Vitamin A promotes healthy surface linings of the eyes and the respiratory, urinary, and intestinal tracts . When those linings break down, it becomes easier for bacteria to enter the body and cause infection. Vitamin A also helps maintain the integrity of skin and mucous membranes, which also function as a barrier to bacteria and viruses.
Retinol is one of the most active, or usable, forms of vitamin A, and is found in animal foods such as liver and whole milk and in some fortified food products. Retinol is also called preformed vitamin A. It can be converted to retinal and retinoic acid, other active forms of the vitamin A family.
Provitamin A carotenoids are darkly colored pigments found in plant foods that can be converted to vitamin A. In the United States, approximately 26% and 34% of vitamin A consumed by men and women, respectively, is provided by provitamin A carotenoids. Common carotenoids found in foods are beta-carotene, alpha-carotene, lutein, zeaxanthin, lycopene, and cryptoxanthin. Of the 563 identified carotenoids, fewer than 10% are precursors for vitamin A. Among these, beta-carotene is most efficiently converted to retinol. Alpha-carotene and beta-cryptoxanthin are also converted to vitamin A, but only half as efficiently as beta-carotene. Lycopene, lutein, and zeaxanthin are carotenoids that do not have vitamin A activity but have other health promoting properties. The Institute of Medicine (IOM) encourages consumption of carotenoid-rich fruits and vegetables for their health-promoting benefits.
Some carotenoids, in addition to serving as sources of vitamin A, have been shown to function as antioxidants in laboratory tests. However, this role has not been consistently demonstrated in humans . Antioxidants protect cells from free radicals, which are potentially damaging by-products of oxygen metabolism that may contribute to the development of some chronic diseases.
I have decided to start an active promotion of this website, so that we get more visitors soon. And it would be a waste of time if we didn't allow for discussion and sharing of informations. In light of that, discussion forum will be officially launched on June 21st.
I hope it will serve a positive function in opening alternative horizons in the area of general information about vitamins.