Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is absolutely necessary for both low-light and color vision. Vitamin A also functions in a very different role, as an irreversibly oxidized form of retinol known as retinoic acid, 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 the 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. The associated acid (retinoic acid), a metabolite that can be irreversibly synthesized from vitamin A, has only partial vitamin A activity, and does not function in the retina for the visual cycle.

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:

What is Vitamin A ?
A is a group of compounds that play an important role in vision, bone growth, reproduction, cell division, and cell differentiation (in which a cell becomes part of the brain, muscle, lungs, blood, or other specialized tissue.) Vitamin A helps regulate the immune system, which helps prevent or fight off infections by making white blood cells that destroy harmful bacteria and viruses. Vitamin A also may help lymphocytes (a type of white blood cell) fight infections more effectively.

Vitamin A promotes healthy surface linings of the eyes and the respiratory, urinary, and intestinal tracts .When those linings break down, it becomes easier for bacteria to enter the body and cause infection. Vitamin A also helps the skin and mucous membranes function as a barrier to bacteria and viruses.

In general, there are two categories of vitamin A, depending on whether the food source is an animal or a plant. Vitamin A found in foods that come from animals is called preformed vitamin A. It is absorbed in the form of retinol, one of the most usable (active) forms of vitamin A. Sources include liver, whole milk, and some fortified food products. Retinol can be made into retinal and retinoic acid (other active forms of vitamin A) in the body.

Vitamin A that is found in colorful fruits and vegetables is called provitamin a carotenoid. They can be made into retinol in the body. In the United States, approximately 26% of vitamin A consumed by men and 34% of vitamin A consumed by women is in the form of provitamin A carotenoids .Common provitamin A carotenoids found in foods that come from plants are beta-carotene, alpha-carotene, and beta-cryptoxanthin. Among these, beta-carotene is most efficiently made into retinol. Alpha-carotene and beta-cryptoxanthin are also converted to vitamin A, but only half as efficiently as beta-carotene.
Of the 563 identified carotenoids, fewer than 10% can be made into vitamin A in the body. Lycopene, lutein, and zeaxanthin are carotenoids that do not have vitamin A activity but have other health promoting properties. The Institute of Medicine (IOM) encourages consumption of all Carotenoid-rich fruits and vegetables for their health-promoting benefits.

Some provitamin A carotenoids have been shown to function as antioxidants in laboratory studies; however, this role has not been consistently demonstrated in humans. Antioxidants protect cells from free radicals, which are potentially damaging by-products of oxygen metabolism that may contribute to the development of some chronic diseases.

Where it is found
Vitamin A comes from animal sources, such as eggs, meat, milk, cheese, cream, liver, kidney, cod, and halibut fish oil. However, all of these sources - except for skim milk that has been fortified with Vitamin A - are high in saturated fat and cholesterol.

Sources of beta-carotene are carrots, pumpkin, sweet potatoes, winter squashes, cantaloupe, pink grapefruit, apricots, broccoli, spinach, and most dark green, leafy vegetables. The more intense the color of a fruit or vegetable, the higher the beta-carotene content. These vegetable sources of beta-carotene are free of fat and cholesterol.

Benefits / uses
Vitamin A helps form and maintain healthy teeth, skeletal and soft tissue, mucous membranes, and skin. It is also known as retinol because it produces the pigments in the retina of the eye.

Vitamin A promotes good vision, especially in low light. It may also be needed for reproduction and breast-feeding.

Retinol is an active form of vitamin A. It is found in animal liver, whole milk, and some fortified foods.

Carotenoids are dark colored dyes found in plant foods that can turn into a form of vitamin A. One such Carotenoid is beta-carotene. Beta-carotene is an antioxidant. Antioxidants protect cells from damage caused by unstable substances called free radicals. Free radicals are believed to contribute to certain chronic diseases and play a role in the degenerative processes seen in aging.

When to take/Type to take
Vitamin A supplement are best to be taken with meal. Vitamin A is available as fish liver oil or as a palmitate (from palmitic acid) However, beta-carotene is a preferential method for supplementing with vitamin A. Unless the user is diabetic, the body can convert beta-carotene into vitamin A as needed. As such, risk for vitamin A toxicity is virtually nonexistent.

Doses
Infants
0 - 6 months: 400 micrograms per day (mcg/day)
7 - 12 months: 500 mcg/day
Children
1 - 3 years: 300 mcg/day
4 - 8 years: 400 mcg/day
9 - 13 years: 600 mcg/day
Adolescents and Adults
Males age 14 and older: 900 mcg/day
Females age 14 and older: 700 mcg/day

Possible Side effects / Precautions / Possible Interactions:
If you don't get enough vitamin A, you are more susceptible to infectious diseases and vision problems.If you get too much vitamin A, you can become sick. Large doses of vitamin A can also cause birth defects. Acute vitamin A poisoning usually occurs when an adult takes several hundred thousand IU. Symptoms of chronic vitamin A poisoning may occur in adults who regularly take more than 25,000 IU a day. Babies and children are more sensitive and can become sick after taking smaller doses of vitamin A or vitamin A-containing products such as retinol (found in skin creams).
Increased amounts of beta-carotene can turn the color of skin to yellow or orange. The skin color returns to normal once the increased intake of beta-carotene is reduced.

Research studies / References