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"Vitamin A" is the blanket term for retinoids, biologically active compounds that occur naturally in both plant and animal tissues.

The vitamin A that comes from animal sources is fat soluble, and in the form of retinoic acid, retinal and retinol. Because these retinoids are very bioavailable and stored in our tissues, too much animal-derived vitamin A can build up in our systems and become toxic.

The vitamin A in fruits and vegetables is in the form of vitamin A precursors or carotenoids, which must be converted by the human body into usable retinoids. There are several distinct classes of carotenoids, and evidence suggests that carotenoids work together to provide their health benefits. Taken together in supplement form, these are commonly called "mixed carotenoids." The body can make all the vitamin A it needs from these plant-derived carotenoids. They are water-soluble and do not accumulate in the body, so toxicity is rare.

More than 500 carotenoids have been identified; fewer than 10 percent can be made into vitamin A in the body. Still, many that cannot be converted to vitamin A have healthful effects - lycopene, for example, has been shown to help prevent prostate cancer.

Why is vitamin A necessary?
Knowing the facts about vitamin A is important, as it plays a vital role in bone growth, reproduction and immune system health. It also helps the skin and mucous membranes repel bacteria and viruses more effectively. It is essential to healthy vision, and may slow declining retinal function in people with retinitis pigmentosa.

What are the signs of a deficiency?
Vitamin A deficiency is common in developing countries, but is quite rare in the United States. One of the earliest signs of a deficiency is night blindness. Permanent blindness can result if the deficiency is left unchecked. Vitamin A deficiency also allows opportunistic infectious diseases such as measles and pneumonia to become deadly. Alcoholics may develop vitamin A deficiencies, and should consequently include rich food sources of vitamin A in their diets (while concurrently sharply curtailing or eliminating alcohol consumption). Supplements may not be wise for alcoholics, however, because vitamin A is stored in the liver, and existing liver damage could make them more susceptible to vitamin A toxicity. In such cases, a doctor's supervision is critical.

How much, and what kind, does an adult need?
Dr. Talebi suggests 15,000 IU of mixed carotenoids daily. The antioxidant formula he recommends contains beta-carotene, alpha-carotene, lycopene, astaxanthin, lutein and zeaxanthin.