Vitamin A
Vitamin A is a family of fat-soluble compounds that play an important role in vision, bone growth, reproduction, cell division, and cell differentiation (in which a cell becomes part of the brain, muscle, lungs, etc.). Vitamin A helps regulate the immune system, which helps prevent or fight off infections by making white blood cells that destroy harmful bacteria and viruses. Vitamin A also may help lymphocytes, a type of white blood cell, fight infections more effectively.
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.
