Vitamin A

Vitamin A

Vitamin A consists of two basic dietary forms: preformed vitamin A, also called retinol, which is the active form of vitamin A; and carotenoids, the inactiveform of vitamin A, which are found in plants.

Functions.Vitamin A is a family of fat-soluble compounds that play an impor-tant role in vision, bone growth, reproduction, and cell division. Vitamin A helps regulate the immune system, which helps fight infections. Vitamin A has been labeled as an antioxidant when, in fact, provitamin A (carotenoids) is the part of the family that functions as an antioxidant. Antioxidants protect cells from freeradicals. Free radicals are atoms or groups of atoms with an odd (unpaired) num-ber of electrons and can be formed when oxygen interacts with certain molecules. Once formed, these highly reactive radicals can start a chain reaction. When they react with important cellular components such as DNA or cell membranes, the most damage occurs. Antioxidants have the capability of safely interacting with free radicals and stopping the chain reaction before vital cells are damaged.

The first organic free radical was discovered in 1900 by Moses Gomberg. In the 1950s, Denman Harman, M.D., was the first to propose the free radical theory of aging.

Sources.There are two forms of vitamin A: preformed vitamin A and pro-vitamin A. Retinol is a preformed vitamin A and is one of the most active and usable forms of vitamin A. Retinol can be converted to retinal and retinoic acid, other active forms of vitamin A.

Provitamin A carotenoids can be converted to vitamin A from darkly colored pigments, both green and orange, in fruits and vegetables. Common car-otenoids are beta-carotene, lutein, lycopene, and zeaxanthin. Beta-carotene is most efficiently converted to retinol. Eating “five-a-day” of fruits and vegetables is highly recommended. The best sources of beta-carotene are carrots, sweet pota-toes, spinach, broccoli, pumpkin, squash (butternut), mango, and cantaloupe.

Research has shown that regular consumption of foods rich in carotenoids decreases the risk of some cancers because of its antioxidant effect. Taking a beta-carotene supplement has not shown the same results.

Preformed vitamin A (retinol) is found in fat-containing animal foods such as liver, butter, cream, whole milk, whole-milk cheeses, and egg yolk. It is also found in low-fat milk products and in cereals that have been fortified with vitamin A, but these are not the best sources.

Requirements.A well-balanced diet is the preferred way to obtain therequired amounts of vitamin A. Vitamin A values are commonly listed as a retinolequivalent (RE). A retinol equivalent is 1 μg retinol or 6μg beta carotene. Referto the inside back cover of this text for the dietary reference intakes of vitamin A as prescribed by the Food and Nutrition Board of the Institute of Medicine.

Hypervitaminosis.The use of a single vitamin supplement should be dis-couraged because an excess of vitamin A can have serious consequences. Signs of hypervitaminosis A may include birth defects, hair loss, dry skin, headaches, nausea, dryness of mucous membranes, liver damage, and bone and joint pain. In general, these symptoms tend to disappear when excessive intake is discontinued.

Deficiency.Signs of a deficiency of vitamin A include night blindness;dry, rough skin; and increased susceptibility to infections. Avitaminosis A can result in blindness or xerophthalmia, a condition characterized by dry, luster-less, mucous membranes of the eye. Lack of vitamin A is the leading cause of blindness in the world (discounting accidents).