaminosis A. Symptoms of hypervitaminosis A include nausea, vomiting, double vision, headache, dizziness, and general desquamation of the skin. Excess retinol intake by pregnant women has been linked to increased risk of birth defects. Also excess intake interfere with vitamin K absorption. Tolerable upper levels (Uls) for retinol for adult males and females are 3,000 mcg or 10,000 IU. No tolerable upper intake level has been established of carotenoids. Supplementation, however is not advised, but a consumption of at least five servings of fruits and vegetables will meet the requirements for carotenoids and is highly recommended. Hypercarotenosis may occur upon ingestion of 30 mg or more of beta-carotene daily, what can cause a yellow discoloration of the skin. The condition usually disappears after carotenoids are removed from the diet. High levels of beta-caratone can also decrease plasma concentration of vitamin E. Natural sources of performed vitamin A include only foods of animal origin, such us liver, dairy products, tuna, sardines, and herring. While carotenoids are present in foods of plant origin, including in general yellow, orange, and red fruits and vegetables. Green vegetables also contain some carotenoids, however the pigment cannot be seen because it is masked by chlorophyll. RDA for vitamin A for adult males is 900 mcg/day, and for females 700 mcg/day.