• Description
• Functions
• Dietary sources
• Required Intakes
• Cautions

Description

Vitamin A is a nutritional term that describes a family of essential compounds that are structurally related. A family includes retinol, retinal or retinaldehyde, retinyl esters, retinoic acid, and provitamin carotenoids such as -carotene. Vitamin A is a fat-soluble vitamin. Vitamin A is essential for growth and life, taking part not only in vision but also in developmental processes that begin early in embryogenesis. Vitamin A continues to be necessary to maintain normal cellular differentiation throughout life. Preformed vitamin A comes from animal origin, such as liver and egg yolk, while the provitamin -carotene is found only in foods of plant origin, particularly carrots and dark leafy vegetables. Vitamin A requires some fat present in the digestive system for absorption and is better absorbed than the carotenoids. Protein enhances the conversion of carotenoids to retinol. The human body stores only limited amounts of vitamin A, mostly in the liver, making dietary intake essential. The liver stores 50 - 80% of body vitamin A. Small amounts are also found in lungs, body fat and kidneys. Unlike most vitamins, vitamin A concentrations tend to increase with age. Excess and deficiency of vitamin A in the mother's diet during pregnancy has been shown to cause malformations of fetal brain and hydrocephalus.

Functions

Vitamin A in its various forms is required for:

• Night vision, due to its involvement in photochemical reactions in the retina.
• Growth and maintenance of epithelial tissue including the cornea, all mucous membranes of gastrointestinal tract, lungs, vagina, urinary tract, bladder and skin.
• Reproductive function in humans.
• Nerve formation and function.
• Epithelial cell proliferation and epidermal differentiation.  Natural and synthetic retinoids have been used increasingly as systemic or topical agents in the treatment of hyperkeratotic disorders, acne, and certain skin cancers.
• Antioxidants may play a preventive role in heart disease and epithelial cancers.
• Both cell-mediated and antibody-mediated immune response.
• Signal transduction, via retinoic acid's hormonal action.

Symptoms of vitamin A deficiency are due to its participation in skin, bone, dental health and immune function. The earliest symptom of vitamin A deficiency is inability to see in dim light, called night blindness or nyctalopia. Other symptoms soon appear including rough, scaly skin (called follicular hyperkeratosis), sinus infection, chronic sore throat and abscesses in mouth and ears. In children, deficiency results in growth retardation, impaired bone and tooth formation. Both deficiency and excess vitamin A cause fetal malformations. Vitamin A deficiency causes spermatogenesis cessation and interrupts estrus cycle.

Symptoms of vitamin A toxicity include loss of appetite, headache, blurred vision, irritability, hair loss, drying and flaking of the skin, swelling in the extremities, drowsiness, diarrhea, nausea, and enlargement of the spleen and liver. Since vitamin A stores increase with age, the elderly are at particular risk for toxicity. Vitamin A excess during the first trimester of pregnancy can result in severe craniofacial and oral clefts and limb defects of the fetus. High doses of vitamin A (retinol and retinyl esters) during pregnancy have been associated with birth defects. It is recommended that women who are pregnant or may become pregnant do not exceed the tolerable Upper Limits.

Dietary sources

Rich sources of vitamin A are liver and cod liver oil.  The major source of vitamin A in the diet is from beta-carotene; provitamin A.  Sources of beta-carotene include yellow and green leafy vegetables such as carrots, spinach, sweet potatoes, squash, and yellow fruits such as peaches and cantaloupe.

Required Intakes

Vitamin A dietary reference intakes (DRIs) are expressed in IU (international units), RAE (retinol activity equivalents) or mcg (micrograms). One RAE is equivalent to 1 mcg of retinol, and one RAE is also equivalent to 3.33 IU of vitamin A activity from retinol. A generally recognized safe upper limit of intake for vitamin A is 8000 to 10,000 IU/day (2400 mcg and 3000 mcg respectively).

Cautions

Make sure to take vitamin A supplements with food; some fat in the diet will enhance absorption. Both vitamin E and zinc aid the body in using vitamin A. In turn, vitamin A facilitates the absorption of iron from foods. A good daily multiple vitamin/mineral will provide the necessary amounts. Don't take vitamin A with isotretinoin or other acne drugs. Together, they may cause high blood levels of vitamin A, which can lead to unwanted side effects. Don't exceed recommended doses of vitamin A. Large doses of preformed vitamin A can build up to toxic levels. If you're pregnant or trying to get pregnant, don't take more than 5,000 IU of vitamin A daily. Higher amounts may result in birth defects. Practice birth control if consuming doses greater than 5,000 IU, and for a month after stopping.