Vitamin A can be confusing because it is not just one vitamin. Rather, it's a generic term that includes many different compounds known by a variety of names. A fat-soluble vitamin that's essential for vision and other critical functions, it's found in eggs and dairy products. The healthiest way to get enough, however, is by eating a variety of yellow, orange and dark-green leafy vegetables.
Vitamin A refers to a large group of compounds that are placed into two categories called retinoids and provitamin A carotenoids. The retinoids come from animal sources of food and include retinol, as well as the two substances that retinol can be turned into: retinal and retinoic acid. When reference is made to "vitamin A," it means retinol because that's the form that's useable by the body. Carotenoids are substances found in plants that can be turned into vitamin A after they're digested. There are about 563 known carotenoids and less than 10 percent can be turned into vitamin A, according to the Office of Dietary Supplements. Alpha-carotene and beta-carotene are two well-known carotenoids that convert to vitamin A. Lycopene, lutein and zeaxanthin are examples of carotenoids that have great health benefits, but that are not changed into vitamin A.
Roles
Vitamin A fills a variety of important roles, but it is best known as being essential for vision. It supports your immune system and helps resist infection by making white blood cells that fight bacteria. Vitamin A is needed for healthy skin and to maintain cells lining the eyes and the respiratory and intestinal tracts. Bones continually replace worn out cells by building new bone and that process depends on the presence of vitamin A. Development of the limbs, heart, eyes and ears rely on the presence of vitamin A.
Food Sources
The top sources of vitamin A are beef liver, chicken liver and chicken and turkey giblets. Other animal sources include milk, eggs and cheese. For the next best sources, think in terms of yellow and orange vegetables or fruits. Good choices include carrots, pumpkin, sweet potatoes, winter squash and cantaloupe. Even though they're not brightly colored, dark green leafy vegetables such as collards, spinach and kale, are also high in vitamin A. Ready-to-eat cereals are fortified with vitamin A.
Recommended Intake
The recommended dietary allowance is 900 mcg/day, or 3,000 IU, for men. Women should have 700 mcg, or 2,333, each day, according to the Linus Pauling Institute. Vitamin A can become toxic, so the upper tolerable intake amount is 3,000 mcg/day, or 10,000 IU. However, toxicity is only caused by too much retinol because it is absorbed quickly and takes a long time for excess to be excreted from the body. High levels of vitamin A can cause birth defects, liver problems, central nervous system disorders and osteoporosis. Carotenoids do not become toxic and have not been associated with health concerns, making beta-carotene a better choice for supplements.
Member Comments