Vitamin A, also known as beta-carotene or retinol,
is a very important vitamin. Preformed vitamin A, as is found in fish liver oil, was the first vitamin officially named and was thereby given the letter A to identify it. Retinol, another name for preformed vitamin A, was so named because of its importance in vision. The rods of the eye, which are located within the retina, contain rhodopsin, or visual purple, and need vitamin A for proper vision. Several carotene pigments found in foods, mainly yellow and orange vegetables and fruits, can be converted to vitamin A in our body and thus are termed provitamin A. Beta-carotene is the most available and also the one that yields the highest amount of A.
Vitamin A is absorbed primarily from the small intestine. Absorption of this fat-soluble vitamin is reduced with alcohol use, with vitamin E deficiency, with cortisone medication, and with excessive iron intake or the use of mineral oil, as well as with exercise. As a fat-soluble vitamin, vitamin A can be stored in the body and used when there is decreased intake. About 90 percent of the storable vitamin A is in the liver; it is also stored in the kidneys, lungs, eyes, and the fat tissue. In addition to reducing absorption, alcohol use also depletes liver stores. The storage of vitamin A is decreased during times of stress or illness unless intake is increased. The body needs the mineral zinc to help release stores of vitamin A for use.
Vitamin A is needed at a level of at least 5,000 IUs (international units) per day, though this may vary due to many factors. Deficiencies of vitamin A are still fairly common worldwide and cause many difficulties. Actually, analysis of the average American diet reveals that it provides only about 4,000 units of vitamin A daily, so the many problems of vitamin A deficiency, such as visual changes, skin dryness, and increased infections, are more common than most people realize.
Sources: The two forms of vitamin A come from different food sources. Preformed A (retinol) is the main animal-source vitamin A. It is found in highest concentrations in all kinds of liver and fish liver oil, which is a common source for supplements. Egg yolks and milk products, such as whole milk, cream, and butter, are also good sources of vitamin A.
Provitamin A, mainly in the form of beta-carotene, is found in a wide variety of yellow- and orange-colored fruits and vegetables, as well as leafy green vegetables. Beta-carotene is actually a double molecule of vitamin A. It may be converted to vitamin A in the upper intestine before absorption; beta-carotene can also be converted to vitamin A in the liver. People with diabetes, low thyroid activity, and those who use a lot of polyunsaturated fatty acids (PUFA) without antioxidants such as vitamin E have lowered ability to convert beta-carotene to A. Assimilation of vitamin A and the carotenes is helped by the presence of bile salts and fatty acids in the intestine.
Functions: Vitamin A performs many important functions in the human body. The following are the most common.
Eyesight. Vitamin A is needed for the formation of rhodopsin, or visual purple, which allows us to see at night. When vitamin A is lacking, there is a lag period in regenerating visual purple and a resulting inability to see well at night, termed “night blindness.” Vitamin A also helps maintain the health of the cornea, the eye covering. Vitamin A deficiency may allow irritation or inflammation of the eye tissue to occur more easily.
Growth and tissue healing. This vitamin is involved in laying down new bone during growth and promoting healthy teeth. After tissue injury or surgery, vitamin A is needed for repair of the tissues and to help protect the tissues from infection.
Healthy skin. Vitamin A stimulates growth of the base layer of the skin cells. It helps the cells differentiate normally (progress from less to more mature cell forms) and gives them their structural integrity. It does this for both the external skin cells and the body’s inner skin, the mucous membrane linings of the nose, eyes, intestinal tract, respiratory lining, and bladder. By this function, it also helps protect these areas from cancer cell development.
By moisturizing the mucous lining cells, which helps proper secretion, and by maintaining their structural integrity, vitamin A helps the body fight off infectious agents and environmental pollutants. The epithelial tissues protected include the linings of the lungs, nose, throat, stomach, intestines, vagina, bladder, and urinary tract, as well as the eyes and skin.
Antioxidation. Vitamin A helps protect the body (particularly cell membranes and tissue linings) from the irritating effects of free radicals (unstable molecules) by neutralizing them. Beta-carotene also protects the tissues from the toxic singlet oxygen radical. This function usually requires an amount higher than the RDA dose, perhaps 10,000–20,000 IUs per day or more. Through its antioxidant effects, vitamin A and beta-carotene help protect the body from the irritating effects of smoke and other pollutants and may also be helpful in preventing problems like ulcers, atherosclerosis and its attendant complications, such as high blood pressure and stroke.
Lowering cancer risk. As discussed previously, vitamin A helps maintain the structural integrity of cells and the healthy functioning of the mucous linings. It also helps with proper cell differentiation of the surface cells. Recently, beta-carotene was shown to improve immune response by stimulating T-helper cell activity. In these ways, it seems to prevent the development of cancer, and a number of studies have shown reduced lung and colon cancer rates in people with higher intakes of beta-carotene.
Uses: With the many functions performed by vitamin A, it has a variety of uses in basic tissue and health maintenance, in clinical treatment for a number of problems (some of which may be vitamin A deficiency symptoms), and in the prevention of many illnesses and diseases. Vitamin A works better when there are sufficient body levels of zinc and an adequate intake of protein.
Infections. Vitamin A helps in many cases to protect tissues during infections and promote rapid recovery, primarily through its support of the health of the skin and mucous lining barriers and its stimulation of mucus production. It also appears to improve antibody response and white blood cell functions. In these ways, vitamin A may be even more helpful in the prevention of infections. By keeping the mucous membranes healthy, it also helps protect against the irritating effects of smoke and pollution.
Eye problems. Vitamin A is often suggested for a variety of eye problems. Night blindness may be an early sign of vitamin A deficiency, but vitamin A functions in many ways to support the health of the eye tissue. It has been used in the treatment of conjunctivitis, blurred vision, nearsightedness, cataracts, and glaucoma, but there is no hard scientific evidence, other than anecdotal, that vitamin A works for these conditions.
Because of its beneficial effects on the skin, vitamin A is used to treat a variety of skin problems, both by local application to rashes, boils, skin ulcers, and so on, and by increased intake to help the skin’s internal healing process. It may have some effect in psoriasis and in periodontal disease. Vitamin A can be supplemented for all kinds of wound healing, including before and after surgery. Its use in healing acne has been controversial, with some studies showing very good results and other studies showing no demonstrable effects. The amount used is usually about 100,000 IUs per day, as 50,000 IUs twice daily, with doses up to 300,000 IUs per day having better success. Used with 800 IUs of vitamin E daily (doesn’t need to be taken at the same time as an A supplement) the vitamin E promotes the activity of vitamin A. One hundred thousand IUs of vitamin A may help alleviate severe acne in many cases. Of course, with these higher doses of vitamin A, some signs of toxicity, such as frontal headaches, can develop and should be watched. If these occur, decreasing the amount of vitamin A will usually alleviate them. Retin A, a new pharmaceutical derivative of vitamin A, appears to help reduce wrinkles (and acne) by restoring skin tissue when applied topically.
Cancer prevention. By its influence in maintaining the cell integrity of the skin and the mucous membrane linings, as in the lungs, digestive system, and urinary and genital tracts, and by its support of proper cell differentiation, vitamin A as beta-carotene has been shown to be helpful in lowering lung cancer risks. It is likely, because of its effects on epithelial cell membranes, adequate beta-carotene intake will reduce risk of many other cancers. Likewise, a deficiency of vitamin A, beta-carotene, or both, increase the risk of cancers.
Pollution protection. The antioxidant function of vitamin A helps to protect the body tissues from the irritating effects of stress, smoke, air pollution, and chemical exposure.
Other uses. Vitamin A has also been tried with some success in treating a variety of other problems, including asthma, fibrocystic breast disease, plantar warts, sebaceous cysts, ulcers, and premenstrual syndrome. In fibrocystic problems, one small study showed a reduction in breast pain and lumps with 150,000 IUs daily, used under medical supervision.
Deficiency and toxicity: A number of difficulties can arise from a deficiency of vitamin A, many of which are based on impairments in the biochemical functions that this important nutrient performs. It is estimated that the diets of approximately 25 percent of the people in the United States are supplying less than the RDA for vitamin A. This commonly occurs in those who avoid the carotene-containing fruits and vegetables or when the diet is filled with processed foods that are depleted of vitamins. The fruits and vegetables are really the most important food groups for intake of the many vitamins and minerals, as well as for fiber and water content. The elderly, teenagers, and alcoholics are the three groups most commonly deficient in vitamin A. Worldwide, vitamin A deficiency is even more common than it is in the United States.
Night blindness, the inability to adapt the eyes to see clearly in the dark, is probably one of the first signs of vitamin A deficiency. Lack of general eye tissue health and vitality may occur, as can decreased vision, irritated, reddened, or dry eyes, and eyes that tire easily; in severe deficiency states, corneal ulcers may develop. Usually, supplemental vitamin A or beta-carotene will correct these problems; the RDA of vitamin A will prevent night blindness and these other eye problems.
Perhaps of greater significance, vitamin A and beta-carotene deficiencies may decrease our protection against infectious agents and the internal process of carcinogenesis. A depletion or deficiency of vitamin A reduces both T lymphocyte (cellular immunity) and B lymphocyte (antibody production) responses; severe vitamin A deficiency has been shown to cause atrophy of the thymus and spleen, both immunologically important organs, and to reduce the number of circulating lymphocytes. Low dietary levels of vitamin A have been associated with an increased risk of many cancers, including breast, cervical, lung, prostate, laryngeal, and stomach cancers, while beta-carotene deficiency has been clearly seen in patients with cancers of the cervix or lungs.
Vitamin A deficiency also affects the skin. Dry, bumpy skin may occur, especially on the backs of the arms. Since vitamin A promotes skin growth, moisture retention, and proper cell differentiation, a deficiency causes decreased skin tone and rapid aging of the skin and a variety of blemishes, acne, or boils. Vitamin A supports the mucous-secreting cells of the internal mucous membranes. Mucus protects these membranes from infection and irritants. When vitamin A is deficient, these internal epithelial cells secrete a protein (keratin) commonly found externally in hair and nails. This keratinization process makes the epithelial cells harder and dryer and thus less protecting.
Vitamin A along with adequate protein intake generates healthy hair. With a lack of A, the hair may lack luster and dandruff is more likely because of the loss of scalp skin moisture. Bone softness or abnormal menstruation may also develop from vitamin A deficiency. Fatigue and insomnia are also possible, as are a decrease in the appetite and some loss of smell and taste. When vitamin A is deficient, vitamin C seems to be lost more rapidly from the body. In addition to the lowered immune function and increased infection rate associated with vitamin A deficiency, periodontal disease, kidney stone formation, ear problems, and acne may occur more frequently.
A number of toxicity symptoms and difficulties may occur when we take too much vitamin A. Since it is stored in the body and is not readily excreted, toxicity may occur from mildly increased doses (say 50,000–100,000 IUs per day) over an extended time, such as a month or two, or from very high doses over a shorter period. Animal liver meats have the highest concentrations of vitamin A; beef liver has about 15,000 IUs per ounce, while polar bear liver has much higher concentrations than this and has been known to cause vitamin A toxicity from one serving. It is highly unlikely that one would get vitamin A toxicity from the diet alone (without lots of liver), since we receive much of it which must then be converted to active vitamin A. The synthetic vitamin A supplements, such as the palmitate or acetate forms, have a greater potential to produce toxic symptoms; high amounts of fish liver oil may produce side effects as well. The levels that cause symptoms vary from person to person, just as the proper optimum dosage does. Under high levels of stress, with illness or trauma, if we smoke or live in a polluted environment, or if we are pregnant or nursing, our vitamin A requirements are higher. When there is depletion or deficiency of vitamin A, higher amounts can be taken for up to a month without the usual risks of toxicity.
The only problem that may arise from high amounts of beta-carotene, as can occur with a high intake of yellow and orange fruits and vegetables and the leafy greens, is an orange-yellow discoloring of the skin. This occurs occasionally when people drink large amounts of carrot juice daily over time. It is of no real consequence and will clear when there is a reduction in carotene intake. Carotenosis can be differentiated from jaundice somewhat by the color and also by the fact that the white parts of the eyes do not turn color as they do with jaundice.
Too much vitamin A intake, however, can lead to slight swelling of the brain and resulting pressure headaches, which may be described as the feeling of a tight band around the forehead. Nausea and vomiting may also occur, as can irritability, dizziness, abdominal pain, and hair loss. Itchy, flaky, or dry skin can also result from too much vitamin A. Anorexia (loss of appetite) and resulting weight loss, liver enlargement, menstrual problems, bone abnormalities or stunted growth, and dry or bleeding lips may also occur. There may be an increased risk of birth defects in pregnant women taking high amounts of vitamin A, say over 400 IUs per pound of body weight. (To be safe, I suggest that pregnant women limit their preformed vitamin A to 15,000 IUs, with additional amounts taken only as beta-carotene.) Children have lower requirements, and toxicity has been seen in babies given adult doses of vitamin A, 20,000–30,000 IUs per day.
|Infants||Under 1 year||1,500–2,000|
|Males||11 years & up||5,000–6,000|
|Females||11 years & up||4,000–5,000|
Requirements: The Recommended Daily Allowance (RDA)—that is, the minimum dietary or supplemental amount to prevent vitamin A deficiency—is approximately 5,000 IUs per day for the adult. About 10,000–15,000 IUs of beta-carotene will probably convert to about 5,000 IUs of vitamin A in the body. Approximately two medium-sized carrots will give us that daily amount. Since this vitamin is fat soluble and stored in the body, daily sources are not absolutely necessary. Requirements, however, depend on body weight, so larger people need more A. Vitamin A needs are also increased with illness, infection, trauma, anxiety or stress, pregnancy, lactation, and alcohol use or smoking.
Approximately 10,000 IUs should be the average adult intake and is a safe amount to consume. The RDA amount will prevent most deficiency symptoms, such as night blindness, but for vitamin A’s antioxidant properties, higher amounts are needed. About 20,000–30,000 IUs daily, preferably as beta-carotene, may be the best range, especially for those of us with some levels of anxiety and stress. If vitamins C and E are used, slightly lower amounts of A are needed, since C and E help prevent the loss of stored vitamin A.
The upper intake, including diet and supplemental vitamin A, ranges from 50,000–100,000 IUs per day for short periods of a week or two. However, these amounts often produce some side effects over time. If no body deficiency or increased body needs of vitamin A are present, 50,000 IUs can cause problems. Infants and children can run into difficulty with doses as low as 10,000–25,000 IUs given over time, depending on their size. This is why it is important to know our food sources and supplement levels of preformed vitamin A (not beta-carotene, which is safe at higher levels), so that we can find the right amounts for each of us and our families.