Ординатура / Офтальмология / Английские материалы / The Eye Care Sourcebook_Lavine_2001

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and quinoa, which are being introduced into this country and are good sources of both calcium and iron.

Children should eat at least four or five servings of calcium-rich foods every day. At least one or two of these should be dark, leafy green vegetables. In older people, a good calcium intake is also important, but it may be even more important to avoid the things that

make you lose calcium in the urine: excess protein, especially animal protein; excess salt and sugars; and, to a minor degree, caffeine.

When you lose more than you take in, we call this a negative calcium balance. The amount lost has to come from somewhere, so it comes from your bones. Supplementation with large amounts of calcium can often create a positive calcium balance, but large doses of calcium can interfere somewhat with absorption of iron, zinc, and other minerals.

Magnesium

Magnesium is the “forgotten” mineral. Although the dairy industry tries to remind us about calcium, and the meat industry about iron, who is there to stand up for magnesium? Magnesium interacts with over 300 enzymes and participates in many of the body’s most important chemical reactions. High magnesium intake from food may help pro-

tect against high blood pressure, diabetes, certain heart problems, kidney stones, and osteoporosis, and deficiency may increase the risk of these diseases. In fact, magnesium is probably at least as important as, if not more

important than, calcium in preventing osteoporosis. In some situations, magnesium may either oppose or augment the effects of calcium, and its body levels must therefore maintain a balance with those of calcium.

Mild magnesium deficiency may be much more common than most people realize. That is because the richest sources are green vegetables (chlorophyll, the green coloring in plants, contains magnesium), whole grains (with emphasis on the “whole”), legumes, nuts, and seeds. Most animal products contain much less magnesium. Therefore, magnesium is one of those miner-

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als in which vegetarians surpass meat eaters, and a meat-based diet without much vegetable and whole grain consumption may actually be considered a risk factor for magnesium deficiency. High sugar intake can

increase loss of magnesium in the urine as well, as can high blood levels of sugar, as seen in diabetics. In fact, some people have speculated that magnesium deficiency may increase the risk of diabetic complications such as diabetic retinopathy.

Phosphorus

Phosphorus, usually in the form of phosphate, is involved in numerous chemical processes in the body, but most of it is located in the bones along with calcium. Deficiency in adults is very rare, although taking aluminum hydroxide antacids for long periods of time can tie up enough phosphate to actually produce a deficiency. In general, the amount of phosphate consumed should equal the amount of calcium consumed. Actually, most people in the United States consume significantly more phosphate than calcium. The reason is that animal products are quite high in phosphate, whereas green vegetables are low. Some phosphate also comes from the phosphoric acid often used in soft drinks. This excess phosphate may cause the parathyroid glands to secrete extra parathyroid hormone, producing a condition called secondary hyperparathyroidism. This extra parathyroid hormone may then cause loss of calcium from bone. Some studies have supported this scenario by showing that higher dietary phosphate intake is associated with lower bone mass. Thus, high phosphate intake may be a risk factor for osteoporosis.

Zinc

Zinc is another trace mineral that most people don’t think about as often as they should. It acts as a cofactor for numerous enzymes and is important for a strong immune system and for growth. The eye and the prostate gland both contain high levels of zinc, and as discussed thoroughly in chapter 12, a high zinc intake may help prevent the progression of age-related macular degeneration, the

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number one cause of poor vision among the elderly. Zinc deficiency causes an impaired immune system, growth retardation in children, skin abnormalities, and other symptoms. The exact amount of zinc required is not certain, but the RDA is 11 milligrams for men and 8 mil-

ligrams for women. Zinc is one mineral that may be a little more difficult to obtain from a plant-based diet as compared with the average American diet.

Meat contains zinc, but good plant sources include whole grains (especially rye), wheat germ, black-eyed peas, sesame and pumpkin seeds, most types of beans, lentils, peas, peanuts, pecans, almonds, cashews, chestnuts, pine nuts, asparagus, spinach, and mushrooms. Although phytate, a substance found in abundance in cereal grains and some vegetables, may inhibit zinc absorption in a plant-based diet, it is probably not a major problem in the United States as compared with countries where flat breads predominate. The reason is that the yeast in leavened breads produces a phytase enzyme that eliminates much of the phytate. Nevertheless, because of the potential for mild zinc deficiency, it is important to pay attention to its dietary sources.

Toxicity from too much zinc can occur as the result of high-dose supplementation. Potential problems include deficiencies of copper, iron, manganese, and other minerals; lowering of HDL cholesterol, the “good” form; immune system impairment; and perhaps accelerated development of Alzheimer’s disease.

Copper

Copper is important for many bodily functions, including blood cell production and maintenance of healthy nervous and cardiovascular systems. Copper deficiency can cause anemia, low white blood cell counts, and possibly even heart disease. Mild deficiency, which cannot be detected by a simple measurement of the copper level in the blood, can depress the activity of important antioxidant and other enzymes, such as superoxide dismutase and glutathione peroxidase (important in the eye). The RDA for adults is 0.9 milligrams per day.

Although some organ meats and some types of seafood are good sources of copper, plant sources provide abundant amounts as well. Soy foods, other beans, lentils, nuts, and seeds are especially rich sources. Consequently, people

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who follow a plant-based diet have a higher copper intake than do people following the typical diet.

Manganese

Manganese is most heavily concentrated in the mitochondria of our cells. Mitochondria are specialized cell parts that act as their powerhouses,creating energy to meet cell demands. One mitochondrial enzyme that contains manganese is superoxide dismutase. This enzyme, which inactivates potentially harmful oxygen free radicals,is different from the other superoxide dismutase (found outside mitochondria), which depends on copper (primarily) and zinc for its activity. Severe manganese deficiency is extremely rare,although it is difficult to determine the incidence of mild deficiency. The blood levels of manganese are fairly low and do not necessarily reflect its concentration in the organs. The liver, kidney, and pancreas are quite rich in manganese, but it is obviously a lot more difficult to biopsy an organ to determine its manganese level than it is to obtain a blood sample!

The richest sources of manganese are whole grains, legumes, nuts, and tea. Fruits and leafy vegetables contain moderate amounts. As expected, then, vegetarians and others who consume a lot of these foods maintain much better levels of manganese in their bodies than do people whose consumption of plant foods is lower. Whether the poorer manganese status in people eating a meat-based diet has any adverse effect on the activity of enzymes like mitochondrial superoxide dismutase remains to be determined.

Selenium

Selenium plays an important role in the body’s antioxidant defense system. It replaces a sulfur atom in the essential amino acid cysteine to form selenocysteine. Selenocysteine becomes part of the glutathione peroxidase enzyme in the eye, which can detoxify hydrogen perox-

ide. Some people have theorized that maintaining adequate amounts of selenium in the body can help prevent certain types of cancer, and several studies support this theory.

Selenium deficiency has also been implicated as a risk factor for an unusual type of heart problem that strikes young people in China.

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Animal products, especially organ meats and seafood, tend to contain more selenium than other foods. Whole grains can be a good source, but the selenium content generally depends on the amount of selenium in the soil, which can vary greatly from one area to another. Nevertheless, the selenium status of vegetarians appears to be about the same as that of nonvegetarians.

Iodine

The thyroid hormones contain iodine. An iodine deficiency results in a hypothyroid (underactive thyroid) state marked by a goiter (enlargement of the thyroid gland). At present, most cases of hypothyroidism are due to a disorder of the thyroid gland itself rather than to an iodine deficiency. Seafood and sea vegetables are rich sources of iodine. Plants contain some iodine, but the amount varies greatly depending on the concentration of iodine in the soil. The use of iodized salt has greatly reduced the incidence of iodine deficiency. It still remains a problem, though, especially in the developing countries. People who consume no sea vegetables or other seafood and who do not use iodized salt are at some risk for deficiency. Therefore, a plant-based diet should include either sea vegetables or iodized salt. Eating large amounts of raw cruciferous vegetables (broccoli, cauliflower, and so on) can also suppress thyroid function because of the presence of chemicals called goitrogens.

Phytochemicals

It is obvious that eating a wide variety of fruits and vegetables in quantity reduces the risk of many diseases, such as cataract, cancer, and cardiovascular disease, but that’s not good enough for some people. They need to know why. This is where phytochemicals (plant chemi-

cals) come into the picture. Fruits and vegetables contain much more than just vitamin C, vitamin E, and beta-carotene. They contain a wide range of substances with diverse

effects, helping to prevent cancer by interfering with virtually every step in the chain of events that leads to the formation of a cancer cell. Many have antioxidant properties that may also protect against heart disease and stroke, keeping the harmful LDL cholesterol from being oxidized to a more atherogenic

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(promoting hardening of the arteries) form. Some have anti-inflammatory properties as well.

Flavonoids and other polyphenols represent a large group of phytochemicals found throughout the plant kingdom. Red wine and grape juice have been found to contain this type of chemical, and some people feel that it may be what is behind the“French paradox,”the low incidence of coronary artery disease among the French. Tea, especially the green variety, contains catechins and other polyphenols that may prevent cancer. Tea consumption was also correlated with a lower incidence of cataracts in one study. Another polyphenol, ellagic acid, is found in high concentration in walnuts, strawberries, and some other berries and is strongly suspected of protecting against cancer by a variety of mechanisms.

Most people are aware that cruciferous vegetables, such as broccoli and cauliflower, may be cancer protective because they contain compounds like indoles and isothiocyanates. But have you heard about the umbelliferous vegetables? This is the group to which carrots, parsley, celery, and parsnips belong. Those of you who are chemists will be glad to know that they contain phthalides, polyacetylenes, polyphenols, monoterpenes, and many other compounds. They are probably at least as important as the cruciferous vegetables. An old-fashioned celery tonic may make you nostalgic, but try a drink combining apple, carrot, and celery juices for a nice refresher.

Soy products such as tofu and tempeh have become more in vogue as a result of the publicity about their cholesterol-lowering and possible cancerprotective effects. They contain substances such as isoflavones (also known as phytoestrogens), saponins, phytates, and lignans, all of which are being actively studied. Many spices also contain powerful phytochemicals. For example, rosemary contains antioxidants that appear to be more potent than vitamin E. And turmeric (an ingredient in mustard) is yellow because of curcumin, another potential cancer preventer.

The list goes on and on, and we have just begun to scratch the surface in discovering the array of phytochemicals present in plants. But we don’t really need more studies to tell us what to eat. Studies of phytochemicals are done to satisfy scientific curiosity, as a way to screen for new drugs, and with the goal of some researchers to supplement unhealthy foods with phytochemicals to make them less unhealthy. For example, people who don’t want to eat their vegetables might one day be able to eat hamburgers fortified with a number of phytochemicals.

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There are a number of drawbacks to this approach, however. First, virtually all the studies done on phytochemicals have been test tube or animal studies. The weakness of animal studies is that they cannot be extrapolated directly to the human situation, since different species

of animals metabolize substances in different ways. And test tube experiments cannot simulate the human condition very well either. Another drawback to studying individual

phytochemicals is that a given phytochemical may not have much of an effect when used alone but may have a synergistic effect when certain other phytochemicals are present. Plants are endowed with certain combinations of phytochemicals, and the human body is adapted to the phytochemical mixtures as they exist in nature.

What all this means is that most of the money being spent on phytochemical research would probably be better spent on educating people to eat their dark leafy greens along with all the other fruits and vegetables, thereby preventing macular degeneration, cataract, and the whole slew of chronic diseases that plague our society. The health benefits derived from eating whole foods greatly exceed that associated with chemical supplementation of unhealthy food products.

Perhaps the most important reason, though, for discussing the subject of phytochemicals is that they represent the most prevalent deficiency in the average American diet. A plant-based diet with minimal or no animal products can easily meet your nutritional needs while avoiding the phytochemical deficiency that characterizes the standard meat-based diet.

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Chapter Nine

1.A. Taylor, P. F. Jacques, T. Nowell, G. Perrone, J. Blumberg, G. Handelman, B. Jozwiak,and D. Nadler.“Vitamin C in Human and Guinea Pig Aqueous,Lens and Plasma in Relation to Intake.”Current Eye Research 16 (1997): 857–64.

2.K.- J. Yeum, F. Shang, W. Schalch, R. M. Russell, and A. Taylor. “FatSoluble Nutrient Concentrations in Different Layers of Human Cataractous Lens.” Current Eye Research 19 (1999): 502–5.

3.C. J. Bates, S. Chen, A. MacDonald, and R. Holden. “Quantitation of Vitamin E and a Carotenoid Pigment in Cataractous Human Lenses, and the Effect of a Dietary Supplement.” International Journal for Vitamin and Nutrition Research 66 (1996): 316–21.

4.L. Chasan-Taber, W. C. Willett, J. M. Seddon, M. J. Stampfer, B. Rosner, G.

A.Colditz, F. E. Speizer, and S. E. Hankinson. “A Prospective Study of Carotenoid and Vitamin A Intakes and Risk of Cataract Extraction in U.

S.Women. The American Journal of Clinical Nutrition 70 (1999): 509–16.

5.L. Brown, E. B. Rimm, J. M. Seddon, E. L. Giovannucci, L. Chasan-Taber,

D.Spiegelman, W. C. Willett, and S. E. Hankinson. “A Prospective Study of Carotenoid Intake and Risk of Cataract Extraction in U. S. Men.” The American Journal of Clinical Nutrition 70 (1999): 517–24.

6.H. W. Skalka and J. T. Prchal. “Cataracts and Riboflavin Deficiency.” The American Journal of Clinical Nutrition 34 (1981): 861–3.

7.S. E. Hankinson, M. J. Stampfer, J. M. Seddon, G. A. Colditz, B. Rosner, F.

E.Speizer, and W. C. Willett.“Nutrient Intake and Cataract Extraction In Women: A Prospective Study.” BMJ (British Medical Journal) 305 (1992): 335–39.

8.J. M. Robertson, A. P. Donner, and J. R. Trevithick. “A Possible Role for Vitamins C and E in Cataract Prevention.” American Journal of Clinical Nutrition 53 (1991): 346S–51S.

9.P. F. Jacques and L. T. Chylack, Jr. “Epidemiologic Evidence of a Role for the Antioxidant Vitamins and Carotenoids in Cataract Prevention.” The American Journal of Clinical Nutrition 53 (1991): 352S–5S.

10.H. W. Skalka and J. T. Prchal. “Presenile Cataract Formation and Decreased Activity of Galactosemic Enzymes.” Archives of Ophthalmology 98 (1980): 269–73.

11.1M. J. Elman, M. T. Miller, and R. Matalon. “Galactokinase Activity in Patients with Idiopathic Cataracts.” Ophthalmology 93(1986): 210–15.

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12.P. F. Jacques, J. Phillips, S. C. Hartz, and L. T. Chylack, Jr. “Lactose Intake, Galactose Metabolism and Senile Cataract.” Nutrition Research 10 (1990): 255–65.

Chapter Ten

1.M. S. Passo, L. Goldberg, D. L. Elliot, and E. M. Van Buskirk. “Exercise Training Reduces Intraocular Pressure Among Subjects Suspected of Having Glaucoma.” Archives of Ophthalmology 109 (1991): 1096–98.

2.E. J. Higginbotham, H. A. Kilimanjaro, J. T. Wilensky, R. L. Batenhorst, and D. Hermann. “The Effect of Caffeine on Intraocular Pressure in Glaucoma Patients.” Ophthalmology 96 (1989): 624–26.

3.K. Lotfi and J. E. Grunwald. “The Effect of Caffeine on the Human Macular Circulation.” Investigative Ophthalmology and Visual Science 32 (1991): 3028–32.

4.C. Pissarello. “La curva giornaliera della tensione nell’occhio normale e nell’occhio glaucomatoso e influenza di fattori diversi (miotici, iridectomia, irido-sclerectomia, derivativi, pasti) determinata con il Tonometro di Schiotz.” Annali di Ottalmologia 44 (1915): 544–636.

5.F. W. Stocker, L. B. Holt, and J. W. Clower.“Clinical Experiments with New Ways of Influencing Intraocular Tension. I. Effect of Rice Diet.” Archives of Ophthalmology 40 (1948): 46–55.

6.N. Naveh-Floman and M. Belkin. “Prostaglandin Metabolism and Intraocular Pressure. British Journal of Ophthalmology 71 (1987): 254–56.

7.J. H. J. Klaver, E. L. Greve, H. Goslinga, H. C. Geijssen, and J. H. A. Heuvelmans.“Blood and Plasma Viscosity Measurements in Patients with Glaucoma.” British Journal of Ophthalmology 69 (1985): 765–70.

8.P. Garcia-Salinas, G. E. Trope, and M. Glynn. “Blood Viscosity in Ocular Hypertension.” Canadian Journal of Ophthalmology 23 (1988): 305–7.

9.H. S. Chung, A. Harris, J. K. Kristinsson, T. A. Ciulla, C. Kagemann, and R. Ritch.“Ginkgo Biloba Extract Increases Ocular Blood Flow Velocity.”

Journal of Ocular Pharmacology and Therapeutics 15 (1999): 233–40.

Chapter Eleven

1.J. Karjalainen, J. M. Martin, M. Knip, J. Ilonen, B. H. Robinson, E. Savilahti, H. K. Åkerblom, and H.-M. Dosch. “A Bovine Albumin

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