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VITAMIN A

Historically, vitamin A has been most well known as a cure for night blindness. In fact, night blindness is a symptom of vitamin A deficiency, which can be alleviated by taking sufficient amounts of that vitamin. Symptoms of night blindness were first recognized in ancient Egypt around 1500 B.C. Although at the time the problem was not linked to a dietary deficiency of any kind, the symptoms were treated by placing roasted or fried liver over the eyes. Later, Hippocrates suggested eating beef liver, a rich source of vitamin A, as a cure for night blindness. Today we known that vitamin A serves several other important functions in the body, and chemical derivatives of vitamin A have important medical benefits, particularly in the area of treatment of skin conditions and in cancer treatment. We also know that vitamin A deficiencies lead to a higher risk for some cancers. Including lung cancer in smokers.

Several compounds with similar chemical structures in nature have vitamin A activity. In animal - derived foods, most vitamin A exists as retinol or retinal. Vitamin A from plants is in the form of carotenoids. Carotenoids are considered "provitamins" because they must be metabolized in the body before they can act as a vitamin. Alone, however, they may serve as free-radical scavengers and quenchers of singlet oxygen. There are many carotenoids in plants, but not all have vitamin A activity. Beta carotene is the most common, and it has the most potent vitamin A activity.

All compounds with vitamin A activity are fat-soluble; like all fats, they are absorbed from the intestinal tract with the help of bile.

FUNCTIONS

Vitamin A in essential to a variety of biochemical and physiological processes, the vitamin must undergo a minor chemical change to allow it to participate in that function. Vitamin A, as retinal, is a part of the pigment called rhodopsin, found in the retina of the eye. Rhodospin in sensitive to small amounts of light and is essential for night vision because it allows us to discern objects in very low light. To function in this way, the retina of the eye must contain a large concentration of vitamin A.

Retinoic acid is present in very small amounts, yet it has an important function in promoting cell growth and cell differentiation. Skin cells and the cornea of the eye are especially affected by vitamin A deficiency. The skin turns rough and scaly, and hair follicles become obstructed. Vitamin A, as retinoic acid, probably plays an important role in preventing cancer because of its ability to regulate cell division and differentiation.

Vitamin A, as retinol, is needed for reproduction. Rats deprived of vitamin A cannot reproduce. Retinoic acid will not cure the infertility.

Vitamin A is also important in maintaining a normal immune system. Vitamin A deficiency results in an increased susceptibility to infections, and it has been called the "anti-infective vitamin". In recent years it has been recognized that measles occurring in vitamin A - deficient children is very severe, often resulting in death. A program started by the World Health Organization gives occasional high oral doses of vitamin A to children in certain developing countries that have a high death rate from measles and other infections. The association between very severe measles requiring hospitalization and low vitamin A status has been made even in United States, and a call has been made to pay more attention to vitamin A intake in children. Some have suggested that all hospitalized children with measles be given a high-dose vitamin A supplement.

Beta carotene, the most potent plant - derived precursor of retinol, retinal, and retinoic acid, ahs a biological activity of its own. It is an excellent free-radical scavenger, and it neutralizes reactive singlet oxygen. However, other plant carotenoids may be even more important in protecting us against the damaging effects of oxygen.

DAILY REQUIREMENTS

The strength of vitamin A can be expressed in terms of a measure of biological activity, international units, or as retinol equivalent, which indicates the amount of retinol to which the substance can be converted in the body. The use of IU is older and has been mostly replaced in nutrition tables by the use of RE values. Commercial vitamin products, however, still use the old IU system and the older U.S. RDA values, which are in IU. To convert between system, simply remember that 1 RE of vitamin A equals 3.3 IU.

The adult RDA for vitamin A is 1,000 RE for men and 800 RE for women. Pregnant or breast-feeding women need an additional 200 and 400 RE, respectively. The U.S. RDA value for adults is 5,000 IU. The RDA for infants and children is smaller than that for adults, but when compared on a pound-for-pound basis, it is considerably larger because of the need for vitamin A in normal processes of growth and development.

Small amounts of vitamin E will increase the capacity of all body tissues, including the retina and liver, to store vitamin A. For this reason, people who must take vitamin A to correct a deficiency state should also take vitamin A to correct a deficiency state should also take a modest amount of vitamin E. Most comprehensive multivitamin formulas have enough vitamin E in the m to satisfy this need for the vitamin.

DIETARY SOURCES

It is easy to recall which foods are rich in vitamin A if you remember that retinols dissolve in fat and carotenes are found in yellow plant pigments. Some rich animal sources of vitamin A are liver, whole milk, cream, and butter. Additional good sources are listed in the accompanying table. Four ounces of beef liver has more than a seven-day supply of vitamin A, and fish liver has even more. Cold liver oil was commonly used as a source of this vitamin and of vitamin D before purified vitamin A was available. A single teaspoon of cod liver oil will fulfill the adult RDA High concentration of carotene are found in dark green vegetables, where the green of chlorophyll usually covers the yellow to-orange carotenoid color, and yellow-orange vegetables such as carrots, pumpkin, and squash. Other sources are listed in the accompanying table.

Vitamin A content of Selected Food
(Average adult RDA is 900 RE, or 4,500 IU)

Food	Approximate content (IU per 3 oz)
Apricots	2,430
Asparagus	810
Butter	2,520
Broccoli	3,150
Cantaloupe	3,060
Carrots	9,000
Cheese	1,170
Cherries	900
Chicken	190
Cod	193
Corn	360
Crab	1,980
Cucumbers	175
Eggs	1,080
Endive	3,000
Green beans	540
Green peppers	380
Herring	135
Kumquats	540
Halibut	765
Lettuce (iceberg)	900
Lettuce (romaine)	1,700
Liver (beef)	33,300
Lobster	828
Mackerel	400
Mango	1,620
Milk (low-fat)*	155
Milk (whole)	200
Nectarine	1,440
Okra	470
Oranges	180
Oysters	280
Papaya	1,530
Peaches	1,170
Persimmons	2,430
Pimentos	2,070
Pistachios	205
Pumpkin	6, 300
Soybeans	630
Spinach	7,200
Sprouts	495
Squash (summer)	360
Squash (winter)	3,600
Swordfish	1,890
Tomatoes	800
Walnuts	270
Watermelon	475
Whitefish	1,800

Carotene is converted to retinol in the wall of the small intestine as it is being absorbed into the blood. The process of conversion continues only as long as retinol is needed. Once the need for retinol has been satisfied, the conversion process stops, so high doses of beta carotene do not result in toxic levels of retinal.

Vitamin A is sensitive to oxygen. Thus, prolonged beating in the presence of air will destroy the vitamin A activity of any food. To minimize this loss while cooking you food, place a lid on the pot or pan used to boil such vegetables as squash or carrots, and use a small pot with just enough water to cover the vegetables. Acid also destroys A, but most food sources of this vitamin are nonacidic, so this is not a practical problem.

DEFICIENCIES

About 15 percent of Americans, mostly infants and children, get less than the RDA value for vitamin A. Fortunately, the average person has about a two-year supply of vitamin A in their liver, so you have to eat a vitamin A - deficient diet for many months before symptoms will develop.

Deficiency Symptoms

Mild vitamin A deficiency may be easily overlooked. Dry, rough skin is common. Sometimes the skin can crack and may even become infected. The infection develops because cracks and rough areas in the skin leave openings for microorganisms to enter and because vitamin A is needed for a strong immune response.

The most recognizable sign of vitamin A deficiency is night blindness. This is followed by further damage to the cornea in the form of a condition called xerophthalmia. If left untreated, severe vitamin A deficiency can lead to permanent blindness. Vitamin A deficiency is the leading worldwide cause of blindness in childhood and is an enormous problem is many developing countries.

People who are deficient in vitamin A have an increased susceptibility to respiratory infections because of changes in the cells that line the respiratory tract. Because of their depressed immune response, deficient children develop more severe forms of common childhood diseases. And a mentioned earlier, measles can be lethal in a vitamin A - deficient child.

Other possible effects of vitamin A deficiency include slow growth, thickening of bone, kidney stone that originate with changes in some of the cells that line the kidney tubules, diarrhea, and reduced production of steroid hormones in the body. Steroids are produced by the adrenal gland and are a part of your natural response to stress and your immune function. Failure to make these important hormones will leave your immune system in a less-than-ideal state. Damage to hearing, taste, and smell, nerve damage, and reduced sweat gland function may also occur.

It is possible to measure vitamin A in your blood, but the results of this test are difficult to interpret because of the large amounts that are stored in the liver. Repeatedly low blood values are a direct indication of a severe vitamin deficiency that demands immediate attention.

TOXICITY

Adults may develop symptoms of vitamin A toxicity after taking more than 50,000 IU a day for long periods of time or after taking a single dose of 300,000 IU or more. Infants given 7,500 to 15,000 RE of vitamin A for thirty days have developed toxicity symptoms. In children, vitamin A toxicity has usually been caused by an overzealous parent giving excessive quantities of supplemental vitamin A. Infants and children who are given 6,000 RE of vitamin A a day and who are not deficient are likely to develop overdose symptoms after several months.

Vitamin A overdose is characterized by vomiting, fatigue, swelling due to fluid accumulation, hydrocephalus, and headache. Vitamin A overdose has, in some cases, been misdiagnosed as a brain tumor. Other symptoms of overdose are liver and lymph gland enlargement, difficult sleeping, joint pains, constipation, and rough skin. The effects of an overdose of vitamin A will usually reverse themselves after you stop taking the vitamin.

Since the body will convert as much carotenoid to retinol as it needs, it is possible to have excess unconverted carotene in the blood. A very high concentration of unconverted carotenoids in the blood is known as hypercarotenosis and can cause a yellow discoloration of the skin. This condition has been confused with jaundice, but the two are not related. Hypercarotenosis is unsightly but usually not dangerous.

Toxicity and Pregnant Women

Pregnant women taking excess vitamin A risk bearing a child with birth defects because of the action of the vitamin on the developing fetus. Some of the possible defects are urinary tract malformations, hydrocephalus, and bone deformities. If you are pregnant, do not exceed your doctor's recommendation for any vitamin or drug, including vitamin A. Current recommendations are to take no more than 10,000 IU per day during pregnancy, and prenatal vitamins contain no more than this amount. But vitamin supplements containing up to 50,000 IU per capsule are readily available, so be careful when selecting a product.

INTERACTIONS

Vitamin A can interact with corticosteroid - type -drugs, oral contraceptives, calcium, zinc, and mineral oil. It can also interfere with certain blood tests.

THERAPEUTIC USES

Vitamin A supplements may be given to counteract any of the standard symptoms of vitamin A deficiency reviewed earlier in the profile. However, such symptoms will not be alleviated by vitamin A unless they are, in fact, caused by vitamin deficiency.

Antioxidant Uses

Plant carotenoids are excellent free-radical scavengers and quenchers of singlet oxygen. As such, they play a role in protecting our cells against oxidative damage and presumably help protect us from a number of chronic diseases linke4d to free-radical damage.

Measles

As discussed earlier, measles can be life-threatening if the child is deficient in vitamin A. It has been recommended that the vitamin A status of all seriously ill patients with measles be checked and a high dose supplement given if needed. A program of providing vitamin A supplements to children to some developing countries has lowered the infant death rate due to infections.

Cancer Prevention

Animals deficient in vitamin A get cancer more often and their tumors spread more quickly than animals without this deficiency. There is evidence that the same thing holds true for people: Those who are deficient in vitamin A may be at a an increased risk of developing cancer, and individuals who do get the disease may find that their cancer spreads more quickly. Low vitamin A intake from vegetable is directly related to tan increase in cancer of the lung, bladder, and larynx,. However, it is not known whether the protective effect of vitamin A can be attributed to beta carotene, other plant carotenoids, retinol , or some other component of green and yellow vegetables. Our best advice is to eat plenty of vegetables, as high vegetable intake is consistently associated with a lower risk of cancer.

Retinoic acid is the form of vitamin A required for normal cell differentiation and thus cancer prevention. Unfortunately, it is metabolized too quickly and is too toxic in high doses to be practical for therapeutic use in cancer treatment. Other synthetic derivatives of retinoic acid show some promise and are under investigation.

Treatment of Acne

One of the symptoms of vitamin A deficiency is an acnelike condition. The observation led to the use of vitamin A capsules as an acne treatment in the 1950s and 1960s and consequently to the discovery that high doses of vitamin A are toxic. After several decades of using vitamin A capsules to treat acne, the consensus is that the treatment is only marginally effective and carries a significant risk of toxicity.

In the 1970s retinoic acid applied to the skin became popular as an acne treatment, and it is still used today. Topical retinoic acid appears to reduce the plugging of sebaceous glands. Plugged sebaceous glands can become infected and turn into pimples. Retinoic acid also mildly irritates the skin and causes peeling, which also helps to free plugged sebaceous glands.

Retinoic acid cream or lotion is not the ideal treatment. It can take months for the treatment to begin working, and it is irritating to the skin. In the search for more potent and less toxic retinoic acid derivatives, researches have discovered several that can be taken by mouth without the usual adverse effects. Isotretinoin, or 13-cis-retinoic acid, has been of some benefit to people who suffer from the most severe from of acne and who have not responded to other forms of treatment. These retinoic acid derivatives are strong teratogens, and patients on the drug must take steps to make sure they do not get pregnant.

Treatment of Psoriasis

Several retinoic acid derivatives are used with some success to treat psoriasis. These drugs are available only by prescription and carry a significant risk for causing birth defects if taken during pregnancy.

UNSUBSTANTIATED CLAIMS

We cannot recommend that you take this vitamin to treat alcoholism, allergies, angina pectoris, arteriosclerosis, arthritis, asthma, bad breath, broken bones, bronchitis, canker sores, cataracts, colitis, the common cold, constipation, cystitis, diabetes, diarrhea, double vision, ear infections, emphysema, epilepsy, eye strain, fever, flu, gout, hair problems, hay fever, headache, heart attack, heart failure, hemorrhoids, hemophilia, hepatitis, infertility, jaundice, kidney stones, learning disabilities, liver cirrhosis, meningitis, mononucleosis, muscular dystrophy, nail problems, osteomalacia, prostate trouble, psychosis, sinusitis, stroke, swollen glands, thyroid disease, tuberculosis, vaginitis, varicose veins, or worms.

AVAILABILITY

Despite its toxicity, vitamin A can be obtained in any strength without a prescription. Attempts by the FDA to attempt to limit the doses that can be purchased without a prescription have been met with fierce opposition by nutritionally oriented consumer groups.

We do not recommended taking more than 10,000 IU a day unless you are under the guidance of a licensed health care professional who is expert in therapy with vitamin A. This is especially true if you are or might become pregnant. Do not take vitamin A doses over 10,000 IU per day if you are pregnant or if there is any chance you may become pregnant.