Vitamin A is a fat-soluble vitamin. Retinol is one of the most active, or usable, forms of vitamin A, and is found in animal foods such as liver, whole eggs, whole milk and in some fortified food products. Retinol is often called preformed vitamin A. It can be converted to retinal and retinoic acid, other active forms of the vitamin A family. Some plant foods contain darkly colored pigments called provitamin A carotenoids that can be converted to vitamin A. In the U.S., approximately 30% of vitamin A consumed is provided by provitamin A carotenoids. Beta-carotene is a provitamin A carotenoid that is more efficiently converted to retinol than other carotenoids.

For example, alpha-carotene and b-cryptoxanthin are also converted to vitamin A, but only half as efficiently as beta-carotene. Lycopene, lutein, and zeaxanthin are other carotenoids commonly found in food. They are not sources of vitamin A but have other health promoting properties.

Vitamin A is essential for innumerable biological functions - growth, night and color vision, healthy skin, immune response, reproductive and adrenal function, bone and cartilage development, epithelial maintenance, cell membrane stability, and mucous secretions.[1}

Unfortunately, many people consider vitamin A to be a "toxic vitamin" or one of little value because its potential dangers have been exaggerated. Toxicity is related to dosage and form -- everything is harmful at some level. Water-soluble vitamin A does not bioaccumulate in the liver as the typical fat-soluble form does. Very high doses (above 100,000 IU per day) of the water-soluble form are routinely used by many alternative doctors for a variety of conditions with a much greater degree of safety.

There is no question that, when taken in excess, fat-soluble vitamins can accumulate in tissues and cause symptoms of toxicity. A 1995 study reported that women who take more than 10,000 IU of preformed vitamin A daily during the first three months of pregnancy run an increased risk of having a child with birth defects.[2] The study neglects to point out, however, that a vitamin A deficiency also puts an unborn child at risk and that vitamin E may prevent vitamin A from oxidizing and becoming toxic.

Although women of child-bearing age must be extremely cautious (consult a doctor before taking over 10,000 IU of either form), in other circumstances vitamin A can have profound therapeutic effects. It fights infection and supports the immune system right along with zinc lozenges, vitamin C and echinacea. The World Health Organization recommends large doses of vitamin A to treat measles -- a leading killer in developing countries. The late nutrition pioneer, Carlton Fredericks, Ph.D., recommended single doses of vitamin A as high as 200,000 IU for three to five days to stop the onset of a cold. Since vitamin A is important for the body's first line of defense, the mucous membrane, it may well be the anti-infective vitamin of choice. Therapeutic doses also help treat glaucoma and conjunctivitis (an inflammation of the mucous membrane that lines the eyelids). In this instance, topical, sterile vitamin A is used.

Healthy adults produce vitamin A from vegetable carotenes, but many people, particularly infants, diabetics and individuals with poor thyroid function, do so less efficiently. Vitamin supplements can be an important source of vitamin A for such people, yet many vegetarian and prenatal products contain only beta carotene, which appears to convert to vitamin A at a fixed rate. Preformed vitamin A delivers the desired dosage more reliably; it is found naturally in foods such as yellow and dark-green vegetables, fruits, dairy foods, eggs and cod liver oil.

Toxicity symptoms include chapped lips, dry skin, headache, fatigue, emotion swings and muscle or joint pain.

References
[1] Dunne, L. Nutrition Almanac, 12-13, McGraw-Hill, NY, 1990
[2] Rothman, K. "Teratogenicity of high vitamin A intake," New England Journal of Medicine 333: pp.1369-73, Nov. 23, 1995