Sources: The two forms of vitamin A come from different food sources. Preformed A (retinol) is the main animal-source vitamin A. It is found in highest concentrations in all kinds of liver and fish liver oil, which is a common source for supplements. Egg yolks and milk products, such as whole milk, cream, and butter, are also good sources of vitamin A.
Provitamin A, mainly in the form of beta-carotene, is found in a wide variety of yellow- and orange-colored fruits and vegetables, as well as leafy green vegetables. Beta-carotene is actually a double molecule of vitamin A. It may be converted to vitamin A in the upper intestine before absorption; beta-carotene can also be converted to vitamin A in the liver. People with diabetes, low thyroid activity, and those who use a lot of polyunsaturated fatty acids (PUFA) without antioxidants such as vitamin E have lowered ability to convert beta-carotene to A. Assimilation of vitamin A and the carotenes is helped by the presence of bile salts and fatty acids in the intestine.
Functions: Vitamin A performs many important functions in the human body. The following are the most common.
Eyesight. Vitamin A is needed for the formation of rhodopsin, or visual purple, which allows us to see at night. When vitamin A is lacking, there is a lag period in regenerating visual purple and a resulting inability to see well at night, termed "night blindness." Vitamin A also helps maintain the health of the cornea, the eye covering. Vitamin A deficiency may allow irritation or inflammation of the eye tissue to occur more easily.
Growth and tissue healing. This vitamin is involved in laying down new bone during growth and promoting healthy teeth. After tissue injury or surgery, vitamin A is needed for repair of the tissues and to help protect the tissues from infection.
Healthy skin. Vitamin A stimulates growth of the base layer of the skin cells. It helps the cells differentiate normally (progress from less to more mature cell forms) and gives them their structural integrity. It does this for both the external skin cells and the body’s inner skin, the mucous membrane linings of the nose, eyes, intestinal tract, respiratory lining, and bladder. By this function, it also helps protect these areas from cancer cell development.
By moisturizing the mucous lining cells, which helps proper secretion, and by maintaining their structural integrity, vitamin A helps the body fight off infectious agents and environmental pollutants. The epithelial tissues protected include the linings of the lungs, nose, throat, stomach, intestines, vagina, bladder, and urinary tract, as well as the eyes and skin.
Antioxidation. Vitamin A helps protect the body (particularly cell membranes and tissue linings) from the irritating effects of free radicals (unstable molecules) by neutralizing them. Beta-carotene also protects the tissues from the toxic singlet oxygen radical. This function usually requires an amount higher than the RDA dose, perhaps 10,000–20,000 IUs per day or more. Through its antioxidant effects, vitamin A and beta-carotene help protect the body from the irritating effects of smoke and other pollutants and may also be helpful in preventing problems like ulcers, atherosclerosis and its attendant complications, such as high blood pressure and stroke.
Lowering cancer risk. As discussed previously, vitamin A helps maintain the structural integrity of cells and the healthy functioning of the mucous linings. It also helps with proper cell differentiation of the surface cells. Recently, beta-carotene was shown to improve immune response by stimulating T-helper cell activity. In these ways, it seems to prevent the development of cancer, and a number of studies have shown reduced lung and colon cancer rates in people with higher intakes of beta-carotene.