Facts
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.Beta-Carotene Facts
Beta-carotene is one of almost 600 carotenoids. Fifty of these compounds have vitamin A activity. Beta-carotene may provide as much as two-thirds of the vitamin A in the human diet. Carotenoids are deposited more widely throughout the body than retinoids and are found in adipose tissues, adrenals and liver.
Six carotenoids found in the highest concentrations in human serum are beta-carotene, alpha-carotene, beta-cryptoxanthin, lycopene, lutein, and zeaxanthin.
Certain carotenoids, such as beta-carotene, alpha-carotene and beta-cryptoxanthin, are dietary precursors of vitamin A, called provitamin A.
Provitamin beta-carotene is found only in foods of plant origin, particularly carrots and dark leafy vegetables.
Beta-carotene becomes an essential nutrient when dietary intake of retinol is inadequate.

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.Beta-carotene functions
Beta-carotene functions as an antioxidant, regulator of cell communication and growth, and has shown immunomodulatory activities thought to be independent of its role as a pro-vitamin A compound.
As an antioxidant, beta-carotene may quench certain free radicals and inhibit lipid peroxidation.
Beta-carotene may exert anti-carcinogenic activity.

Deficiency signs and symptoms
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.

Toxicity
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.Beta-Carotene toxicity
Chronic doses of 30 mg/day or higher of beta-carotene may cause carotenodermia. Carotenodermia is characterized by yellowish discolorization of the skin and is considered harmless and reversible with the discontinuation of beta-carotene supplementation.
Supplementation of 20 mg/day or greater of beta-carotene has been linked to increased lung cancer incidence in smokers. Smokers should avoid >15 mg/day beta-carotene supplementation pending further research.

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.