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