Vitamin A is a group of unsaturated nutritional organic compounds that includes retinol, retinal, retinoic acid, and several provitamin A carotenoids (most notably beta-carotene). Vitamin A has multiple functions: it is important for growth and development, for the maintenance of the immune system and good vision. Vitamin A is needed by the retina of the eye in the form of retinal, which combines with protein opsin to form rhodopsin, the light-absorbing molecule necessary for both low-light (scotopic vision) and color vision. Vitamin A also functions in a very different role as retinoic acid (an irreversibly oxidized form of retinol), which is an important hormone-like growth factor for epithelial and other cells.
In foods of animal origin, the major form of vitamin A is an ester, primarily retinyl palmitate, which is converted to retinol (chemically an alcohol) in the small intestine. The retinol form functions as a storage form of the vitamin and can be converted to and from its visually active aldehyde form, retinal.
All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group. Both structural features are essential for vitamin activity. The orange pigment of carrots (beta-carotene) can be represented as two connected retinyl groups, which are used in the body to contribute to vitamin A levels. Alpha-carotene and gamma-carotene also have a single retinyl group, which give them some vitamin activity. None of the other carotenes have vitamin activity. The carotenoid beta-cryptoxanthin possesses an ionone group and has vitamin activity in humans.
Vitamin A can be found in two principal forms in foods:
• Retinol, the form of vitamin A absorbed when eating animal food sources, is a yellow, fat-soluble substance. Since the pure alcohol form is unstable, the vitamin is found in tissues in a form of retinyl ester. It is also commercially produced and administered as esters such as retinyl acetate or palmitate.
• The carotenes alpha-carotene, beta-carotene, gamma-carotene; and the xanthophyll beta-cryptoxanthin (all of which contain beta-ionone rings), but no other carotenoids, function as provitamin A in herbivores and omnivore animals, which possess the enzyme beta-carotene 15,15′-dioxygenase which cleaves beta-carotene in the intestinal mucosa and converts it to retinol.
Vitamin A deficiency is estimated to affect approximately one third of children under the age of five around the world. It is estimated to claim the lives of 670,000 children under five annually. Approximately 250,000–500,000 children in developing countries become blind each year owing to vitamin A deficiency, with the highest prevalence in Southeast Asia and Africa. Vitamin A deficiency is “the leading cause of preventable childhood blindness,” according to UNICEF. It also increases the risk of death from common childhood conditions such as diarrhea. UNICEF regards addressing vitamin A deficiency as critical to reducing child mortality, the fourth of the United Nations’ Millennium Development Goals.
A meta-analysis of 43 studies showed that vitamin A supplementation of children under five who are at risk of deficiency reduced mortality by up to 24%.
The World Health Organization estimated that vitamin A supplementation averted 1.25 million deaths due to vitamin A deficiency in 40 countries since 1998. In 2008, it was estimated that an annual investment of US$60 million in vitamin A and zinc supplementation combined would yield benefits of more than US$1 billion per year, with every dollar spent generating benefits of more than US$17.
Low blood vitamin A levels have been associated with rapid HIV infection and deaths.
Excessive vitamin A consumption can lead to nausea, irritability, anorexia (reduced appetite), vomiting, blurry vision, headaches, hair loss, muscle and abdominal pain and weakness, drowsiness, and altered mental status. In chronic cases, hair loss, dry skin, drying of the mucous membranes, fever, insomnia, fatigue, weight loss, bone fractures, anemia, and diarrhea can all be evident on top of the symptoms associated with less serious toxicity.
The role of vitamin A in the visual cycle is specifically related to the retinal form. Within the eye, 11-cis-retinal is bound to the protein “opsin” to form rhodopsin in rods and iodopsin (cones) at conserved lysine residues.
Vitamin A, in the retinoic acid form, plays an important role in gene transcription. Once retinol has been taken up by a cell, it can be oxidized to retinal (retinaldehyde) by retinol dehydrogenases and then retinaldehyde can be oxidized to retinoic acid by retinaldehyde dehydrogenases.
Vitamin A plays a role in many areas of the immune system, particularly in T cell differentiation and proliferation.
Vitamin A promotes the proliferation of T cells through an indirect mechanism involving an increase in IL-2 In addition to promoting proliferation, Vitamin A, specifically retinoic acid, influences the differentiation of T cells.
Vitamin A, and more specifically, retinoic acid, appears to maintain normal skin health by switching on genes and differentiating keratinocytes (immature skin cells) into mature epidermal cells.
Vitamin A is key for good vision, a healthy immune system, and cell growth. There are two types of vitamin A. In this case our cover is primarily about the active form of vitamin A — retinoids — that comes from animal products. Beta-carotene is among the second type of vitamin A, which comes from plants.
The American Heart Association recommends obtaining antioxidants, including beta-carotene, by eating a well-balanced diet high in fruits, vegetables, and whole grains rather than from supplements until more is known about the risks and benefits of supplementation.
High doses of antioxidants (including vitamin A) may actually do more harm than good. Vitamin A supplementation alone, or in combination with other antioxidants, is associated with an increased risk of mortality from all causes, according to an analysis of multiple studies.
Topical and oral retinoids are common prescription treatments for acne and other skin conditions, including wrinkles. Oral vitamin A is also used as a treatment for measles and dry eye in people with low levels of vitamin A. Vitamin A is also used for a specific type of leukemia.
Vitamin A has been studied as a treatment for many other conditions, including cancers, cataracts, and HIV. However, the results are inconclusive.
The amount of vitamin A you need depends on your age and reproductive status. Recommended intakes for vitamin A for people aged 14 years and older range between 700 and 900 micrograms (mcg) of retinol activity equivalents (RAE) per day. Recommended intakes for women who are nursing range between 1,200 and 1,300 RAE. Lower values are recommended for infants and children younger than 14.
However, the vitamin A content of foods and dietary supplements is given on product labels in international units (IU), not mcg RAE. Converting between IU and mcg RAE is not easy. A varied diet with 900 mcg RAE of vitamin A, for example, provides between 3,000 and 36,000 IU of vitamin A depending on the foods consumed. See our Health Professional Fact Sheet on Vitamin A for more details.
For adults and children aged 4 years and older, the U.S. Food and Drug Administration has established a vitamin A Daily Value (DV) of 5,000 IU from a varied diet of both plant and animal foods. DVs are not recommended intakes; they don’t vary by age and for men and women, for example. But trying to reach 100% of the DV each day, on average, is useful to help you get enough vitamin A.
High intakes of some forms of vitamin A can be harmful.
Getting too much-preformed vitamin A (usually from supplements or certain medicines) can cause dizziness, nausea, headaches, coma, and even death. High intakes of preformed vitamin A in pregnant women can also cause birth defects in their babies. Women who might be pregnant should not take high doses of vitamin A supplements.
Consuming high amounts of beta-carotene or other forms of provitamin A can turn the skin yellow-orange, but this condition is harmless. High intakes of beta-carotene do not cause birth defects or the other more serious effects caused by getting too much-preformed vitamin A.
Vitamin A supplements can interact or interfere with the medicines you take. Here are several examples:
Orlistat (Alli®, Xenical®), a weight-loss drug, can decrease the absorption of vitamin A, causing low blood levels in some people.
Several synthetic forms of vitamin A are used in prescription medicines. Examples are the psoriasis treatment acitretin (Soriatane®) and bexarotene (Targretin®), used to treat the skin effects of T-cell lymphoma. Taking these medicines in combination with a vitamin A supplement can cause dangerously high levels of vitamin A in the blood.
Tell your doctor, pharmacist, and other healthcare providers about any dietary supplements and medicines you take. They can tell you if those dietary supplements might interact or interfere with your prescription or over-the-counter medicines or if the medicines might interfere with how your body absorbs, uses, or breaks down nutrients.
Vitamin A provides a wide range of nutritional support for the entire body, including vision, growth and bone development, reproductive function, cell growth, immune function, and the integrity of mucosal and epithelial surfaces.
Immune health: Vitamin A is also an essential nutrient for the body’s natural defense systems.
Eye health: Vitamin A is a fat-soluble vitamin required for healthy vision.
Vitamin A taken daily may help to maintain healthy eyesight and enhance immune system function for overall well-being.
Vitamin A is essential for maintaining a healthy reproductive system in both men and women.