What Is Vitamin A?

By Gary Heiting, OD

Vitamin A actually is a group of antioxidant compounds that play an important role in vision, bone growth and health of the immune system. Vitamin A also helps the surface of the eye, mucous membranes and skin be effective barriers to bacteria and viruses, reducing the risk of eye infections, respiratory problems and other infectious diseases.
In general, there are two types of vitamin A, depending on the type of food source it comes from:
Vitamin A from animal-derived foods is called retinol. This “pre-formed” vitamin A can be used directly by the body. Good food sources of retinol vitamin A include beef and chicken liver, whole milk and cheese.

Vitamin A obtained from colorful fruits and vegetables is in the form of “provitamin A” carotenoids, which are converted to retinol by the body after the food is ingested. Good food sources of provitamin A carotenoids include carrots, sweet potatoes, spinach, kale and cantaloupes.

Beta carotene is one of the most prevalent and effective provitamin A carotenoids.

Eye Benefits of Vitamin A and Beta Carotene

Because vitamin A helps protect the surface of the eye (cornea), it is essential for good vision.

Studies show vitamin A eye drops are effective for the treatment of dry eyes. In fact, one study found that over-the-counter lubricating eye drops containing vitamin A were as effective for the treatment of dry eye syndrome as more expensive prescription eye drops formulated for dry eye relief.

Vitamin A eye drops also have been shown effective for the treatment of a specific type of eye inflammation called superior limbic keratoconjunctivitis.
Vitamin A, at least when in combination with other antioxidant vitamins, also appears to play a role in decreasing the risk of macular degeneration (AMD). In the landmark Age-Related Eye Disease Study (AREDS) sponsored by the National Eye Institute, people at high risk for the disease who took a daily multiple vitamin that included vitamin A (as beta carotene), vitamin C, vitamin E, zinc and copper had a 25 percent reduced risk of advanced AMD during a six-year period.
It also appears that a combination of vitamin A and lutein may prolong vision in people suffering from retinitis pigmentosa (RP). A recent four-year study conducted by researchers from Harvard Medical School and other prominent universities found that individuals with retinitis pigmentosa who took daily supplements of vitamin A (15,000 IU) and lutein (12 mg) had a slower loss of peripheral vision than those who did not take the combined supplements.
Because beta carotene is converted into vitamin A in the body, it’s likely beta carotene (provitamin A) offers similar eye benefits as the pre-formed retinol type of vitamin A, though more research is needed to confirm this.

In early 2011, researchers at Columbia University Medical Center (New York) found that a synthetic, altered form of vitamin A might be able to slow the progression of Stargardt’s disease, an inherited eye disease that causes severe vision loss in young people.

When given to mice with the same genetic defect as humans with Stargardt’s disease (also called juvenile macular degeneration), the modified vitamin A inhibited the growth of clump-like deposits in the retina called “vitamin A dimers” that are associated with degenerative changes and vision loss.
The National Eye Institute has awarded the researchers a $1.25 million grant to further investigate the link between vitamin A dimers and various retinal degenerations, which could lead to new approaches to treat these diseases.
Also, Alkeus Pharmaceuticals (Cambridge, Mass.) has licensed certain patents from Columbia University related to the research and is planning to launch clinical trials for the treatment of Stargardt’s disease and macular degeneration with modified vitamin A in the near future.

Vitamin A Deficiency
Vitamin A deficiency is rare in the United States, but it is common among the poor in developing countries. It’s estimated that approximately 250,000 to 500,000 malnourished children worldwide become blind each year due to vitamin A deficiency that could have been prevented with a proper diet.
One of the first signs of vitamin A deficiency is night blindness. In ancient Egypt, it was discovered that night blindness could be cured by eating liver, which later was found to be a rich source of vitamin A.

A lack of vitamin A causes the cornea to become very dry, leading to clouding of the front of the eye, corneal ulcers and vision loss. Vitamin A deficiency also causes damage to the retina, which also contributes to blindness.
Because vitamin A also is important for resistance to infection and a healthy immune system, vitamin A deficiency can lead to death from respiratory and other infections.

Vitamin A – Daily Value
In most cases, it’s best to obtain vitamins and minerals from a healthy, balanced diet. The concept of the Daily Value (DV) was developed to help consumers determine if a food contains a lot or a little of a nutrient, based on its Recommended Dietary Allowance. The DV for vitamin A is 5,000 IU.
The following tables provide DV percentages for some of the best vitamin A foods:

Vitamin A Toxicity
Vitamin A that comes from animal food sources is not water-soluble and therefore is not readily excreted from the body. Instead, it is stored in body fat and, if ingested in excess amounts, can build up in the body and become toxic.
Beta carotene and other provitamin A carotenoids found in fruits and vegetables don’t pose the same vitamin A toxicity risk. These compounds are water-soluble and are easily eliminated from the body, so vitamin A toxicity from vegetarian food sources is rare.
Beta carotene supplements, however, may have serious risks for smokers. Two studies have found that smokers taking daily supplements containing 20 to 30 mg of beta-carotene had an increased risk of lung cancer compared with smokers who did not take the eye supplements. (These studies are controversial, however, and a large study of more than 22,000 male physicians found no adverse health effects when these doctors took beta carotene supplements of 50 mg every other day.)
The Institute of Medicine has established the following upper intake levels for the animal-based, retinol form of vitamin A to reduce the risk of vitamin A toxicity:
• Children (ages 4 to 8): 3,000 IU
• Children (ages 9 to 13): 5,610 IU
• Teenagers (ages 14 to 18): 9,240 IU
• Adults (age 19 and older): 10,000 IU
Possible toxicity reactions from long-term daily consumption of vitamin A above these levels include birth defects, liver abnormalities, reduced bone mineral density that can lead to osteoporosis, and central nervous system disorders.

Learn more:
http://www.allaboutvision.com/nutrition/vitamin_a.htm