see also Beta-Carotene
Vitamin A is a fat-soluble nutrient involved in a wide variety of bodily functions. In its pure, isolated form, vitamin A appears as a yellow, crystalline substance known as retinol or preformed vitamin A. The body readily converts retinol into the active forms retinal (or retinaldehyde) and retinoic acid. Beta-carotene, a pigment found in green, orange, and yellow vegetables, can be metabolized in the body to yield retinol and is the best nonanimal source of the vitamin. For this reason, beta-carotene is often described as a provitamin A carotenoid.
Vitamin A is known to play an essential role in eye function. Vitamin A is also required for normal growth and development of the body, hormone production, immune system function, and for maintaining healthy skin. It is also important for maintaining the integrity of the mucous membranes in the respiratory, urinary, and digestive tracts.
Vitamin A is a key component of rhodopsin, a pigment needed by the eye to initiate biochemical responses to visual stimuli. Low serum vitamin A levels have been associated with night blindness, and numerous studies have shown night blindness to improve with vitamin A supplementation.
Vitamin A has traditionally been measured in international units (IU), but can also be measured in micrograms (mcg) or retinol equivalents (RE). Although optimum intake of vitamin A may vary with age and gender, 5,000 IU per day is considered adequate for most people. Because vitamin A is a fat-soluble compound that is stored in the liver, long-term, high-dose supplementation (over 50,000 IU per day for adults, over 25,000 IU per day for children) can result in toxic accumulations. For adult men and post-menopausal women, up to 25,000 IU per day is generally considered safe. Because high retinol intake has been associated with increased risk for birth defects, women who are or may become pregnant should not consume more than 10,000 IU per day. Beta-carotene may be a safer source of vitamin A for these women because it is not associated with birth-defect risk.