Vitamin A is clear yellow oil and one of the four fat-soluble vitamins, along with vitamins D, E and K. This means it dissolves in organic solvents, such as ether or cleaning fluid and, in the body, is absorbed and transported in a manner similar to that of fats, If you have problems absorbing fat in your intestines, you run a higher-than-normal risk of developing a vitamin A deficiency.
Q. If vitamin A is fat-soluble; does that mean it's found in foods that contain fat?
A. Yes. Vitamin A occurs naturally only in foods of animal origin, such as liver, which is the storage place for vitamin A in animals and humans; some seafood; butter; whole milk and egg yolks. This form of vitamin A is usually called pre-formed vitamin A, or retinol. Retinol is also added to skim and low-fat milk.
Vitamin A deficiency also causes weakening of epithelial cells, which form sheets covering the internal and external surfaces of the body, including the lining of blood vessels and other small cavities. Epithelial cells occur in the skin, lungs, developing teeth, inner ear, cornea, gonads, glands and their ducts, gums, the front of the lens of the eye, the sensory part of the nose, the cervix and other areas of the body.
Vitamin A must be present for these cells to differentiate - to go from their immature to their mature states. If these cells fail to mature because of a vitamin A deficiency, often the result is an alteration in skin and mucous membranes that resembles a precancerous condition, and which, given the proper conditions, could lead to cancer. Weakened epithelial tissues also pave the way for a variety of infections; Infection is, in fact, far more likely than cancer.
Q. How would I know if I were low in vitamin A?
A. Signs of vitamin A deficiency include:
• poor night vision, or impaired adaptation of the eyes to darkness
• lack of normal mucus secretion, including dry eyes and mouth
• susceptibility to infection, such as sinus trouble or sore throats
• exophthalmia, an eye condition characterized by swollen lids and a sticky discharge from the eyes
• A condition called follicular hyperkeratosis, where the skin feels like coarse sandpaper.
Recent studies have even found hearing disorders in people who are deficient in vitamin A.
Q. Does beta-carotene do anything that's different from vitamin A?
A. Even though beta-carotene itself is not considered a vitamin, it does seem to have activity in the body independent of its conversion to vitamin A, and much of that activity seems to revolve around cancer prevention and protection against heart disease. That's why researchers now distinguish beta-carotene from preformed vitamin A.
Q. How did researchers discover that there is a difference between the two?
A. Studies done over the years seem to indicate that people who get lots of vitamin A from plants, as carotenoids, are at less risk of developing cancer of the lungs, cervix and gastrointestinal tract, while people who get most of their vitamin A from animal foods are not protected in this way. In animal studies, beta-carotene has provided protection against chemical- or radiation-induced cancer. And in human studies, it also seems to offer some cancer protection.
Q. What about heart disease and beta-carotene?
A. Several recent studies also indicate that beta-carotene provides some protection against heart disease.
One American research project, the Physicians' Health Study, a long-term study involving 22,000 male doctors, found that among 333 men who already had evidence of cardiovascular disease, those taking beta-carotene supplements over six years had half as many strokes, heart attacks, sudden cardiac deaths or operations to open or bypass clogged coronary arteries as those taking placebos, in this study, the doctors took 50 mg of beta-carotene every other day (equivalent to 83,720 IU).
Q. Any studies on women?
A. large study of nurses, by the same researchers, found a 22 per cent reduction in the risk of heart attack and a 40 per cent reduction in stroke risk for women with high intakes of fruits and vegetables rich in beta-carotene, compared with women whose intakes were low. The low beta-carotene group was getting fewer than 6 mg of carotene a day, the amount found in less than half a carrot. The high intake group was getting more than 15 to 20 mg a day — one to two on the carrot scale. So the difference between the two groups was only about 10 to 15 mg, about the amount found in a single helping of a beta-carotene-rich fruit or vegetable.
Q. How does beta-carotene provide cancer and heart-disease protection?
A. Like vitamins C and E, beta-carotene works as an antioxidant, but in its own unique way.
Q. And what again is an antioxidant?
A. A molecule that helps to limit an oxidative reaction (a reaction that involves oxygen) by neutralizing free radicals. Oxidative reactions take place all the time in our bodies. For instance, they help our bodies to generate energy. Oxidative reactions also turn butter rancid, cause wet iron to rust, and turn a slice of apple brown. While necessary in the body, Oxidative reactions are potentially harmful. The reactions form highly reactive, chemically unstable atoms or atom groups called free radicals. Free radicals lack electrons, and attempt to steal electrons from other molecules to regain their balance, as it were. A victimized molecule will itself, in turn, become a free radical, initiating a chain reaction of multiplying free radicals which oxidize a large amount of cells very quickly.
Q. Are you saying that these reactions can harm cells!
A. Yes, oxidative reactions can damage the fatty outer membranes of cells, impairing the ability of molecules to move in and out of cells. This means the cell may not work right, or may even die. Free radicals which get inside a cell can damage the cell's genetic material, DNA, causing mutations which have the potential to lead to cancer. Free radicals can also damage a cell's energy source, the mitochondria, destroying the cell's ability to produce the energy it needs to function.
Q. So antioxidants such as beta-carotene and vitamins E and C halo to limit oxidative chain reactions?
A.Yes. Antioxidants help stop these chain reactions by offering up one of their electrons to free radicals and, thus, neutralizing them. Nor do the antioxidants become free radicals themselves as a result. Instead, they simply become inactive.
Q. OK, So beta-carotene and vitamins E and C help to limit the cell damage that can be caused by oxidative reactions. But what does this mean in terms of health and disease prevention?
A. It means that anywhere in the body that oxidative reactions are contributing to a disease process, these nutrients might help to prevent or limit that disease.
Q. Can you give me some examples?
A. Take eyes, for instance. The lenses of the eyes are exposed to oxidative damage from sunlight. This oxida-tive damage can lead to cataracts, a condition where the normally clear lens of the eye becomes milky white and opaque. Worldwide in incidence, cataracts are one of the most common causes of blindness among older people. In both animal and human studies, antioxidant nutrients help to prevent cataracts. They work by protecting proteins in the lens from oxidative damage from sunlight. In one study, extra amounts of either vitamin E or vitamin C reduced the incidence of cataracts by about 30 per cent Lungs are also vulnerable to oxidative damage from pollutants in the air, such as ozone and nitrogen dioxide. Animal studies indicate that low levels of vitamin and other antioxidants increase pollutant damage to the lungs, and that high levels of vitamins E and C and beta-carotene help protect lungs.
Q. It sounds like there is really strong evidence that this stuff protects against cancer. Is that right?
A. Let's say that researchers are excited, but until they get results of several clinical trials currently in
progress, the case won't be entirely proven. Still, researchers suggest you eat plenty of fruits and vegetables, and include those containing beta-carotene,
Q. Are any other carotenoids thought to help protect against cancer?
A. Yes. Other carotenoids besides beta-carotene are known to act as antioxidants, but they are still much less studied than beta-carotene. Exactly how they offer protection is unknown. These carotenoids include lutein, zeaxarrthin, beta-cryptoxanthin, lycopene, alpha carotene and others. Studies of cells in culture and with animals demonstrate that these carotenoids can protect against cancer.
Q. Does the substance called retinol!, which you earlier called pre-formed vitamin A, play any role in cancer prevention?
A. Most of the dietary studies which have looked for an association between retinol and cancer have not found an association. One study even found that people whose diets were high in some retinol-containing foods seemed to have an increased risk of cancer, but it seems more likely that the association was due to the high fat content of the foods - eggs, butter, sour cream - than to the vitamin A content.
Q. Aren't some acne drugs made from vitamin A? I heard they were also being used to treat cancer. Are they?
A. There are two types of acne drugs made from derivatives of vitamin A. Both have similar structures. One is Retin-A (tretinoin), an externally applied gel used for the treatment of acne, wrinkles and a potentially precancerous skin condition called actinic keratosis.
The other drug is Accutane (isotretinoin), a drug taken orally in the form of soft gelatin capsules. Accutane is used for the treatment of severe acne. Accutane — but not Retin-A - has been used experimentally to reverse potentially precancerous changes in the cells lining the inside of the mouth, a condition called oral leukoplakia.
Q. Is treatment with Accutane successful?
A It certainly shows promise. In one study Acutance was given in large doses to 70 people with leukoplakia. After three months, 24 out of 53 of the people who responded well to this treatment were changed to a lower, maintenance dose of the drug, while the rest were given beta-carotene. After nine months, the researchers found that only two of the people still taking the drug (8 per cent) had seen their conditions worsen. However, 16 (55 per cent) of those taking beta-carotene experienced a worsening of their condition.
Q. Are there any side-effects associated with Acutance?
A. Yes. Dry eyes and mouth, headaches and depression are common in people taking large doses. That's why
Some researchers believe its use should be reserved for people who have severe leukoplakia or who have previously had oral cancer. Just what percentage of people is permanently cured of their leukoplakia by a course of Acutance over several months is unknown. Most people cannot take it in large doses for more than a year or so, but smaller, maintenance doses may be effective with fewer side-effects.
We should mention here that large doses of beta-carotene have also been tried against oral leukoplakia, but beta-carotene doesn't seem to be as effective as Acutance in treating this condition. Still, some researchers are hopeful it will prove to be of some use, since it has no side-effects.
Q. Are people supposed to get a certain amount of beta-carotene?
A. There is no RNI for beta-carotene. To meet the RNI for vitamin A, you'd need to get about 6 mg of beta-carotene a day.
Q. I've heard you can turn orange if you eat too many carrots or take too much beta-carotene as supplements. Is that true?
A. Yes, your skin can turn a yellow-orange color if you take more than 30 mg a day of beta-carotene. But this condition appears to be harmless and goes away if you reduce the amount of beta-carotene you are consuming.
Q. Does anyone need greater-than-average amounts of vitamin A?
A. Some people may require more. People with absorption problems may need more than the RN1; so may people who are recovering from extensive burns.
VITAMIN A AND BETA-CAROTENE QUICK-REFERENCE GUIDE
RNI
Men: 1,000 RE (5,000 IU) Women: 800 RE (4,000 IU)
Sources
Cod-liver oil, beef liver, oysters, butter, whole milk, egg yolks and orange and green leafy vegetables, such as carrots, sweet potatoes, butternut squash, mangoes, apricots, spinach, turnip greens, broccoli and cos (romaine) lettuce.
Signs of Deficiency
Night blindness; dry eyes and mouth; susceptibility to infection; swollen eyelids and sticky discharge from the eyes; coarse, rough skin.
Possible Toxicity Problems
A single large dose of 250,000 to 300,000 IU, or smaller amounts of 50,000 IU for long periods of time, can cause symptoms of toxicity, such as bone and joint pain, hair loss, skin dryness, itching and flaking, weakness, headache and vision problems.