The Encyclopedia Of Nutrition And Good Health - XYZ pps

Zinc also aids the interaction of insulin with its target tissues to facilitate the uptake of blood sugar.. Furthermore, zinc deficiency in the womb can lead to a weakened immune system

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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 TOLERANCEin 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

-CAROTENEthat cannot be converted to vitamin A

XANTHOPHYLLbelongs 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 RIBOFLAVINfor 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 MANNITOLand

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

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 condifermenta-tions, storage condifermenta-tions, 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 perper-cent; 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-on-again DIETING, some people may gradually adapt to using food more efficiently, possibly mak-ing weight loss more difficult with repeated dietmak-ing 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.)

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677

zeaxanthin A yellow-pigmented XANTHOPHYLL

derived from BETA-CAROTENE This CAROTENOIDhas

powerful ANTIOXIDANTproperties 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 RADICALSand 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

DNApolymerase, 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 synsyn-thesis 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 SUGARregulation Thus zinc

is classified as an ANTIOXIDANTwhen 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 BRANcontain 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

-DERSmay 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

-CIUMdisplace 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 STOMACHto form PEPSINfor protein digestion in the stomach; chymotrypsinogen and trypsinogen secreted by the PANCREAS to form CHYMOTRYPSIN

and TRYPSIN for protein DIGESTIONin the intestine Secretion of these DIGESTIVE ENZYMESas zymogens normally assures their safe transit through the cell before activation so they do not attack the tissue that is their source

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