The Encyclopedia Of Nutrition And Good Health - U ppsx

U
634
ubiquinone
See
COENZYME Q.
ulcerative colitis See
COLITIS.
ulcers See DUODENAL ULCER; PEPTIC ULCER.
ultraviolet light (UV light) A form of radiant
energy that lies between visible light and X rays in
the electromagnetic spectrum. Ultraviolet light has
a shorter wavelength than visible light and there-
fore has more energy than visible light. In fact, UV
light can induce chemical reactions in the body and
in manufacturing processes. UV light from the sun
converts a cholesterol product in the skin called 7-
dehydrocholesterol to vitamin D
1
. Commercial
treatment of the plant
STEROL, ERGOSTEROL, with
UV light produces vitamin D
2
. UV light is used to
sterilize milk and foods and has other industrial
uses because it can destroy viruses and bacteria.
UV light is potentially harmful to eyes and
SKIN.
Lifelong exposure increases the risk of
CATARACTS
due to damage to the lens of the eye. UV exposure is

also implicated in damage to the cornea and the
retina. Sunburn is an immediate symptom of the
dangers of overexposure to UV light. Prolonged
exposure to UV light in sunlight can cause prema-
ture aging of the skin; loss of the skin’s elasticity
causes wrinkles. Long-term exposure to sunlight can
cause rough, brownish, scaly growth on the skin, a
precancerous condition called actinic keratosis. Tan-
ning increases the risk of skin cancer, the most com-
mon type of cancer; malignant melanoma is the
most serious form. A second type of skin cancer,
basal cell carcinoma, does not spread to other parts
of the body and is more prevalent. Above 1 million
new cases were diagnosed in 2001. Those at greatest
risk are fair-skinned people with light-colored eyes;
those who have a high incidence of moles or who
have a family history of cancer; and people who
spend long periods of time out of doors.
Preventing skin cancer involves protection from
direct exposure to sunlight. Specialists recommend
wearing UV-absorbing sunglasses and using appro-
priate sunscreen lotion whenever individuals are
out in the sun. Nutrition seems to play an impor-
tant role in limiting the effects of long-term UV
exposure. The reason for this lies in the nature of
the damage induced by UV light. UV light causes
the formation of free radicals, highly reactive
chemical fragments that randomly attack cell com-
ponents like protein. When free radicals attack
DNA, they can cause mutations, which can be a

prelude to cancer. Nutritional
ANTIOXIDANTS are
agents that counter the effects of free radicals by
either squelching them or by preventing their for-
mation. Included in the family of antioxidants are
VITAMIN C, VITAMIN E, and BETA-CAROTENE. Certain
trace minerals like selenium and zinc can also
effectively protect cells against oxidative damage.
(See also
MACULAR DEGENERATION; CARCINOGEN.)
undernutrition See MALNUTRITION.
unsaturated fat FAT that contains a relatively
high percentage of
FATTY ACIDS containing chains of
carbon atoms that are deficient in hydrogen atoms
and possess double bonds. Each double bond lacks
two hydrogen atoms:
Fish oils and most vegetable oils are unsaturates.
In contrast, animal fat and shortening are saturated
fats, enriched in saturated fatty acids whose carbon
atom chains are filled up with hydrogen atoms.
These important chemical differences govern the
physical and chemical properties of fats. Unsatu-
rated fats are liquids at room temperature and are
sensitive to attack by
OXYGEN, which can lead to
RANCIDITY (decomposition), while saturated fats are
solid at room temperature and are more stable to
heat and oxidation.
Unsaturated fats fall into two groups.

MONOUN-
SATURATED OILS
are rich in the monounsaturated
fatty acid,
OLEIC ACID; this building block lacks a
single pair of hydrogen atoms. Such oils are called
“monounsaturates” or “oleic-rich” oils;
OLIVE OIL is
a typical example. Monounsaturates seem to lower
the risk of
HEART DISEASE by lowering blood CHO-
LESTEROL (a desirable effect) without lowering
HIGH-DENSITY LIPOPROTEIN (HDL), the beneficial
form of cholesterol. Monounsaturates are not usu-
ally partially hydrogenated.
Polyunsaturates are the second type of unsatu-
rated fats. These oils are rich in polyunsaturated
fatty acids, which lack many hydrogen atoms.
Because they are less saturated, polyunsaturated
are more susceptible to oxidation. Vitamin E pro-
tects polyunsaturates against oxidation, and the
requirement for vitamin E increases with increased
consumption of polyunsaturates.
An industrial process called hydrogenation is
used to harden unsaturated fats by adding hy-
drogen atoms and making the fatty acids more
saturated. Partially hydrogenated vegetable oils
maintain some of their unsaturated character and
possess some double bonds; such oils are less sus-
ceptible to rancidity and have a longer shelf life

than untreated oils.
Polyunsaturates provide varying amounts of
ESSENTIAL FATTY ACIDS, required in the diet because
they cannot be manufactured by the body. Most
vegetable oils like soybean oil, corn oil, and saf-
flower oil provide the essential fatty acid,
LINOLEIC
ACID
, a polyunsaturated fatty acid of the omega-6
family, the most prevalent in the diet. These polyun-
saturated oils seem to lower blood cholesterol (a
desirable effect), but in large amounts they also
lower the beneficial HDL. Polyunsaturates lower the
risk of
HEART ATTACK and STROKE because they block
the formation of dangerous blood clots. On the
other hand, in excess they may increase the risk of
CANCER
and lower immunity, at least in lab animals.
Certain seed oils like
FLAXSEED OIL and fish oil
supply the second essential fatty acid,
ALPHA
LINOLENIC ACID
, and other omega-3 polyunsatu-
rated fatty acids. Fish oils supply large polyunsatu-
rated fatty acids that may help prevent or decrease
the symptoms of heart disease and
HYPERTENSION
and of inflammatory conditions like RHEUMATOID

ARTHRITIS
.
Unsaturated fats produce essentially the same
number of calories as saturated fats when burned
for energy, nine calories per gram—more than
twice the amount derived from carbohydrate or
protein. Both saturated and unsaturated fats when
consumed in excess can lead to weight gain. Cur-
rent dietary guidelines call for selecting a diet that
is lower in total fat (less than 30 percent of total
daily calories, with saturated fat representing less
than 10 percent of calories) in order to lower the
risk of cancer and heart disease. (See also
FAT
METABOLISM
.)
Judd, J. T. et al. “Effects of Margarine Compared with
Those of Butter on Blood Lipid Profiles Related to Car-
diovascular Disease Risk Factors in Normolipidemic
Adults Fed Controlled Diets,” American Journal of Clin-
ical Nutrition 68, no. 4. (1998): 768–777.
urea The major nitrogen-containing waste pro-
duced by the body. Urea production is the body’s
way of safely disposing of ammonia, the potentially
dangerous product released when
AMINO ACIDS are
broken down. Urea is nontoxic; it accounts for 60
percent to 90 percent of total excreted nitrogen.
Urea production occurs in the
LIVER as part of

the body’s detoxification function; the amount pro-
duced is proportional to the amount of protein
consumed. The liver uses a group of specialized
enzymes (the
UREA CYCLE) to produce urea. Urea is
released into the bloodstream and transported to
the kidney for excretion in the urine. Normally 20
to 35 g of urea are excreted each day. Blood urea
nitrogen (BUN) refers to the measurement of blood
urea levels and is used as a diagnostic tool to assess
liver and kidney function. The amount of urea
nitrogen in a 24-hour urine sample can be multi-
urea 635
plied by 6.25 to estimate the amount of protein
used per day. (See also
AMINO ACID METABOLISM.)
urea cycle A collection of enzymes responsible for
transforming
AMMONIA to UREA. Ammonia is a toxic
by-product of amino acid breakdown (
AMINO ACID
METABOLISM
) that is released into the bloodstream.
The brain is extremely sensitive to ammonia; there-
fore, its disposal is important. To detoxify this sub-
stance and simplify its elimination from the body,
ammonia is efficiently removed from the blood by
the liver, where it is combined with bicarbonate to
create a safe product, urea, to be excreted by the kid-
neys. The liver also disposes of ammonia produced

by gut bacteria and absorbed by the intestine.
ORNITHINE
, a nonprotein amino acid that accepts
ammonia, is regenerated when ammonia is released.
It is worth noting that the urea cycle produces the
amino acid
ARGININE. However, the production of
this amino acid can be limited during illness and
periods of rapid growth, and dietary sources are
important. (See also
GLUTAMINE; KREB’S CYCLE.)
urethane A cancer-causing agent in alcoholic
beverages. Urethane has been found to form spon-
taneously in
WINE, BEER, and hard liquors. Bourbon
whiskey, fruit brandies, and some desserts and
table wines sold in the United States may contain
high levels, exceeding levels permitted in a number
of countries, including Canada. Research is under-
way to lessen urethane contamination of alcoholic
beverages. Because the
PRESERVATIVE DEPC (die-
thylpyrocarbonate) spontaneously breaks down
into urethane, it was banned by the FDA in 1972.
(See also
CANCER.)
uric acid A waste product form the degradation
of
RNA and DNA. Uric acid is a nitrogen-containing
carbon compound derived from

PURINES, which are
building blocks of DNA, the genetic material of the
cell, and of RNA, which directs the manufacture of
proteins. Because the body cannot decompose the
purine ring system, enzymes convert purines to
uric acid instead. Between 0.5 and 1.0 g of uric acid
is secreted in the urine daily.
A high blood level of uric acid is associated with
GOUT, a form of ARTHRITIS that is characterized by
deposits of uric acid in joints. The risk of gout
increases with a high protein diet, a family history
of gout and an acidic blood pH. Uric acid can form
kidney stones, particularly in patients with gout.
People with gout should minimize their consump-
tion of foods with a high purine content: liver and
organ meats; fish like anchovies,
MACKEREL, and
SARDINES; and yeast. Excessive alcohol can raise
blood uric acid levels and increase the risk of kid-
ney stones and gout. Extra fluid, 10 to 12 glasses of
nonalcoholic, noncaffeinated beverages daily, can
minimize uric acid deposits.
Uric acid performs a useful function for the
body: It is a powerful
ANTIOXIDANT that can destroy
dangerous free radicals, which are linked to degen-
erative diseases and
AGING. Evolution may have
favored humans’ ability to accumulate relatively
high blood levels of uric acid in order to assure the

presence of an antioxidant that is unrelated to diet.
(See also
VITAMIN C; VITAMIN E.)
urinary tract A system made up of two kidneys,
two ureters (tubes leading to the urinary bladder)
and a single urethra, the duct that conveys urine to
outside the body. The primary function of the uri-
nary tract is to assist the body in maintaining a con-
stant internal environment (homeostasis) by
controlling the composition and volume of the
BLOOD. Kidneys aid in eliminating toxic materials,
drugs, normal waste products, and even excesses of
essential nutrients as urine. Kidneys assume a role
in red blood cell production by secreting erythro-
poietin, stimulatory protein. They help regulate
blood pH and form
RENIN, a hormone to regulate
BLOOD PRESSURE
. Finally, the kidneys activate VITA-
MIN D to its hormone form, dihydroxycalciferol
(calcitriol).
The two kidneys are located in the back just
above the waistline. Usually each kidney is
encased in a heavy cushion of fat for protection.
An extremely thin person may lack this fatty sup-
port, and the kidneys may drop, placing a kink in
the ureter. The rate of blood flow through the kid-
ney is extremely high; about 10 percent of the
total blood pumped each minute enters the kid-
neys. This high volume, together with a normal

blood pressure, is required to filter the blood and
to form urine.
636 urea cycle
Each kidney contains more than a million
microscopic functional units called
NEPHRONS.A
nephron resembles a tiny funnel with an extremely
long, twisted stem. Nephrons filter blood: some
materials, like water, pass into the tubules, small
tubes that drain the kidney. Others stay in the
blood. Useful nutrients like
GLUCOSE and minerals
(
ELECTROLYTES) are reabsorbed into the blood. The
sum of these activities is urine.
Filtration usually occurs at a rate of 7,500 ml per
hour, equivalent to 190 quarts of fluid filtered per
day. Normally between 97 percent and 99 percent
of the water is reabsorbed, together with all of the
glucose because this nutrient, the primary brain
fuel, is too precious to waste. However, in
DIABETES
MELLITUS
the blood glucose concentration may ex-
ceed the rate at which it can be reabsorbed by
kidney cells and is excreted in urine. Glucose in
the urine (glucosuria) is a sign of uncontrolled
diabetes.
Sodium reabsorption by the kidneys varies with
blood pressure: When the sodium concentration in

the blood is low, blood pressure drops and renin is
released. This enzyme activates
ANGIOTENSIN
, a hor-
mone that constricts minute arteries located at the
nephron, increasing blood pressure. Angiotensin
further stimulates the
ADRENAL GLAND to release
ALDOSTERONE, which increases sodium and water
reabsorption by the tubules. Conditions that limit
urine output are inadequate fluid consumption,
stress, kidney disease, and
CARDIOVASCULAR DISEASE.
Usually urine is sterile; the closer it gets to the
urethra, the greater the possibility of bacterial con-
tamination. The daily flow of urine out of the body
and the immune defenses are usually adequate to
assure a sterile urinary tract. However, bladder
infections are common among women. Because
over half of these infections involve the kidneys,
they can be serious. Partial blockage of the urinary
tract, for example by stone formation or scarring,
can reduce urine flow and favor infection. Cran-
berry juice contains materials that prevent bacteria
from sticking to the walls of the bladder or urethra,
thus inhibiting bacterial infection. Garlic and
onions also contain substances that limit bacterial
growth. Sugar and sweetened juice can help pro-
mote bacterial growth, however. Drinking plenty
of water (5 pints daily) is recommended.

Stone formation is a second frequent problem of
the urinary tract; 10 percent of men and 5 percent
of women in the United States will experience
stone formation. Calcium as calcium carbonate or
calcium oxalate can become insoluble and form
crystalline deposits when water consumption is
inadequate. Alternatively, uric acid can form stones
when certain anti-cancer drugs are taken, or when
excessive purines (yeast sardines, organ meats like
liver, caviar) are part of the diet. Stone formation
correlates with inadequate water intake and fiber
consumption; high intake of foods like cocoa,
spinach, parsley, rhubarb, and certain nuts contain-
ing oxalate, can favor stone formation in the uri-
nary tract. (See also
AMINO ACID METABOLISM;
ERYTHROPOIESIS; KIDNEY STONES; UREA CYCLE.)
urticaria The medical term for hives, welts that
form in response to irritation or allergy. Urticaria is
typical of an immediate allergy reaction (immedi-
ate hypersensitivity) and is often accompanied by
ASTHMA, itchy eyes, and runny nose. Animal dan-
der, pollen, house dust, and certain foods can cause
urticaria in susceptible people. (See also
ALLERGY,
IMMEDIATE.)
USDA (U.S. Department of Agriculture) A fed-
eral agency responsible for regulating food quality
and safety. The USDA operates under the authority
of the Federal Meat Inspection Act and the Poultry

Products Act, which assign the USDA the responsi-
bility for assuring the safety of
MEAT and POULTRY.
The USDA inspects meat and meat products, as
well as poultry and poultry products destined for
human consumption. It inspects slaughtering and
processing practices. State laws for meat and poul-
try inspection must equal USDA standards.
The USDA sets guidelines for meat labeling and
grades beef according to eating quality for food
companies that pay for this service. Programs for
health and nutrition, nutrition education and
research are carried out by the eight divisions of
the USDA. These divisions are the Agricultural
Research Service; Consumer and Marketing Ser-
vice; Cooperative State Research Service; Annual
and Plant Health Inspection Service; Federal Exten-
sion Service; Cooperative State Research Service;
Food and Nutrition Service; Veterinary Services;
USDA 637
Labeling and Registration Section. (See also
PROCESSED FOOD.)
USRDA (U.S. recommended daily allowances) A
standard for nutrient intake for healthy people,
designed by the U.S. Department of Agriculture
(
USDA) for use on food labels through 1994. The
USRDAs represent the highest
RECOMMENDED
DIETARY ALLOWANCES

(RDAs), which were devel-
oped by the Food and Nutrition Board of the
National Academy of Sciences for several nutrients
in 1968. The USRDAs are not to be confused with
the RDAs, which are not used on food labels. The
list of RDAs is much more extensive now, and sev-
eral values differ from those specified in 1968.
The USRDA was replaced in 1994 by the RDI
or
REFERENCE DAILY INTAKE (RDI) on food labels. For
the time being, the RDI is identical to the USRDA.
Individual USRDAs were condensed into just four
population groupings: adults and children over
four years; infants up to one year; children under
four years; and pregnant or lactating women. Gen-
erally the highest values for a given age group were
used. Thus the USRDAs for adults and children
over four years generally were the dietary
allowances recommended for a teenage male.
When nutritional claims about a food were
made on a food label, or if the food was labeled
“enriched,” or if extra nutrients were added, the
manufacturer was required to list the content of
several nutrients as percentages of the USRDAs.
The values food manufacturers used for the most
common category, adults and children over four,
are shown in the accompanying table.
Nutrient USRDA
Vitamin A 1,000 mcg
Vitamin C 60 mg

Calcium 1,000 mg
Iron 18 mg
Vitamin D 10 mcg
Vitamin E 30 IU
Thiamin 1.5 mg
Riboflavin 1.7 mg
Niacin 20 mg
Vitamin B
6
2.0 mg
Folate 400 mcg
Vitamin B
12
6 mcg
Biotin 300 mcg
Pantothenic Acid 10 mg
Phosphorus 1,000 mg
Magnesium 400 mg
Zinc 15 mg
Iodine 150 mcg
Selenium N/A
Copper 2 mg
For this category the protein value is 45 g of
high-quality protein (equivalent to milk protein,
CASEIN), or 65 g of lower quality protein (less than
casein). (See also
FOOD LABELING.)
National Research Council. Recommended Dietary
Allowances, 10th ed. Washington, D.C.: 1989.
638 USRDA