Ординатура / Офтальмология / Английские материалы / The Eye Care Sourcebook_Lavine_2001

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maintain adequate levels of the B vitamins niacin and pyridoxine. Therefore, to maintain a good antioxidant status, you have to consume a sufficient amount of riboflavin. Deficiency can produce skin changes, including soreness near the corners of the mouth (the most

common cause of this, however, is the consumption of acidic drinks like orange juice), and cataract. Dairy and other animal prod-

ucts contain riboflavin, but mushrooms, asparagus, broccoli, collard greens, spinach, turnip greens, avocados, brewer’s yeast, and grains (whole or fortified) are good sources as well.

Niacin (Vitamin B3)

Niacin (vitamin B3) is part of a coenzyme that participates in the production and breakdown of carbohydrates, fatty acids, and amino acids. Niacin comes from the diet, but the body can also manu-

facture it from the amino acid tryptophan, with riboflavin helping out in the process.

Deficiency of niacin is called pellagra and is characterized by diarrhea and inflamma-

tion in the skin and mucous membranes. Meat contains niacin, but good plant sources include asparagus, avocados, broccoli, brussels sprouts, corn, kale, mushrooms, okra, peas, potatoes, pumpkin, rutabagas, squash, tomatoes, and brewer’s yeast.

Pyridoxine (Vitamin B6)

Pyridoxine (vitamin B6) also participates in many chemical reactions, including those involved in the production of neurotransmitters, the chemical sig-

naling agents of the nervous system. Pyroxidine

aids in the formation of niacin (vitamin B3) Vitamin B6 may help and is important for the functioning of red

blood cells. It plays an especially important

role in chemical reactions involving amino acids. The more protein you consume, the more pyridoxine you require. Therefore, someone who follows a healthy diet that avoids the excess protein characteristic of the typical Western

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diet may need a little less pyridoxine than the average person needs. Deficiency in our society is uncommon but can cause seizures, skin changes, and anemia. It can also raise blood levels of the amino acid homocysteine, and such elevated levels represent an important risk factor for heart and other blood vessel diseases. There is some evidence that a diet rich in pyridoxine may even help prevent heart disease independent of its effect on homocysteine levels. Besides animal products, good sources of pyridoxine include whole grains and rice, soy products, peanuts, and walnuts.

Cobalamin (Vitamin B12)

Vitamin B12 is involved in manufacturing protein as well as DNA. Deficiency of this vitamin can cause irreversible damage to the brain and spinal cord. The earliest sign may be cognitive dysfunction—problems with memory and thinking. In the eye, damage may occur to the

optic nerve, causing a decrease in vision. Anemia is often a late sign of the disease. The toxicity to the nervous system is attributed to a buildup of the amino acid homocysteine

and a reduction in the amino acid S-adenosyl methionine that occur when vitamin B12 levels become too low. Although a test can measure the vitamin B12 level in the blood, some people develop high homocysteine levels in the fluid bathing the brain and spinal cord before the level of vitamin B12 in the blood falls below the “normal” range (and sometimes even before the homocysteine level in the blood rises). Nevertheless, in people who have a borderline vitamin B12 level, the blood homocysteine level should be checked.

The dietary concern about vitamin B12 is that it is present almost exclusively in animal products, with beef and fish being the best sources. Dairy products and eggs contain some vitamin B12, but the vitamin B12 from eggs does not appear to be as well absorbed as that from chicken. Small amounts may be present on some plants as a result of bacterial contamination, and organically grown vegetables may contain small amounts as well. Before the era of modern food processing, many people probably obtained adequate amounts of vitamin B12 from plant sources. For example, strict vegetarians in India may have no problem there but become vitamin B12 deficient when they move to England, where they consume a sanitized version of the same types of food.

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Another potential source for vitamin B12 is the bacteria in our intestines. We have a large number of bacteria in our large intestines, and they do make some active vitamin B12. Unfortunately, the large intestine cannot absorb this vitamin. Some people may have bacteria in their small intestines as well that can produce small amounts of vitamin B12, but in this location it can be absorbed, aided by a protein called intrinsic factor that floats down from the stomach and facilitates vitamin B12 absorption. It would be foolish for most people to depend on these bacteria as their sole source of the vitamin, however.

Most people who develop vitamin B12 deficiency do so because of agerelated changes in the stomach that reduce or eliminate acid production. Without acid, vitamin B12 cannot be cleaved from the protein in foods and is not absorbed. The lack of acid also allows bacteria to grow in the stomach, and these bacteria can consume any vitamin B12 that comes their way. Less commonly, the production of intrinsic factor may be reduced as well. With a normal stomach and small intestine, the amount of vitamin B12 required in the diet is extremely small. The recommended dietary allowance is 2.4 micrograms a day. Further, the body is extremely efficient at recycling vitamin B12, so it can take years to use up the body’s stores.

Nevertheless, since the consequences of vitamin B12 deficiency are serious, everyone should make sure to include a source of this vitamin in their diet. A nonanimal source

that contains the active vitamin is the Red Star brand of nutritional yeast (vegetarian formula). Nutritional yeast has a nutty, cheesy taste and is good in soups and as a cheese substitute.

Cereals and soy milks may be fortified as well. The vitamin comes from bacterial cultures in these cases. Organically grown vegetables, as mentioned before, contain minute amounts of vitamin B12. Dried sea vegetables and bluegreen algae probably do not contain significant amounts of active vitamin B12 and should not be relied on as a source of this vitamin. They contain analogues of vitamin B12, substances that are very similar to the actual vitamin but that do not function in the same way.

For people with no vitamin B12 sources in their diet, I recommend taking 1,000 micrograms of vitamin B12 once a week or 2,000 micrograms every two weeks. People who obtain a little from dairy and eggs but not from meat or fish should probably supplement with about half of the above amounts.

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Folate

Folate (also folic acid or folacin) is another of the B vitamins that is active as a coenzyme in chemical reactions involving amino acids and DNA. Deficiency can cause anemia. Pregnant women who are

deficient are at increased risk for certain birth defects (neural tube defects) in their offspring. Leafy green vegetables, asparagus, beets, orange juice, and many beans are all

rich sources of folate, although processing and heat can destroy some of it. Because of the problem with pregnant women, refined flour is now fortified with this vitamin. Predictably, studies show that vegetarians maintain significantly higher levels of folate than do nonvegetarians.

Biotin

Biotin is also a vitamin and is a coenzyme in chemical reactions involving sugars and fatty acids. Deficiency is rare, although it can be induced by eating a great deal of raw egg white, which contains a substance that binds to biotin. Some biotin is produced by the bacteria in our intestines, but it is also present in a number of foods. Animal products had been felt to be the best sources, but a study showed that strict vegetarians who ate no animal products, including dairy or eggs, had higher levels of biotin than vegetarians who ate dairy and eggs, who in turn had higher levels than people who ate mixed diets that included meat. Whether the extra biotin in the vegetarians came from their diets or from “healthier” bacteria in their intestines is unknown.

Pantothenic Acid

Pantothenic acid is a B vitamin that plays a role in the production of energy from carbohydrates and in fatty acid metabolism especially. Deficiency is virtually unknown. It is in a wide variety of foods, and intestinal bacteria may produce it as well. As with many other vitamins, food-processing techniques such as freezing and canning can destroy a significant amount of the pantothenic acid present in various foods.

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Ascorbic Acid (Vitamin C)

Vitamin C has many functions in the body. It is necessary for the formation of collagen, a structural protein important for wound healing, and it plays a role in the formation of certain neurotransmitters.

Vitamin C also influences white blood cell function and is important for a strong immune system. Its antioxidant properties are well known, and it restores vitamin E to its

active state. Deficiency of vitamin C is called scurvy, which is marked by bleeding gums, fatigue, muscle ache, tiny hemorrhages in the skin, and joint pains.

The best food sources of vitamin C are fruits and vegetables, including citrus fruits; red and green peppers; chili peppers; strawberries; kiwifruits; tropical fruits like papayas, mangoes, and guavas; cantaloupes; cruciferous vegetables; tomatoes; potatoes; sweet pota-

toes; and many more. Animal products contain minimal to no vitamin C. The current RDA is 75 milligrams a day for women and 90 milligrams a day for men, but the optimal

level is probably much higher. I normally recommend at least 200 milligrams a day. People who consume a great deal of fruits and vegetables obviously obtain much more vitamin C than people who don’t.

Two other warnings—vitamin C is easily destroyed by cooking, and smokers have significantly lower levels of vitamin C in their bloodstream than do nonsmokers.

Vitamin D

Vitamin D is really a steroid hormone rather than a vitamin. That is because the skin—the largest organ in the body—manufactures vitamin D from

7-dehydrocholesterol, a form of cholesterol, after exposure to sunlight. Vitamin D aids in the absorption of calcium from the intestine, helps the kidney regulate calcium lev-

els, and helps create and maintain strong bones. It helps support the immune system as well. One study showed that adequate sunlight exposure

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may reduce the risk of breast cancer, a finding that may be explained by the immune-enhancing effects of vitamin D. Deficiency of vitamin D can cause a bone problem called rickets in infants, and it may also contribute to osteoporosis in adults. Although vitamin D can be obtained from foods, sunlight exposure is a much more important factor for maintenance of adequate vitamin D levels. Hence, it is preferable to obtain vitamin D from sunlight rather than from dietary supplementation.

There are few dietary sources of vitamin D. Fatty fish, egg yolks, and mushrooms contain moderate amounts, but the main food source in the United States has been fortified food products. Dairy products naturally contain very little vitamin D, but they have usually been fortified by the time they reach the consumer, at least in the United States. Unfortunately, this fortification is not always done accurately, and consumers have occasionally been exposed to excessive amounts of vitamin D. Other foods and beverages, such as soy milk, are often fortified with vitamin D as well.

Vitamin E

Vitamin E is a major component of the body’s antioxidant defense system. Deficiency can lead to sterility as well as muscle and nerve problems. Vitamin E is in the lens of the eye and may help pre-

vent cataract. It is also present in the retina, where it may help prevent macular degeneration. Vitamin E in nature actually comprises

eight different compounds: alpha-, beta-, gamma-, and delta-tocopherol; and alpha-, beta-, gamma-, and delta-tocotrienol. No one food source is rich in all of them, but wheat and soy oils together cover the spectrum fairly well. Most animal products provide small amounts of alpha-tocopherol and hardly any of the other forms.

The average diet contains more gamma-tocopherol than alpha-tocopherol, but the bloodstream maintains higher levels of alpha-tocopherol. As a result, it has been assumed that alpha-tocopherol is the more important and the more biologically active. This may not always be the case, however. One study showed that gamma-tocopherol was more effective than alpha-tocopherol in detoxifying nitrogen dioxide, a powerful prooxidant in cigarette smoke. Gamma-tocopherol may also be more effective in preventing the development of cancer under

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some circumstances. A recent study found that higher blood levels of gammatocopherol may provide protection against heart disease. In addition, some of our body tissues may preferentially take up gamma-tocopherol as compared with alpha-tocopherol. The tocotrienols have important antioxidant properties as well and are being investigated for possible cholesterol-lowering effects.

We should try to ingest the full range of vitamin E compounds that we find in foods, as opposed to alpha-tocopherol alone, as is found in supplements (even most so-called mixed tocopherol vitamin E supplements contain over 95 percent alpha-tocopherol). Another problem with vitamin E supplements is that large dosages (over 400 IU a day) of alpha-tocopherol drastically lower the levels of gamma-tocopherol in the bloodstream, an action that may have harmful consequences. These findings may explain why a few studies have found beneficial effects from consuming vitamin E from foods as opposed to vitamin E from supplements.

Vitamin E is carried in the blood by the proteins that carry cholesterol. The ratio of vitamin E to cholesterol in the bloodstream may be important, especially with regard to the risk for developing hardening of the arteries. One study found that vegetarians have higher levels of vitamin E relative to cholesterol than do nonvegetarians. This can be explained by the fact that the foods richest in vitamin E are whole grains (containing the germ), sweet potatoes, green vegetables, soy products, peanuts and most nuts, mangoes, and, to some degree, apples, pears, and other fruits.

Vitamin K

Vitamin K is a group of substances involved in producing certain proteins, most notably those having to do with blood clotting. These substances, called phylloquinone and menaquinones, occur in plants, animals, and bacteria (including those in our intestines). Interestingly, there are vitamin K receptors on our bones, and maintaining an adequate vitamin K status may help prevent osteoporosis. Deficiency is marked by bleeding problems. Since vitamin K is a fat-soluble vitamin, deficiency may occur in people who have fat malabsorption problems. Chronic disease and long-term antibiotic therapy that wipes out many of the normal intestinal bacteria increase the risk of deficiency. Leafy green vegetables are considered the best sources of vitamin K.

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Minerals

Minerals are crucial for all bodily functions. In addition to their obvious contribution to bone and cartilage development, minerals are required for the proper functioning of many enzymes and hormones. We require more of certain minerals than of others, but a deficiency of any of them can lead to serious problems. Therefore, maintaining adequate levels in the body is just as important for minerals as for vitamins. In some cases, it may be hard to determine whether a deficiency of a particular mineral is present, because the level of the mineral in the bloodstream may not reflect the level in the body tissues where it is most active. Let us now look at the major minerals and see what role they play in promoting good health, including the health of the eyes.

Iron

Iron is best known for being a part of the hemoglobin molecule, the oxygen carrier of red blood cells. However, it is also part of myoglobin, a similar type of protein in muscle cells, and it acts as a cofactor in conjunction with many enzymes. Fatigue and compromise of the immune system are early signs of mild iron deficiency. For example, an immune system slightly compromised from iron deficiency may increase the likeli-

hood of recurrences of herpes infections in the eye. As the deficiency becomes more pronounced, a type of anemia occurs marked by small, pale red blood cells. Iron deficiency can

occur as the result of blood loss or because of inability to meet the increased need for iron caused by rapid growth or pregnancy. In the elderly, iron may not be absorbed as well as it is in younger people. And dietary factors can play a role. However, excessive iron intake and storage can cause serious problems. About 10 to 15 percent of the population has a tendency to store too much iron. This excess iron may promote oxidation reactions in the body, and some feel it is a major risk factor for heart disease. Thus, it is important to avoid both deficiencies and excesses of iron.

Absorption of iron from foods depends on many factors, including the amount and form of iron in food products, the amount of iron stores in an

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individual, and the presence of inhibitors or promoters of iron absorption in the foods consumed. Iron from animal products is in the form of heme iron, which is more readily absorbed than the nonheme iron derived from plant foods. Therefore, people who eat red meat often absorb more iron from their foods than people who do not eat red meat. There is evidence that people who switch to diets containing primarily nonheme iron adjust rapidly and begin to absorb a larger percentage of the nonheme iron than they were absorbing before. People at higher risk for iron deficiency who want to avoid meat consumption can improve their iron stores by paying attention to the factors that promote or inhibit iron absorption, which we will soon discuss. Even if not eating meat tends to lower the body’s iron stores somewhat, it probably also decreases the risk of toxicity from too much iron.

Plant foods that contain good amounts of iron include whole and fortified grains, lentils, garbanzo beans, Swiss chard, spinach, potatoes, watermelon, strawberries, almonds, blackstrap molasses, kale, and broccoli. Seitan, a meat analogue (fake meat) made from the gluten (protein) portion of wheat, is often a very rich source.

Vitamin C, including that present naturally in foods, greatly enhances the absorption of nonheme iron. That is why foods like potatoes, watermelon, and strawberries are such good sources: They not only have a fair amount of iron but also contain vitamin C. Eating other vitamin C–rich foods at the same meal has the same effect.

Soy products contain moderate amounts of iron, but the iron is not well absorbed. Fermented soy products such as tempeh, which have less of an inhibitory substance called phytate, allow slightly better iron absorption than other soy products such as tofu, but it is still not very good. Some other inhibitory factor associated with the protein fraction of soy products seems to be responsible for the poor iron absorption. One study found that some young Buddhist adults in China who ate a diet very rich in soy products showed compromised iron status.

Tea contains inhibitors of iron absorption called tannins, so at-risk individuals should avoid drinking tea with meals. Excess fiber, as might be seen in bran-enriched breads or muffins, can also have an inhibitory effect.

Dairy products may promote iron deficiency in several ways. First, they contain almost no iron and are being substituted for foods that do contain iron. Second, their calcium content is high, and high concentrations of cal-

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cium in foods may inhibit iron absorption. Third, there has even been the suggestion that the types of proteins in dairy products may inhibit iron absorption somewhat. Therefore,

minimizing or eliminating dairy products should improve your iron status. Remember, there are many good sources of calcium besides dairy products.

Calcium

Calcium is the most abundant mineral in the body, and almost all of it is in the skeleton. However, it has other important functions as well, playing a role in blood clotting, nerve and muscle activity, and other areas. A calcium deficiency can lead to bone problems in both the young and the old. Adequate calcium intake is important in children because that is the period of life in which the bone mass, which must last a lifetime, is determined. A great deal of attention has been focused on calcium intake with regard to the development of osteoporosis in adults. However, there is a lack of evidence indicating that higher dietary calcium intakes in adults, especially in the form of dairy products, has any real impact on the incidence of bone fractures. Calcium intake is just one of many factors involved in osteoporosis.

Absorption of calcium from foods depends on the amount in the food and the type of food. About 30 percent of the calcium from milk is absorbed. By comparison, many of the dark, leafy green vegetables that are rich in calcium, such as kale, mustard greens, broccoli, and turnip greens, have a fractional absorption (percent absorbed) at least

as high if not higher. Spinach is the exception: Although rich in calcium, only about 5 percent is absorbed. It contains some calcium absorption inhibitors. The majority of the

calcium from whole wheat bread is absorbed, indicating that the amount of fiber and other substances present does not interfere with absorption. Other good dietary sources include collard greens, bok choy (Chinese cabbage), almonds, figs, tofu processed with calcium sulfate, and blackstrap molasses. This form of molasses is rich in both iron and calcium, although bear in mind that the sugar in blackstrap molasses makes you lose much of the calcium in your urine. It is also worth mentioning the alternative grains amaranth, teff,

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