Vitamin A (a.k.a. retinol, or preformed retinol) plays a varied role in the human body. This vitamin assists in keeping the immune system working properly, helps to ensure the healthy formation and maintenance of body tissue, particularly skin and hair, but also bone, is involved in the maintenance of tissue lining in the respiratory, gastrointestinal and urogenital tracts (i.e., lungs, intestine and urinary tracts), and of course, vitamin A is a requisite compound in the maintenance of vision, particularly low-light (night) vision.
      There is also evidence, the degree of which is controversial, that certain precursor forms of vitamin A, beta carotene in particular, have an antioxidant effect in human tissue and may help in the prevention of a number of diseases.

      Vitamin A is somewhat unusual in that it is not measured by quantity, but on the basis of how much of a particular food behaves as vitamin A once it has been metabolized in the body. The standard unit of measurement is the retinol equivalent (RE), that is, how much of a particular food is equivalent to pure vitamin A (retinol) after consumption.
      An important point to keep in mind is that numerous dietary and supplemental sources of vitamin A are in the form of beta carotene and other carotenoids, which are not equivalent to pure vitamin A.
      Why do food labels still use the older International Unit (IU) as a standard reference? In the U.S., food manufacturers are required by law to label their products according to guidelines established in the government's Title 21 of the Code of Federal Regulations, a document that is decades old. CFR Title 21 says the unit of measurement for vitamin A shall be the IU.
      The retinol equivalent (RE) and microgram (mcg) are equivalent units of measurement, so the figures from the recent National Academy of Sciences (NAS) Dietary Reference Intakes study and the previous NAS RDAs (1989) are comparative.

      Vitamin A (retinol) is found primarily in dairy products (whole milk, eggs, cheese, and butter), chicken, liver (especially pork, lamb, chicken, turkey or beef), and in fish oils. Additionally, a large number of food products are commonly fortified with vitamin A, such as milk products, cereals, soups and many other packaged foods.
      Vitamin A is also manufactured in the human body from its precursor form, compounds known as carotenoids. As a group, there are some 600 known variants of carotenoids, only six of which commonly occur in normal diets (in North America): alpha-carotene, beta-carotene, lycopene, lutein, zeaxanthin, and beta-cryptoxanthin. Only three of these are actually converted into vitamin A: alpha-carotene, beta-carotene, and beta-cryptoxanthin. These are collectively referred to as provitamin A carotenoids, and are found in various fruits and vegetables such as cantaloupe, broccoli, spinach and other leafy green vegetables (primarily as beta-carotene), carrots, tomatoes and tomato products, and spinach or collard greens (contain other carotenoids as well).

Vitamin A deficiency is relatively unknown in the developed world, primarily because of the prevalence of the provitamin A carotenoids in normal diets, and the addition of these compounds to many foods in which they do not normally occur. Due to the fat-solubility of this compound, tissue reserves are normally high and prolonged dietary deprivation would be required to induce deficiency. Such deficiency, however, can lead to poor night vision (ultimately night blindness), as well as xerophthalmia (abnormal dryness and thickening of the cornea), depression of immune system reactions to infections, impaired bone growth, and hyperkeratosis (a plugging of hair follicles with keratin).

      The form in which supplements are ingested is significant. Excessive amounts of vitamin A (as retinol) can be toxic (remember, this vitamin is fat-soluble and it is normally retained in the body for long periods of time). Without specific medical advise to the contrary, vitamin A (as retinol) should not be taken in excessive amounts (see Health Risks).
      Provitamin A (vitamin A as its carotenoid precursor, primarily beta carotene), is the form more commonly encountered in dietary supplements. Not only is this precursor a much less concentrated source of vitamin A (approximately 6 mcg of beta carotene is converted to 1 mcg of retinol), the carotenoid form falls into that category of compounds known as the antioxidants, and may well have additional health benefits, though this remains a somewhat controversial issue.

      As with other fat-soluble vitamins, vitamin A tends to build up in the body, particularly in the liver, and can be toxic in large doses if taken in its natural form (as retinol). Individuals who also consume supplemental fish oil (omega-3, other fatty acids, and cod liver oil, in particular) are at greater risk due to a higher retention of fat soluble vitamins. Levels above 50,000 IU (15,000 RE) of vitamin A (as retinol) per day in children (somewhat higher in adults), have lead to toxicity and hypervitaminosis. Symptoms of this condition include irritability, loss of appetite, headache, blurred vision, nausea and vomiting, diarrhea, abdominal cramps, nose bleeds, joint pain, itching, rashes, and dry skin and hair. Among the potential dangers are serious liver damage, red blood cell damage, alopecia (hair can be easily pulled out), and birth defects.
      Of course, it is obvious that most of these symptoms and health dangers can also be the result of many other disorders unrelated to vitamin A.
      The effects of vitamin A poisoning from excessive carotenoids (beta carotene), on the other hand, are milder by comparison and relatively harmless. The most common symptom is a yellowing of the skin, particularly on the palms of the hands and soles of the feet. By all accounts, Bugs Bunny should be yellow.

      There have been recent studies that have suggested that increased levels of antioxidants (including provitamin A) may decrease the risk of macular degeneration of the eye and cataracts, as well as the risk of some cardiovascular events and some cancers.
      Although the current evidence appears to be consistent, there is much more research needed in this area. It is our intent to update this section over time as more studies are completed and more data become available.