Understanding Vitamin A

Vitamin A is the moisturizing nutrient that keeps your skin and mucous membranes (the slick tissue that lines the eyes, nose, mouth, throat, vagina, and rectum) smooth and supple. Vitamin A is also the vision vitamin, a constituent of 11-cis retinol, a protein in the rods (cells in the back of your eye that enable you to see even when the lights are low) that prevents or slows the development of age-related macular degeneration, or progressive damage to the retina of the eye, which can cause the loss of central vision (the ability to see clearly enough to read or do fine work). Finally, vitamin A promotes the growth of healthy bones and teeth, keeps your reproductive system humming, and encourages your immune system to churn out the cells you need to fight off infection.
Two chemicals provide vitamin A: retinoids and carotenoids. Retinoids are compounds whose names all start with ret: retinol, retinaldehyde, retinoic acid, and so on. These fat-soluble substances are found in several foods of animal origin: liver (again!) and whole milk, eggs, and butter. Retinoids give you preformed vitamin A, the kind of nutrient your body can use right away. The second form of vitamin A is the vitamin A precursor, a chemical such as beta-carotene, a deep yellow carotenoid (pigment) found in dark green and bright yellow fruits and vegetables. Your body transforms a vitamin A precursor into a retinol-like substance. So far, scientists have identified at least 500 different carotenoids. Only 1 in 10 — about 50 altogether — are considered, like beta-carotene, to be sources of vitamin A.
Traditionally, the recommended dietary allowances of vitamin A are measured in International Units (IU). However, because retinol is the most efficient source of vitamin A, the modern way to measure the RDA for vitamin A is as retinol equivalents, abbreviated as RE. One microgram (mcg) RE = 3.3 IU. However, many vitamin products still list the RDA for vitamin A in IUs.