The Encyclopedia Of Nutrition And Good Health - XYZ pps

X 673 xanthan gum A food additive used as an emulsi- fier in salad dressing, syrup, and pie fillings where viscous mixtures are desired. Xanthan gum is sta- ble to heat and acidic conditions, making it a ver- satile stabilizer. A water-soluble POLYSACCHARIDE, xanthan gum is produced by bacteria and is con- sidered a safe additive. As a supplement, xanthan gum is a source of water-soluble FIBER, and it improves the body’s ability to use glucose ( BLOOD SUGAR), decreasing the need for INSULIN, the hormone responsible for lowering blood sugar. Xanthan gum improves GLU- COSE TOLERANCE in diabetics, apparently by slowing starch digestion and preventing glucose from being dumped into the bloodstream. Xanthan gum may also help lower blood CHOLESTEROL. (See also DIA- BETES MELLITUS; FOOD ADDITIVES.) xanthophyll A yellow pigment related to BETA- CAROTENE that cannot be converted to vitamin A. XANTHOPHYLL belongs to the family of CAROTENOIDS , widely distributed yellow-orange pigments found in many plants. It is often added to poultry feed to color poultry skin yellow and to deepen the color of egg yolks. Consumers often associate these color changes with quality. xenobiotic Any compound found in the body that originated from the external environment. Xenobiotics represent a diverse group of chemicals, ranging from PESTICIDES to drugs, industrial sol- vents and cigarette tar. Xenobiotics are either stored in fatty tissues, such as adipose (fat) tissue and nerves, or they are processed by enzymes and eliminated through the bile, urine, and, to a certain extent, in sweat. However, long-term exposure to environmental chemicals in the home or the work- place can lead to their buildup in the body, ulti- mately leading to illness. Chemical modification of xenobiotics occurs chiefly in the LIVER; lungs, kidneys, and intestines also play a role. Enzymes of these organs convert foreign compounds to more water-soluble forms, so that they can be excreted. To accomplish this feat, the body employs a battery of oxidizing enzymes, called Phase 1 enzymes. These enzymes require IRON, NIACIN, and RIBOFLAVIN for their activity. A sec- ond battery of enzymes, called Phase 2 enzymes, then attaches compounds—including amino acids, sulfate, and sugar acids—to the modified xenobi- otic. The “conjugated” product is generally much more soluble in body fluids, is more easily excreted and is less harmful than the unprocessed xenobiotic. Key nutrients support Phase 2 processes: the sulfur- containing amino acid CYSTEINE, VITAMIN C, PAN- TOTHENIC ACID and niacin. (See also CANCER; DETOXICATION ; GLUTATHIONE.) McFadden, S. A. “Phenotypic Variation in Zenobiotic Metabolism and Adverse Environmental Response: Focus on Sulfur-Dependent Detoxification Pathways,” Toxicology 111 (1996): 43–65. xylitol A sugar alcohol used as a sweetener. Xyl- itol is a derivative of a common pentose (small sugar) that is as sweet as SUCROSE (table sugar). Small amounts occur naturally in PLUMS, RASPBER- RIES, STRAWBERRIES, and vegetables like CAULI- FLOWER and EGGPLANT. The commercial source is birch bark. Xylitol is used in sugar-free gum and diet foods like jams and jellies. It has little effect on BLOOD SUGAR. Like other sugar alcohols, such as MANNITOL and SORBITOL, xylitol resists fermentation by oral bacte- ria. Early studies indicated that chewing sugarless gum sweetened with xylitol after eating sugar-rich foods reduced the rate of dental cavities. More recently, children given xylitol daily seemed to develop less tooth decay. Xylitol may interfere with decay-causing bacteria by blocking their utilization of other sugars. However, the safety of this additive is questionable. Bladder and adrenal gland tumors can form and organs can be damaged when ani- mals consume xylitol for long periods. (See also ARTIFICIAL SWEETENERS ; NATURAL SWEETENERS; TEETH.) 674 xylitol Y 675 yam (Dioscorea spp.) A starchy tuber of a tropi- cal plant. Yams are unrelated to the rich SWEET POTATOES sold in the United States, which they resemble. Yams were cultivated in Africa and Asia as early as 8000 B.C. and are now a staple among people living in tropical regions. They are produced in West Africa, Southeast Asia, and the Polynesian islands. The cush cush yam (D. trifida), the Asiatic yam (D. alata), and the white yam (D. rotunda) are important food yams. Yams can be baked, broiled, roasted, or fried. Unlike sweet potatoes, true yams contain little BETA-CAROTENE. Compared to POTA- TOES, yams contain the same amount of protein, but 50 percent more starch. They contain half as much vitamin C as potatoes. Several varieties of yams contain ALKALOIDS, plant substances that possess powerful physiologic effects. The Asiatic bitter yam, D. hispida, contains diosorine, a toxin that can be removed by soaking or boiling it in water. Chinese yams, D. opposita and D. batatas, are used in Oriental medicine. Certain wild types of yams yield STEROIDS, hormone-like substances that have been used to prepare oral contraceptives and arthritis medications. yeast See BAKER’S YEAST; BREWER’S YEAST; CANDIDA ALBICANS . yellow no. 5 and no. 6 See ARTIFICIAL FOOD COL- ORS. yogurt A fermented milk product. Yogurt is one of the oldest cultured dairy foods. Commercially, a mixture of fresh, skimmed, and nonfat dry MILKS are inoculated with bacteria that ferment milk sugar to LACTIC ACID, which both curdles the milk and acts as a PRESERVATIVE. In terms of nutrients, yogurt resembles the milk from which it was prepared, including the calcium content. The nutritional quality varies with the type of milk, type of lactic acid cultures, fermenta- tion conditions, storage conditions, and additives. Producers may add thickeners ( CARRAGEENAN, modified starch, PECTIN , or AGAR), nonfat dry milk solids and SUGAR, as well as fruit flavor, to yogurt. Acid-producing bacteria such as LACTOBACILLUS ACI- DOPHILUS, L. bulgaricus, and Streptococcus thermophilus are sometimes added as well. Various claims have been made regarding the possible health benefits of yogurt. Studies have shown that yogurt with live cultures of lactic acid- producing bacteria is tolerated by lactose-sensitive people, presumably due to the presence of bacter- ial enzymes capable of degrading LACTOSE (milk sugar) to simple sugars. The conjecture that yogurt offers protection against coronary heart disease by lowering blood CHOLESTEROL arose from observa- tions of Masai tribesmen of East Africa who con- sume large amounts of yogurt-like foods, yet maintain low blood cholesterol levels. Subsequent studies have failed to support this hypothesis. Data on the effects of yogurt on gastrointestinal infec- tions are conflicting. Yogurt’s role in increasing longevity remains unproven. Yogurt has become popular in the United States since the 1960s due to its convenience, variety of flavors, and its image as a health food. Many vari- eties of yogurt are available: fat and nonfat, plain and flavored, frozen yogurts, and yogurt drinks. Yogurt may be pasteurized, so the live bacteria are destroyed. Manufacturers may add back acid- producing bacteria. Nonfat yogurt contains 6 per- cent of its calories as fat; low-fat yogurt, 24 percent; and whole milk yogurt, 48 percent. The nutrient content of 1 cup (227 g) of nonfat yogurt is: 127 calories; protein, 13 g; carbohydrate, 17.4 g; fat, 0.41 g; cholesterol, 4 mg; calcium, 452 mg; potas- sium, 579 mg; thiamin, 0.11 mg; riboflavin, 0.53 mg; niacin, 0.28 mg. One cup of yogurt made from whole milk (227 g) provides 138 calories; protein, 7.9 g; carbohydrate, 10.6 g; fat, 7.6 g; cholesterol, 29 mg; calcium, 275 mg; potassium, 216 mg; thiamin, 0.07 mg; riboflavin, 0.32 mg; niacin, 0.17 mg. Shalev, E. et al. “Ingestion of Yogurt Containing Lacto- bacillus Acidophilus Compared with Pasteurized Yogurt as Prophylaxis for Recurrent Candidal Vagini- tis and Bacterial Vaginosis,” Archives of Family Medicine 5 (1996): 593–596. yo-yo dieting (diet-induced obesity) A pattern of repeated losing and regaining of weight. This pat- tern of weight fluctuation may carry added health risks for overweight people. People whose weight fluctuates repeatedly suffer increased mortality from heart disease as well as other causes. With on- again off-again DIETING, some people may gradually adapt to using food more efficiently, possibly mak- ing weight loss more difficult with repeated dieting cycles, although there is no consensus on the long- term consequences. There are other possibilities: Yo-yo dieting may increase a dieter’s desire for fatty foods and it may lower self-esteem, leading to other ineffective weight-loss strategies. Fat regained after drastic weight loss tends to cluster in the upper body and the abdominal cavity. The so-called android pat- tern, with a waist to hip ratio greater than 1.0, increases the risk of CARDIOVASCULAR DISEASE. The National Task Force on the Prevention and Treat- ment of Obesity concluded that yo-yo dieting may not pose serious health risks (1994). OBESITY is a complex phenomenon due to many factors, in- cluding inheritance and environmental influ- ences. More study is needed to define long-term health effects of repeated dieting. (See also WEIGHT MANAGEMENT .) 676 yo-yo dieting Z 677 zeaxanthin A yellow-pigmented XANTHOPHYLL derived from BETA-CAROTENE. This CAROTENOID has powerful ANTIOXIDANT properties and is plentiful in corn and dark green leafy vegetables such as spinach and collard greens. It is typically found in combination with lutein, a closely-related caro- tenoid. Neither zeaxanthin nor lutein can be con- verted to vitamin A. Recent studies have linked consumption of zeaxanthin and LUTEIN, from foods, with reduced risk of eye diseases such as macular degeneration and cataracts. It is believed that these carotenoids protect delicate eye tissue from damage by FREE RADICALS and that they may act as light fil- ters, preventing harmful blue light from the sun from reaching inner eye structures. zein A protein from CORN. Zein is deficient in two dietary essential AMINO ACIDS, LYSINE, and TRYPTO- PHAN. High lysine varieties of corn have been devel- oped to partially remedy this deficiency. Zein is commercially produced by treating cornmeal with alcohol. It is used as an edible coating for CANDY and shelled nuts and finds many industrial uses, ranging from paper coating to microencapsulation. (See also GLUTEN.) Zen macrobiotic diet See MACROBIOTIC DIET. zinc An essential trace mineral nutrient required for a wide array of metabolic processes. The body contains about 2.2 g of zinc, more than any trace mineral except iron. The highest concentrations occur in the skin, prostate gland, eyes, nails, and hair, although it is widely distributed among tis- sues. Over 100 different ENZYMES (biological cata- lysts) require zinc. For example, zinc is required by DNA polymerase, an enzyme required for the syn- thesis of DNA (responsible for inheritance), and by RNA polymerase, an enzyme required for the syn- thesis of RNA. RNA guides the synthesis of proteins using the genetic information stored in DNA. Gene activators that regulate the expression of genetic information often utilize proteins containing zinc to bind to specific regions on the DNA molecule. Possible Roles in Maintaining Health Other roles range from protection against oxidation to digestion and BLOOD SUGAR regulation. Thus zinc is classified as an ANTIOXIDANT when it functions as a cofactor for SUPEROXIDE DISMUTASE, the enzyme that disarms a particularly reactive form of oxygen. In digestion, the pancreatic PROTEASE, CARBOXYPEP- TIDASE, requires zinc for its protein-degrading action. Furthermore, the hormone INSULIN is processed and packaged as a zinc complex. Zinc also aids the interaction of insulin with its target tissues to facilitate the uptake of blood sugar. Zinc supports normal cell division and growth, the function of cell membranes, the IMMUNE SYS- TEM, BONE calcification and the development and function of male reproductive organs. Many trace minerals and vitamins, including zinc, are required for normal growth and development. Zinc defi- ciency can cause birth defects, complicated deliver- ies and low birth weight, as well as impaired learning and delayed sexual development. Immune System It is well established that zinc stimulates the IMMUNE SYSTEM. Zinc activates T- lymphocytes, the soldiers and generals of the immune system. Furthermore, zinc deficiency in the womb can lead to a weakened immune system at birth and enhanced risk of infection in newborn infants. Furthermore, zinc deficiency may be par- tially responsible for the weakened immunity that so frequently accompanies AGING. Zinc supplemen- tation can improve white cell counts and antibody production in healthy, elderly people. There are intriguing hints that the zinc status of some AIDS patients is marginal; these patients have a severely imbalanced immune system. Zinc may help lessen symptoms of such autoimmune diseases as RHEUMATOID ARTHRITIS , when the body begins to attack its own tissues. Zinc’s role in combating the common cold is controversial. Some studies have shown that zinc supplements can reduce the length and severity of cold symptoms, while other studies have been inconclusive. Additional research is needed to determine whether zinc can help cold sufferers lessen their misery. Cancer A healthy immune system helps pre- vent cancer, and zinc-deficient animals are more sensitive to carcinogens (cancer-causing agents). Patients with prostate cancer have significantly lower zinc levels, as do those with esophageal can- cer. Whether zinc supplementation alone corrects prostate enlargement or slows prostate cancer is unproven. Male Fertility Zinc is implicated in normal prostate functions and male infertility. The male sex hormone, TESTOSTERONE, may regulate zinc metabolism in the prostate, and zinc, in turn, may affect testosterone metabolism in the prostate. Zinc deficiency leads to a lowered sperm count as well as impotence, and initial studies suggest that zinc can be used therapeutically in these cases. Vision Zinc plays a role in vision. Conversion of VITAMIN A to its biologically active form, retinal, requires zinc. Zinc-deficient alcoholics may suffer from night blindness, and zinc has been used in this situation. Aging frequently brings blindness. In cer- tain cases, zinc supplementation may slow the progress of vision loss due to this condition. Tissue Repair Zinc seems to promote wound healing, particularly in zinc-deficient individuals. Zinc is necessary for tissue repair and growth. Zinc ointments have been used to treat ACNE. Taste Zinc deficiency leads to altered taste and smell sensitivity; diminished taste acuity may respond to zinc. Requirements The RECOMMENDED DIETARY ALLOWANCE for adult men is 11 mg and for adult women is 8 mg. Pregnancy and lactation increase a woman’s requirements. Sources The best sources of zinc are animal products. SEAFOODS, MEAT, and POULTRY provide readily absorbed forms of zinc. BREWER’S YEAST, whole GRAINS, and BRAN contain zinc. Zinc in veg- etables and grains is tightly bound, limiting its BIOAVAILABILITY . Recently zinc has been added to parenteral (IV) nutrient formulations and to break- fast cereals. Marginal (subclinical) zinc deficiency can be a problem for many Americans. Early symptoms of a subclinical deficiency include loss of appetite, altered taste and smell, decreased appetite, as well as slow growth in children. Lethargy, white spots on fingernails, slow wound healing, impotence, and delayed sexual development may follow. Chronic dieters, alcoholics, strict vegetarians, and young children with diets compromised by junk food often consume inadequate zinc and other trace nutrients. Some patients with EATING DISOR- DERS may be zinc deficient. Zinc may help patients recovering from injury or infection. Pregnant and lactating women require zinc and other trace min- erals. Elderly people may rely on zinc-deficient foods, a situation that is compounded by their reduced ability to absorb trace minerals like zinc. Strenuous exercise increases zinc loss through sweating and increased excretion, consequently an athlete’s need for zinc increases. Factors that increase the need for zinc include kidney disease, diabetes, cystic fibrosis, INFLAMMA- TORY BOWEL DISEASE , inherited zinc deficiency, and the use of diuretics and laxatives. A high-fiber diet and foods containing PHYTIC ACID can bind trace minerals and limit zinc uptake when large amounts are taken. Likewise, excessive COPPER, IRON, or CAL- CIUM displace zinc and limit its uptake. Safety Zinc is relatively nontoxic, and modest zinc supplementation for insurance may be appro- priate particularly when the diet is compromised or there is maldigestion or malabsorption. The ratio of zinc to copper should be about seven to one, the ratio of the RDAs. Symptoms of zinc excess include nausea, bloating, abdominal cramps, diarrhea, and fever. A high zinc intake (100 to 300 mg daily) may suppress the immune system, lower HIGH-DENSITY LIPOPROTEIN (HDL, believed to protect against heart disease) and block the absorption of copper, creat- ing a copper deficiency. Copper deficiency in turn 678 zinc can increase blood CHOLESTEROL, LOW-DENSITY LIPOPROTEIN (LDL, the undesirable form) and lower HDL, thus increasing the risk of CARDIOVASCULAR DISEASE . (See also ATHEROSCLEROSIS.) Mares-Perlman, J. A. et al. “Zinc Intake and Sources in the U.S. Adult Population 1976–1980,” Journal of the American College of Nutrition 14, no. 4 (1995): 349–357. Mossad, S. B. et al. “Zinc Gluconate Lozenges for Treating the Common Cold,” Annals of Internal Medicine 125 (1996): 81–88. zymogen (proenzyme) An inactive form of an enzyme that is converted in the body to an active enzyme. Examples include pepsinogen, secreted by the STOMACH to form PEPSIN for protein digestion in the stomach; chymotrypsinogen and trypsinogen secreted by the PANCREAS to form CHYMOTRYPSIN and TRYPSIN for protein DIGESTION in the intestine. Secretion of these DIGESTIVE ENZYMES as zymogens normally assures their safe transit through the cell before activation so they do not attack the tissue that is their source. zymogen 679 . inheritance), and by RNA polymerase, an enzyme required for the syn- thesis of RNA. RNA guides the synthesis of proteins using the genetic information stored in DNA. Gene activators that regulate the expression. living in tropical regions. They are produced in West Africa, Southeast Asia, and the Polynesian islands. The cush cush yam (D. trifida), the Asiatic yam (D. alata), and the white yam (D. rotunda). variety of flavors, and its image as a health food. Many vari- eties of yogurt are available: fat and nonfat, plain and flavored, frozen yogurts, and yogurt drinks. Yogurt may be pasteurized, so the