The Encyclopedia Of Nutrition And Good Health - E pot

The typical American diet, indeed the common pattern in industrialized nations, is low in fresh FRUIT and VEGETABLES and high in meat and processed food that is high in FAT, sodium, and

218

eating disorders A class of diseases based on

obsessive behavior related to eating: ANOREXIA NER

-VOSA, BULIMIA NERVOSA, and compulsive

overeat-ing Eating disorders affect people from all

socioeconomic classes, and ages range from 3 to 90

Although both men and women can develop

eat-ing disorders, predominantly women are affected

Though factors vary among individuals, the

cycle of eating disorders seems to be initiated by

psychological injury, including physical and

psy-chological abuse among family members, reduced

self-esteem, oppression, social isolation, and

nutri-tional insults, including faulty diet, abuse of drugs,

alcohol, and medications, and food intolerance

The resulting behavioral changes and altered eating

can lead to a cycle of altered diet, altered APPETITE,

and hunger mechanisms worsened by nutritional

imbalances, leading to further eating changes and

compulsive behavior

Anorexia nervosa, self-induced starvation out of

an intense fear of becoming obese, was first

described 100 years ago It now occurs in an

esti-mated 1 percent of American women Although

there is a genetic predisposition, social factors play

key roles in determining the occurrence of

anorexia nervosa Symptoms include a markedly

distorted body image and self-restricted dieting,

leading to extreme weakness, muscle wasting, and

loss of 25 percent of original body weight, and

ces-sation of menstruation Anorexia nervosa is

poten-tially life-threatening

Bulimia nervosa is characterized by recurring

episodes of binge eating (rapid, excessive eating),

followed by purging Most bulimics are women

Bulimia was first described in the United States in

1980 and the incidence in America is increasing It

is characterized by frequent attempts to lose weight

by severely restricted diets, followed by episodes ofbingeing, followed by deliberate vomiting or abuse

of diuretics or laxatives, to lose weight Bulimicsgenerally possess low self-esteem and fear aninability to stop eating Frequent fluctuations inbody weight are common

The third category is compulsive overeating,which refers to episodes of excessive overeating orbingeing in secret Compulsive overeating is ac-companied by repeated attempts to lose weight anddiet The increased occurrence of compulsive over-eating among American teenagers parallels theirincreased prevalence of OBESITY Compulsive over-eating can substitute for confronting life issues anddealing with emotions and their sources

Hazards associated with eating disorders otherthan obesity include starvation, electrolyte andfluid imbalances, liver damage, kidney damage,stroke, cessation of menstruation, diabetes, inter-nal bleeding, and ultimately death

Eating disorders are complex conditions thatprofoundly affect health, and therapy entails mul-tiple approaches including psychological counsel-ing, support groups, nutrition counseling, and, inextreme cases, medical intervention Strategies thatfocus on weight reduction or diet modificationalone are of limited effectiveness in treating eatingdisorders because they fail to resolve underlyingpsychological issues The most comprehensivetreatment programs address emotional, social,physical, and spiritual components Extendedaftercare services as well as active participation inself-help groups support recovery

Claude-Pierre, Peggy The Secret Language of Eating ders: The Revolutionary Approach to Understanding and Curing Anorexia and Bulimia New York: Random

Disor-House, 1998.

eating patterns 219

eating patterns Trends in food consumption in

the United States; patterns of eating behavior,

reflecting food choices based on personal habits,

taboos, customs, and family traditions Food costs

and food availability profoundly influence

day-to-day food choices Many personal values affect food

choices: political conviction, such as conviction

about the environment, or religious preferences

(for example Catholic, Muslim, or Jewish);

percep-tions of social status; conditioning through

adver-tising Food purchases are often made according to

health concerns, thus recent trends in eating fewer

EGGS, less whole MILK, and less red MEATare based

in part upon consumers’ concerns about CHOLES

-TEROLand clogged arteries

The typical American diet, indeed the common

pattern in industrialized nations, is low in fresh FRUIT

and VEGETABLES and high in meat and processed

food that is high in FAT, sodium, and sugar Such a

diet does not supply adequate FIBER, TRACE MINER

-ALS, VITAMINS, and substances in plant foods whose

antiaging and anticancer properties are still being

discovered Sweets and alcohol displace

nutrient-rich foods, and lifestyle choices such as cigarette

smoking and lack of EXERCISEalter nutrient needs

Meal time has changed from eating at home

with family to eating quick meals away from home

For many Americans, half of every dollar spent on

food goes to outside meals Generally, the odds of

eating a less nutritious meal increase the more

meals are eaten away from home Often the

choices rely on convenience foods and snack foods

that contain high levels of fat, sugar, and sodium

Teenagers often obtain 20 percent of their food

from such snacks, which has a long-term impact on

their health

The typical U.S diet is imbalanced due to

nutri-ent inadequacy and nutrinutri-ent excess The typical

diet supplies too many CALORIESand too much FAT

Sweeteners in SOFT DRINKS, snacks, candy, ice

cream, and pastry supply 25 percent of the calories

Each succeeding generation of Americans appears

to get fatter The typical diet supplies more fat than

recommended to lower the risk of cancer and of

heart disease Paradoxically, women often have a

problem in finding diets that will provide all the

vitamins and minerals they need, while lowering

their fat and calorie intake

Results of Surveys

Surveys provide insight about what Americans areeating and how they make food choices Recentpolls to discover whether Americans are becomingmore concerned about nutrition offer conflictingresults On the one hand, many Americans realizethat diet is a factor in health risks On the otherhand, some results suggest that Americans havebecome less concerned about limiting high choles-terol food and obtaining adequate nutrients likevitamins While heads of households have becomemore interested in many foods high in fiber andlower in fat and CHOLESTEROL, many also say thattheir health habits have not significantly changed.Sixty percent of American adults do not get enoughphysical activity and more than 25 percent are notactive at all in their leisure time Only about one-fourth of adults eat the recommended five or moreservings of fruits and vegetables each day

Recent data indicate that Americans are gettingfatter; obesity has reached epidemic proportions A

2000 survey by the U.S Centers for Disease trol and Prevention (CDC) found that more than

Con-45 million adults (about 60 percent of the adultpopulation) are obese The number of young peo-ple (age 6 to 17) who are considered overweighthas more than doubled in the last two decades.That the average American now weighs morethan 20 years ago is perhaps related to the fact thatfood consumption has increased 20 pounds perperson The DIETARY GUIDELINES FOR AMERICANSrec-ommend a diet that is low in saturated fat and cho-lesterol, yet more than half the adult populationfails to meet the recommendations for saturated fatintake The guidelines recommend no more than

30 percent of daily calories come from fat; the age adult gets 33 percent Adults also consume toomany foods high in sugar Americans consume onaverage 20 teaspoons of sugar daily Sodium con-sumption, too, exceeds recommended levels Theguidelines suggest limiting intake to no more than2,400 mg a day On average, men consume 4,000

aver-mg and women consume 3,000 aver-mg

Eating healthier means changing one’s lifestyle

A number of experts recommend gradual changeand moderation, with ample information to enablepeople to make and support wise choices A variety

of food substitutions can be implemented in a

step-wise fashion to achieve a gradual change to

health-ier alternatives The following illustrates initial

substitutions:

Food Healthy Alternative Healthier Alternative

whole milk low-fat milk nonfat milk

cottage cheese low-fat cottage nonfat cottage cheese

cheese sugar cereal nonsugar cereal whole-grain cereal,

non-processed fruit drink orange juice whole orange

soft drink with diet soft drink sparkling water or

sugar, caffeine fruit juice

white bread brown bread 100 percent

whole-wheat bread

(See also FAT, HIDDEN.)

Liebman, Bonnie “The Changing American Diet,”

Nutri-tion AcNutri-tion Healthletter, 22:5 (1995): 9–10.

Physical Activity and Good Nutrition: Essential Elements to

Pre-vent Chronic Diseases and Obesity Atlanta, Ga.: Centers

for Disease Control and Prevention, 2000.

Woteki, Catherine “Consumption, Intake Patterns and

Exposure,” Critical Reviews in Food Science and Nutrition,

35:1,2 (1995): 143–147.

echinacea (Echinacea angustifolia; E purpurea) A

perennial herb that stimulates the body’s immune

system Echinacea is native to North America, from

Texas to the prairies of Canada Native American

tribes have long used this herb as an antiseptic and

as an analgesic (pain reliever) The dried root is

used most often It contains echinacin, a

polysac-charide (complex carbohydrate) that speeds wound

healing and helps maintain connective tissue

Echi-nacea contains inulin, a polysaccharide composed

of fructose that activates the “alternative”

comple-ments pathway, a system that increases defenses

against viruses and bacteria, and speeds the

migra-tion of defensive white cells (such as neutrophils,

monocytes, and lymphocytes) to injured or infected

regions Echinacea stimulates T-lymphocytes,

which help direct immune defenses It also

stimu-lates the production of interferon (antiviral

defense) and of lymphokines, chemicals of the

immune system that stimulate natural killer cellsand scavenger cells (macrophages) Echinacea isregarded as a safe herb

eclampsia (toxemia of pregnancy) A seriousmedical condition that can accompany pregnancy

“Preeclampsia” is the early stage characterized byhigh blood pressure, headache, protein in urine,and swelling (EDEMA) of legs and feet If untreated,the patient may develop true eclampsia

Eclampsia develops in 0.5 percent of patientswith preeclampsia Convulsive seizures can occurbetween the 20th week of pregnancy and the firstweek after birth; eclampsia is usually fatal ifuntreated In addition to the symptoms ofpreeclampsia, symptoms of eclampsia includesevere headaches, dizziness, abnormal pain, nau-sea, convulsions, and possibly coma The kidney,liver, brain, and placenta are affected The causes ofeclampsia are unknown HYPERTENSION and kidneydisease contribute to the problem Treatment ofeclampsia is medical Salt restriction is not part ofthe treatment, and the use of DIURETICSmay simplymask signs and symptoms

Poor nourishment may predispose a woman topreeclampsia Some studies indicate that poorlynourished women develop preeclampsia moreoften, but studies of calcium supplementation forpreventing preeclampsia have had mixed results Arecent study showed that supplemental vitamin Cand vitamin E may reduce preeclampsia in high-risk women, but the authors caution that it is toosoon to recommend supplementation Pregnantwomen should make sure their diet is adequate infood sources of these vitamins and take only thesupplements prescribed by their prenatal careprovider

Anyaegbungm, A., and C Edwards “Hypertension in

Pregnancy,” Journal of the National Medical Association

220 echinacea

a discharge; the condition is not infectious

Aller-gies may be involved; many eczema patients test

positive for allergies and they may exhibit hay

fever symptoms Emotional upset and stress can

aggravate itching Contact with chemical irritants

can trigger eczema Inheritance is a risk factor for

eczema, and dietary factors are involved

The Role of Diet

Often, eczema symptoms improve with an ELIMINA

-TION DIET in which suspected allergy-producing

foods are avoided, suggesting a linkage to food

allergy in some cases In infants, cow’s MILKis the

most common food allergy associated with eczema

ESSENTIAL FATTY ACIDScan help limit the

inflamma-tory process Omega-3 fatty acids are the essential

fatty acids found in flaxseed oil and fish oils, while

omega-6 fatty acids are found in EVENING PRIMROSE

OIL and BORAGE oil Vitamin C and FLAVONOIDS,

plant substances that serve as ANTIOXIDANTS, appear

to help control or limit inflammatory processes

also Vitamin A plays a key role in skin

develop-ment and maintenance, and deficiencies should be

corrected Many eczema patients are ZINCdeficient,

and zinc supplementation may be helpful This

may be linked to the fact that low stomach acid is

very common in patients with eczema, and low

stomach acid contributes to mineral malabsorption,

ultimately leading to eczema Zinc is necessary to

convert essential fatty acids to PROSTAGLANDINS,

hormone-like substances that regulate

inflamma-tion, among other processes

The diet of eczema sufferers should minimize

convenience foods that supply high levels of fat,

sugar, and other additives, while emphasizing

whole foods, particularly fresh fruit and vegetables

Increasing consumption of polyunsaturated FAT

(like SAFFLOWERoil) and FISH OILmay help decrease

inflammation and boost the immune system (See

also ACNE; ALLERGY, FOOD; BREAST-FEEDING;

LEUKOTRIENES.)

edema The swelling of any part of the body due

to fluid accumulation Allergies can cause edema

when the allergic response releases HISTAMINEand

other inflammatory agents that make capillaries

porous so that fluid from BLOODcan then leak into

surrounding tissue, causing puffy eyes and a

swollen face, for example Edema is also one of thesymptoms of preeclampsia in pregnancy General-ized edema can result from serious medical condi-tions such as HYPOTHYROIDISM, kidney failure, liverdisease, and congestive heart failure

Nutritionally-related causes of edema includeexcessive SODIUM, certain nutrient deficiencies, andfood allergies Excessive sodium can cause edema

in about 20 percent of the American populationwho are sensitive to high salt intake Edema is alsoassociated with severe nutritional deficiency dis-eases such as BERIBERI(due to a deficiency of THI-

AMIN) and with protein MALNUTRITION.Edema is often treated with DIURETICS (waterpills), which increase urine production to removewater and salt from the body The use of diuretics

is considered potentially dangerous during nancy Reduced salt intake can be helpful and low-sodium diets may be prescribed (See also

preg-ALDOSTERONE; ECLAMPSIA; HYPERTENSION; KWASH

-IORKOR; SODIUM.)

edible portion The portion of food usually sumed For example, husks, hulls, rinds, peels,seed, bones and gristle are usually excluded fromweight measurements and nutrient compositions

con-EDTA (ethylenediaminetetraacetic acid; disodium EDTA) A FOOD ADDITIVE used as an ANTIOXIDANT

and a preservative to trap unwanted metal ion taminants As an antioxidant, EDTA blocks the for-mation of highly reactive forms of oxygen thatattack fats and oils Attack of food molecules byoxygen requires metal ions as catalysts Whenmetal ion contaminants are trapped by EDTA, they

con-do not form free radicals EDTA is often combinedwith butylated hydroxytoluene (BHT) and PROPYL GALLATE, which work together as antioxidants inprocessed foods

EDTA is added to salad dressings, MARGARINE,

MAYONNAISE, potatoes, peas, and vegetables EDTAtraps metal ions in canned SHELLFISH that wouldpromote off-color and altered taste In BEERit pre-vents excessive foaming and turbidity SOFT DRINK

producers use EDTA to stabilize ARTIFICIAL FOOD COLORS Excessive EDTA would trap essential nutri-ents in the body, such as trace mineral nutrientsand CALCIUM, and therefore typical usage is 0.01

EDTA 221

percent in foods and beverages The body absorbs

only about 5 percent of an oral dose, and absorbed

EDTA is excreted in the urine EDTA is considered

a safe additive In medicine, EDTA is used to treat

metal (LEAD) poisoning and as an alternative

treat-ment in cardiovascular disease (See also CHELATE.)

EFA See ESSENTIAL FATTY ACIDS

egg As food, typically, the ovum of domestic

fowl Eggs are an inexpensive, nutrient-rich food

that provides high-quality PROTEINwith all essential

amino acids Egg protein is often used as a

refer-ence protein in nutritional studies because it is

readily digested and supplies a well balanced

mix-ture of amino acids that are readily absorbed A

popular food, nonfertile chicken eggs are

mass-produced, inexpensive, readily available, and easily

prepared Hens’ eggs contain an inner fluid (the

egg white) which contains half of the protein but

no cholesterol; proteins protect the yolk from

bac-teria, such as AVIDIN, which tenaciously binds the B

vitamin BIOTIN Bacteria requiring this vitamin

can-not grow in the egg white

The yolk provides many nutrients such as

min-erals, vitamins, all of the CHOLESTEROL (213 mg),

and 5 g of fat The yellow color is due to the plant

pigment xanthophyll (lutein) The egg yolk is also

rich in LECITHIN, a phospholipid used to emulsify

other lipids

As food, eggs have limitations: Their CALCIUM

content is low and they lack VITAMIN C In recipes,

egg yolks can often be substituted by using double

the amount of egg white Most commercial EGG

SUBSTITUTESuse egg whites only

Eating more than a few eggs a week can raise

blood cholesterol in sensitive people On the other

hand, moderate consumption seems safe for most

healthy people Perhaps as many as one-third of

Americans are extremely sensitive to dietary

cho-lesterol; their blood levels rise significantly when

they eat cholesterol-rich foods These individuals

should limit their dietary cholesterol to no more

than 300 mg per day Particularly those with a

fam-ily history of early heart disease or high cholesterol

levels need to be watchful More important than

monitoring cholesterol in the diet is limiting fat

consumption, especially consumption of SATURATED

FAT With a low-fat diet, egg cholesterol seems tohave little impact on blood cholesterol levels.Organic chicken eggs, farm fresh eggs, andmass-produced eggs contain the same amount ofcholesterol, although mineral and vitamin contentcan vary according to the chicken feed Efforts arebeing made to increase the content of ESSENTIAL FATTY ACIDSof eggs by varying the hens’ diets.Eggs can be contaminated by trace amounts of

PESTICIDES from feed, so purchasing organic eggsmight be desirable Because raw or undercookedeggs can transmit food poisoning (SALMONELLA),raw cracked eggs should be discarded, and eggsshould be cooked thoroughly Eggs should berefrigerated to prevent bacterial contamination andage-related changes One chicken egg (58 g) con-tains 79 calories; protein, 6.1 g; fat 5.6 g; choles-terol, 213 mg; calcium, 28 mg; iron, 1.04 mg; zinc,0.61 mg; vitamin A, 78 retinol equivalents; thi-amin, 0.04 mg; riboflavin, 0.15 mg; niacin, 0.03

mg (See also BIOLOGICAL VALUE; ORGANIC FOODS.)

egg allergies A very common type of foodallergy, usually due to a sensitivity to egg whites.Egg allergies may trigger symptoms quickly, includ-ing hives, asthma, watery eyes, swelling, and nau-sea shortly after the egg is consumed Alternatively,

a delayed sensitivity with symptoms such asheadaches, DIARRHEA, or CONSTIPATION, can develophours after the food is consumed Because eggs are

so common in processed foods, food labels of bakedgoods should be read carefully (See also ALLERGY,

FOOD; BIOLOGICAL VALUE; FOOD SENSITIVITY.)

eggplant (Solanum melongena; aubergine) Alarge, pear-shaped member of the NIGHTSHADE FAM-

ILY that includes potatoes, tomatoes, and peppers.This vegetable probably originated in India and hasbeen cultivated since antiquity Eggplant is oftendark purple with a glossy skin, but white, yellow,and striped varieties are also cultivated It is a goodsource of FIBERand is an ingredient in many Mid-dle Eastern dishes Ratatouille, a dish based on egg-plant, is part of southern French cuisine

Unripened eggplant may contain the toxic loid SOLANINE, which also occurs in green potatoes.Solanine can cause neurological symptoms anddamage red blood cells

alka-222 EFA

One cup of cooked eggplant (160 g) contains 45

calories; protein, 1.3 g; carbohydrate, 10.6 g; fiber,

6 g; calcium, 10 mg; iron, 0.56 mg; potassium, 397

mg; vitamin A, 10 retinol equivalents; thiamin,

0.12 mg; riboflavin, 0.03 mg; niacin, 0.96 mg;

vita-min C, 2 mg

egg substitutes Cholesterol-free alternatives to

eggs By eliminating the yolk, food manufacturers

have developed several cholesterol-free products

Egg substitutes usually contain egg white (egg

albumin); a partially HYDROGENATED VEGETABLE OIL

like CORN OIL; together with ARTIFICIAL FOOD COL

-ORS, GUMS, EMULSIFIERS, and several VITAMINS and

MINERALSfound in a typical egg Soy protein–based

egg substitutes are also marketed

eicosapentaenoic acid (EPA) A large, complex

POLYUNSATURATED FATTY ACIDfound in FISHand fish

oils EPA belongs to the omega-3 family of

polyun-saturates, derived from the ESSENTIAL FATTY ACID,

ALPHA LINOLENIC ACID EPA, with 20 carbons and

five double bonds, is the parent compound for

PROSTAGLANDINS (PGE3 series) and thrombaxane

A3, hormone-like substances that help

counterbal-ance inflammatory processes triggered by other

prostaglandins (PGE2series) and other

thrombox-ane A2 The latter come from ARACHIDONIC ACID, an

omega-6 polyunsaturate prevalent in meat

There-fore EPA and fish oil tend to balance some of the

effects of a meat-heavy diet Recent research has

focused on the relationship between EPA and heart

disease

Population studies indicate that death due to

heart disease is lower among those who consume

an average of 30 g of fish daily, as compared with

those who eat meat daily Other studies have

yielded mixed results While the results are

sugges-tive, it is not yet clear which of the constituents of

fish oils are more important EPA could reduce the

risk of coronary heart disease by several

mecha-nisms: EPA can inhibit clot formation indirectly It

blocks the formation of thromboxane A2, a potent

factor that causes platelets to clump Clumping of

these cellular fragments in the blood helps form

blood clots within vessels EPA is also converted to

PGI3, a prostaglandin that directly blocks platelet

aggregation EPA and related lipids seem to lower

blood fat levels They may lower serum cholesterol,

if initial levels are elevated and diets are high insaturated fat, by depressing the formation of LOW-

DENSITY LIPOPROTEIN(LDL), the undesirable form ofcholesterol EPA and its relatives may also block theearly stages of atherosclerosis The improvement ofglucose utilization in diabetics who use fish oil sup-plement remains controversial

Using fish oil as a supplemental source of EPAcarries potential hazards Depending on the source,

it may contain industrial pollutants Fish liver oilmay contain high levels of VITAMIN A and vitamin

D, which can be toxic in high doses, as well as ticides and other contaminants Guidelines regard-ing the optimal intake of EPA or of fish oil have notyet been established (See also ESKIMO DIET; OMEGA-

pes-3 FATTY ACIDS.)

elastin A fibrous PROTEIN found in the lung, inlarge blood vessels and in the other elastic connec-tive tissue such as ligaments Elastin differs fromother fibrous proteins like COLLAGEN in that it iscapable of undergoing a two-way stretch, like atrampoline net Synthesis of elastin requires cop-per, and an inherited inability to absorb copper(Menke’s syndrome) causes defective arteries

electrolyte replacement See SPORT DRINKS

electrolytes Electrically charged atoms or cules occurring in the blood and other body fluidsand in solutions in general Electrolytes control thedistribution of water among the blood, cells, and tis-sues, and the spaces between them; thus, electro-lytes help regulate blood volume and composition.The electrical charge of electrolytes enables them tofunction in the transmission of nerve impulses and

mole-in muscle contraction, mole-includmole-ing heart muscle trolytes also help regulate ACID-base balance tomaintain the pH of body fluids close to neutrality.Electrolytes consist of positively charged ions(cations) and negatively charged ions (anions) Thepredominant cations are POTASSIUM, SODIUM, and

Elec-MAGNESIUM, while the major anion is CHLORIDE.These electrolytes are not evenly distributed:Sodium and chloride are concentrated outsidecells, while potassium and magnesium are concen-trated in the cytoplasm (within cells)

electrolytes 223

Sodium, potassium, and chloride are nutrients.

Low levels are excreted daily, and they must be

regularly replaced through foods and beverages

The kidneys reabsorb these electrolytes and play an

important role in regulating electrolyte balance

Electrolytes are secreted by the GASTROINTESTINAL

TRACT in digestive juices and bile For example,

chloride is secreted by stomach glands as

hydrochloric acid and pancreatic secretion, and the

small intestine releases sodium and bicarbonate

However, much of sodium and chloride is

reab-sorbed and recycled

The body malfunctions when electrolytes are

lost—through vomiting, diarrhea, excessive

urina-tion caused by certain medicaurina-tions (diuretics) or

uncontrolled diabetes, or through excessive

perspi-ration Too little sodium causes fatigue, muscle

weakness, or even convulsions Potent diuretics

most frequently cause potassium losses because

they rid the body of excess water Diabetes, severe

burns, and dietary deficiencies can also cause low

potassium Symptoms of potassium loss include a

weak pulse, general weakness, and low blood

pres-sure Magnesium deficiency can be the result of

diarrhea, chronic alcoholism, inflamed pancreas

(pancreatitis), kidney disease, and inadequate diet

Chloride loss can occur with diarrhea and intestinal

disease Because chloride helps prevent excessive

bicarbonate, which is alkaline, low chloride can

cause the body to become excessively alkaline

Mild losses of sodium and chloride are easily

replaced by common beverages and processed

foods Potassium and magnesium are obtained

from whole grains, green leafy vegetables, and

fruits and juices The loss of electrolytes and water

can lead to life-threatening situations more rapidly

than for losses of any other nutrient With

pro-longed electrolyte imbalance, the brain, heart, and

lungs do not function normally, and medical

atten-tion is required to replace fluids and electrolytes

intravenously (See also BULIMIA NERVOSA; DEHY

-DRATION; KETONE BODIES.)

electron transport chain A collection of ENZYMES

responsible for the final stages of oxidation of FATTY

ACIDS, CARBOHYDRATES, AMINO ACIDS, and other fuels

in the presence of oxygen The electron transport

chain is part of the machinery of the MITOCHOND

-RIA, the cell’s powerhouses The tion enzymes are called CYTOCHROMES Electronspass sequentially from one cytochrome to the nextlike a bucket brigade Nutrients like IRONand COEN-

oxidation-reduc-ZYME Qare required for this process In the last step,the enzyme cytochrome oxidase transfers electrons

to molecular oxygen to yield water Cyanide sons cytochrome oxidase, accounting for the toxic-ity of this substance

poi-An essential feature of electron transfer betweencytochromes is the simultaneous synthesis of ATP,which is the cell’s energy currency Chemicalenergy trapped in ATP meets almost all energyrequirements of cells (See also KREB’S CYCLE;

by the body Elemental diets are prescribed for

INFLAMMATORY BOWEL DISEASEand other conditions

in which the digestive and absorptive functions ofthe gastrointestinal tract are severely compro-mised: preparation for gastrointestinal surgery,treatment of burn victims, pancreatitis, and severediarrhea (See also DIGESTION; GASTROINTESTINAL DISORDERS.)

elimination diet/challenge test A simple way ofdetecting food allergies During the initial phase of

an elimination diet, all foods suspected of causingallergies are avoided for five to 14 days, to providetime for the IMMUNE SYSTEM to recover from theirritant During the recovery phase, the diet is lim-ited to “safe” foods and might include foods gener-ally considered to be nonallergenic, such as lamb,rice, pears, and pure spring water or a chemicallydefined meal replacement that incorporateshydrolyzed protein such as rice protein

When symptoms have diminished or peared, individual foods are added back to thediet, one at a time, for example, every two days.During this challenge period, the patient is in-structed to record the recurrence of allergy symp-

disap-224 electron transport chain

toms when a questionable food is added back to

the diet This is a practical way to identify the

offending food

However, patients should not stay on an

elimi-nation diet for a long time because it is too

restric-tive, and patients on such a diet risk inadequate

intake of vitamins and minerals This method is

also cumbersome when there are multiple food

allergies and requires patience and persistence For

severe food allergy symptoms, medical supervision

should be obtained before suspected foods are

eaten (See also ALLERGY, FOOD.)

ELISA ( Enzyme-linked immuno-absorbent assays)

These are sensitive analytical methods used in

clin-ical diagnostic testing These highly sensitive lab

tests use manufactured ANTIBODIES (monoclonal

antibodies), which bind with great specificity to a

particular class of molecules, such as steroid HOR

-MONES, tumor markers, viral antigens, parasitic

antigens, and even other types of antibody

mole-cules The antibody used in the test is tagged with

marker enzymes to permit measuring the degree of

its binding ELISA assays provide a previously

unattainable window for assessing nutrient

condi-tions and fundamental physiologic mechanisms,

like circadian rhythm of hormone secretion, food

allergies, and disease processes such as HIV-related

(human immunodeficiency virus) conditions (See

also ALLERGY, FOOD.)

Emden–Meyerhoff pathway See GLYCOLYSIS

emesis Vomiting of stomach contents, and

possi-bly of intestinal contents in severe situations

Potential causes of emesis include food poisoning;

chemical poisoning; viral infections; drug side

effects; nervous conditions like brain injury,

migraines, meningitis, seasickness; stimulation of

the brain’s vomiting center; gastric conditions such

as cancer and peptic ulcers; and intestinal disorders

like intestinal obstruction Aggravating factors

include excessive pain, sensitivity to certain foods,

and toxic chemicals Odors, shock, nervousness,

anxiety, hysteria, morning sickness, coughing, and

irritation of the pharynx can cause reflexive

vom-iting Chronic vomiting can lead to ELECTROLYTEloss

and tooth erosion due to gastric acid, DEHYDRATION

and MALNUTRITION (See also BULIMIA NERVOSA; EAT

-ING DISORDERS.)

emetic Any agent causing vomiting Chemicalssuch as ipecac syrup, mustard, and zinc sulfate in-duce vomiting by local stimulation Chemical emet-ics may be dangerous in pregnancy, GASTRIC ULCERS,

CARDIOVASCULAR DISEASE, or hernia Drinking largeamounts of warm water can also induce vomiting.(See also BULIMIA NERVOSA; STOMACH ACID.)

empty calories Calories derived from burningfood that provide excessive FAT, sugar, and/or whiteflour but contain far fewer key nutrients than thosefound in minimally processed foods In particular,trace MINERALS, VITAMINS, PROTEIN, and FIBER arelikely to be deficient Foods supplying empty calo-ries are said to have low NUTRIENT DENSITY Alco-holic beverages, SOFT DRINKS, natural sweetenerslike syrup and sugar, many manufactured foods,pastry, DESSERTS, CHIPS, and similar high-fat snackslack the balance of nutrients found in whole foodsand tend to crowd out more nutritious foods.Most healthy people can tolerate an occasionalsplurge on junk food However, problems arisewhen eating empty calorie foods becomes a habit.The more junk food a person eats, the more impor-tant it is for the remainder of the diet to supplyimportant nutrients to make up for the deficiency.All too often, nutrient deficiencies such as folicacid, iron, calcium, even fiber occur High-fat foodsprovide excessive calories, which favors weightgain OBESITYis a growing problem for both Amer-ican adults and children Excessive fat and inade-quate ANTIOXIDANTnutrients like VITAMIN C, VITAMIN

E, and BETA-CAROTENEare linked to CARDIOVASCU

-LAR DISEASE, AGING, and CANCER (See also CONVE

-NIENCE FOOD; DEGENERATIVE DISEASES; EATING PATTERNS; FAST FOOD; REFINED CARBOHYDRATES.)

emulsified vitamins Fat-soluble VITAMINSlike amins A, D, and E that are processed as water-soluble emulsions While not true solutions, emul-sions consist of microscopic particles uniformly dis-persed and stabilized so that oil and water do notseparate Like FAT, these vitamins are normally

vit-emulsified vitamins 225

insoluble in water Emulsified fat-soluble vitamins

are absorbed directly by intestinal cells

Emulsifica-tion facilitates their uptake, an important

consider-ation for individuals with MALABSORPTION (See also

VITAMIN A; VITAMIN D; VITAMIN E; VITAMIN K.)

emulsifiers (stabilizers, surfactants) A class of

FOOD ADDITIVESwidely used in manufactured foods

to suspend oily materials in water These chemicals,

related to detergents, can suspend oils and lipids

(water-insoluble materials) such as dyes in water as

tiny droplets that do not coalesce or separate upon

standing

The most common commercial emulsifiers are

DIGLYCERIDES, MONOGLYCERIDES, LECITHIN, POLYSOR

-BATES, and sorbitan mono-stearate Emulsifiers are

used to keep bread from becoming stale; to stabilize

fat in NONDAIRY CREAMERSfor COFFEE; to keep cakes

fluffy; to suspend flavors and food coloring in

processed foods; and to stabilize ice cream Egg

lecithin is used to emulsify vegetable oils and

vine-gar to create MAYONNAISE (See also CONVENIENCE

FOOD.)

endive (Cichorium endivia) This bitter biennial

or annual herb is a member of the aster family It

was used by ancient Egyptians and Romans and

may have originated in India Endive was adapted

as food in France in the 14th century and is

culti-vated as a salad plant Closely related to CHICORY,

which it resembles, endive is a slightly bitter salad

green The leaves are finely divided and curly and

are clustered in a loose head To decrease their

bit-terness, the leaves are covered several weeks

before harvest It is an excellent source of BETA

-CAROTENE Nutrient content of 1 cup (50 g) is 28

calories; protein, 0.6 g; carbohydrate, 1.7 g; fiber,

0.46 g; calcium, 30 mg; 1.026 retinol equivalents;

vitamin C, 3.2 mg; niacin, 0.49 mg; and small

amounts of other nutrients

endocrine system The collection of ductless

glands that secrete HORMONES, chemical regulators

of the body that travel to their target tissues via the

bloodstream As members of the body’s system of

checks and balances, ENDOCRINE glands respond to

signals from the brain and to changes in chemicals

in the blood, including BLOOD SUGAR, CALCIUM, andhormones

The endocrine system consists of the ADRENAL GLANDS, PITUITARY (hypophysis), endocrine PAN-

CREAS, THYROID, PARATHYROID, testes and ovaries,the pineal gland, and the thymus The placenta alsosecretes hormones Each endocrine gland produces

a characteristic hormone, or set of hormones.Endocrine malfunctions create serious imbalances.Hyposecretion, in which inadequate levels of hor-mones are secreted, and hypersecretion, in whichexcessive amounts of hormones are secreted, causemany pathological conditions ranging from dia-betes to excessive FATIGUEand CATABOLIC STATE.The hypothalamus is the nerve tissue that linksthe brain with the pituitary gland The hypothala-mus indirectly controls many functions It inte-grates and controls the autonomic nervous system,the grouping of nerves that regulates involuntaryprocesses such as smooth muscle contractions (forexample, the movement of food through the GAS-

TROINTESTINAL TRACT and glandular secretions) Inresponse to signals from the brain, and its ownsensing mechanisms, the hypothalamus releaseshormones that regulate the release of six hormonesfrom the pituitary The hypothalamus also formstwo hormones, oxytocin and ANTIDIURETIC HOR-

MONE (ADH), which travel to the posterior itary where they are released

pitu-The functions of individual endocrine glands can

be summarized as follows:

Pituitary Although it is only the size of a pea,the pituitary is considered the “master” endocrinegland because it regulates the activity of so many dif-ferent glands throughout the body ADH regulates

SODIUM and WATER balance while oxytocin lates uterine contraction and the ejection of milkfrom mammary glands The anterior pituitary makes

stimu-“trophic hormones,” hormones that activate otherglands of the endocrine system Trophic hormonesinclude melanocyte-stimulating hormone (MSH) toregulate pigmented cells (melanocytes); PROLACTIN

for milk secretion in women and for TESTOSTERONE

production in males; interstitial cell stimulating mone (ICSH), for testosterone production (men);luteinizing hormone (LH), for ovulation and proges-terone production (women); follicle stimulatinghormone (FSH), for development of ova and estro-

hor-226 emulsifiers

gen production (women) and sperm production in

testes (men); thyroid stimulating hormone (TSH) for

release of hormones from the thyroid gland; GROWTH

HORMONE for tissue repair and maintenance; and

ADRENOCORTICOTROPIC HORMONE(ACTH) for cortisol

release from the adrenal cortex The hypothalamus

regulates the release of growth hormone, thyroid

stimulating hormone, ACTH, FSH, and LH, and

pro-lactin from the pituitary

Adrenal Glands In response to ACTH, the

adrenal glands produce the stress hormones EPI

-NEPHRINE and NOREPINEPHRINE from the inner

region of the adrenals to increase blood sugar, raise

blood pressure, increase pulse rate, and similar

effects that adapt the body to stress The outer

region of the adrenals produce steroids, like GLUCO

-CORTICOIDS, which raise blood sugar, increase the

rate of breakdown of protein and fuels, and MINER

-ALOCORTICOIDS, which conserve sodium and water

Thyroid Gland Thyroid hormones regulate the

rate of energy production of cells, the rates of tissue

growth and development, and activity of the

ner-vous system Calcitonin lowers blood calcium levels

Parathyroid Glands Parathyroid hormone

raises calcium and phosphate ions and activates

bone degradation

Thymus Thymic hormones, thymosins,

acti-vate the immune system by promoting the

prolif-eration of T cells, a major type of white blood cell

Islets of Langerhans Clusters of endocrine

cells scattered throughout the pancreas The islets

contain the alpha cells that produce GLUCAGON to

increase blood glucose; and the beta cells that

pro-duce INSULINto lower blood glucose

Ovaries ESTROGENS (from the follicles) are

female sex hormones that control the growth and

development of female reproductive organs and

secondary female characteristics and protein

syn-thesis They also produce PROGESTERONE(from the

corpus luteum), which helps prepare the

endo-metrium for implantation of a fertilized egg, and

promotes milk production

Gonads Produce testosterone in men This

hormone controls growth of male sex organs, body

growth, secondary male characteristics, and sperm

development

Pineal Gland Produces melatonin, which has

an inhibitory effect on ovaries The pineal gland

helps regulate circadian rhythms and may inhibitreproductive activities

Placenta Serves as a temporary endocrinegland until birth During pregnancy the placentasecretes chorionic gonadotropin, a hormone capa-ble of stimulating the growth and function of thegonads, as well as estrogen and progesterone (Seealso HOMEOSTASIS.)

endogenous carbohydrate Glucose that is thesized in the body from noncarbohydrate com-pounds, excluding FATTY ACIDS, rather thanobtained from the diet The process, called GLUCO-

syn-NEOGENESIS, converts most AMINO ACIDSto glucose;thus, approximately 50 percent of dietary PROTEIN

may be metabolized to glucose under normal fasting) conditions GLYCEROL from FATbreakdownyields glucose as does LACTIC ACID, PYRUVIC ACID,and such carboxylic acids of the KREB’S CYCLE as

(non-CITRIC ACID, SUCCINIC ACID, and OXALOACETIC ACID.(See also CARBOHYDRATE METABOLISM; FASTING.)

endoplasmic reticulum A membrane systemfound within the cytoplasm of many cell types Theendoplasmic reticulum functions in the synthesis,transport, and storage of a wide variety of exportedproducts, such as DIGESTIVE ENZYMES The endoplas-mic reticulum of the LIVERalso houses enzymes forcholesterol synthesis, for completion of long-chain

FATTY ACIDS, and for powerful oxidizing enzymes(detoxifying enzymes) needed to modify toxic mol-ecules and products of normal metabolism such

as steroid hormones, to convert them to soluble materials that are more readily excreted.Long-term adaptation to repeated drug exposureproduces a growth of “smooth” endoplasmic retic-ulum with increased levels of detoxifying enzymes

water-in the liver The portion of the endoplasmic lum devoted to protein synthesis machinery iscalled the “rough” endoplasmic reticulum Thecytoplasm of cells that manufacture huge amounts

reticu-of protein, such as the PANCREASand liver, is packedwith rough endoplasmic reticulum to support theirexport of proteins (See also CHOLESTEROL; CYTO-

CHROME P 450; DETOXIFICATION.)

endorphins Brain chemicals involved in a widevariety of body processes, including pain control

endorphins 227

and emotions Endorphins are produced by the

body in response to pain, stress, and emotions, and

they regulate the release of pituitary hormones,

including growth hormone, ADRENOCORTICOTROPIC

HORMONE (which triggers hormone release from

the adrenal glands), and thyroid-stimulating

hor-mone, among others Endorphins also regulate

cells of the immune system, white blood cells,

anti-body producing cells, and natural killer T-cells

Endorphins are 50 to 100 times more potent

than morphine as painkillers Beta-endorphins may

suppress pain by blocking the release of substance P,

a neuropeptide that conducts pain-related nerve

impulses to the central nervous system

Endorphins bind to specific sites in the brain

associated with pain perception and sedation They

also play a role in memory, learning, sexual drive,

regulation of body temperature, and decreased

blood pressure Increased endorphin levels may be

related to an increased sense of well-being or

euphoria, much like the effects of morphine and

other opiates According to one hypothesis,

indi-viduals with chronic pain may have fewer brain

endorphins than usual EXERCISEmay cause a

nat-ural “runner’s high,” a sense of euphoria and

insensitivity to pain, when the brain is stimulated

to form endorphins

Endorphins may regulate APPETITE; and possibly

certain foods like sweets may trigger the brain to

produce endorphins, causing an “eater’s high.”

Bulimics often binge on food, especially CARBOHY

-DRATEand sweets, and their CRAVINGmay represent

the need to feel better by eating food that triggers

endorphin production in the brain Thus, a craving

may be a form of “self-medication.” Similarly,

peo-ple with chemical dependencies may be deficient in

certain neuropeptides and thus crave substances

that stimulate endorphin production or opiate

drugs, which substitute for them, because of their

increased need for endorphin stimulation

Enkephalins, brain compounds that block pain,

were first discovered in 1975 Enkephalins are

smaller than endorphins and contain only a few

amino acids Enkephalins are concentrated in the

hypothalamus, parts of the limbic system of the

brain, and in the spinal cord They too are more

potent painkillers than morphine It is believed

they inhibit pain pathways of the nervous system

like endorphins (See also ADDICTION; BULIMIA NER

-VOSA; NEUROTRANSMITTER.)

endosperm The starchy portion of kernels of

CEREAL GRAINSthat is concentrated in flour Duringgermination, the endosperm would normally sup-ply the growing plant embryo with energy In theproduction of flour, the milling of grains removes

FIBERand GERMfrom kernels to yield a STARCHtion that can be decolorized and bleached to yieldwhite flour (See also BREAD; WHEAT.)

frac-energy The capacity to carry out work In tion, energy is measured in large CALORIES(or kilo-calories), defined as the amount of heat required toincrease the temperature of 1 liter of water by 1°C–or in terms of kilojoules, a less common measure

nutri-of energy used in medical literature

There are two principal kinds of energy: tial energy (stored energy, such as chemicalenergy) and kinetic energy (motion) Energy takesmany forms Radiant energy is characterized bywave motion; heat and light are common exam-ples Chemical reactions involved in the break-down of nutrients release some energy as heat,which cannot be used for cellular work Electricalenergy, based upon the flow of ions or electrons, isthe form of energy essential for nerve impulses andfor muscle cells

poten-The law of conservation of energy states thatenergy cannot be created or destroyed, onlychanged to other forms This holds true for energyconsiderations of the body, where energy storageequals the difference between food consumed andenergy output

Energy is released when organic compounds areoxidized or broken down On the other hand, theformation of bonds when biomolecules are assem-bled requires energy Reactions involved in build-ing processes (biosynthesis) require the input ofenergy supplied by the breakdown and oxidation

of foods Therefore, the body requires fuel to duce energy to drive all of the processes of growth,maintenance, and repair of the body

pro-The major fuels are FATS, CARBOHYDRATE, and, to

a lesser extent, PROTEIN Their oxidation yields alarge amount of energy released as heat or trapped

228 endosperm

as chemical energy in the form of ATP, the energy

currency of the cell The end products of the

com-plete oxidation of fuels are water and carbon

diox-ide The process is quite efficient: Up to 40 percent

of the energy released during glucose oxidation is

trapped as chemical energy in the form of ATP The

amount of ATP present at any moment is very

small; it is not a storage form of energy Instead the

body stores energy primarily as fats (triglycerides)

in adipose tissue

The total number of calories in a food can be

determined in the laboratory by a process called

direct calorimetry In this procedure, a food is

com-pletely oxidized in a sealed chamber immersed in

an insulated water tank The heat released or the

oxygen consumed are measured to obtain total

calories However, the calories in a food

deter-mined by direct calorimetry exceed the caloric

yield in the body because not all food constituents

are oxidized by the body to carbon dioxide and

water Tables of caloric values of foods are corrected

for this discrepancy Food energy values can be

cal-culated using general energy conversion factors (4

calories per gram of protein or carbohydrate; 9

calories per gram of fat) and adding up the calories

contributed by carbohydrate, fat, and protein Most

tables list physiologically available energy values

based on specific energy conversions

Energy Balance

Energy balance refers to the difference between

calories consumed in food and the energy used for

work and lost to the environment as heat and an

increased state of disorder or randomness The

body requires a steady input of chemical energy to

maintain all energy-requiring processes of the cell

At “energy equilibrium,” energy released from

food consumed equals the work performed There

is no surplus, and body weight is stable If the

num-ber of calories consumed exceeds energy needs, the

excess energy is stored primarily as body fat and,

therefore, weight increases

Weight loss occurs when ingested food provides

fewer calories than required to fuel the body, that

is, when fuel expenditure exceeds intake This fact

is exploited by calorie-reduction diets, which are

designed to assure that fuel intake is less than

energy needs The difference is made up by

burn-ing fat deposited in ADIPOSE TISSUE; muscle proteinmay also be broken down when calories arerestricted With gradual weight loss, the poundslost represent the consumption of body fat

The total number of calories in food consumedrepresents the energy intake Energy expenditureconsists of three categories The resting energyexpenditure (REE) is defined as the energyrequired to breathe, to pump blood, to keep theneurons of the brain active; in general, to maintainall body functions and normal temperature while

at rest, awake, and at a typical room temperature

It represents the largest contribution to total energyrequirements REE accounts for approximately 60percent of the energy output and reflects lean bodymass This accounts for the observed differences inREE between men and women, and betweenyounger and older adults In practice, the REE iseasier to measure than the BASAL METABOLIC RATE

(BMR), which is measured 12 hours after a meal,and soon after awakening in the morning REE dif-fers from BMR by only 10 percent, and for conve-nience REE is often used

A second category of energy expenditure is aheat factor called the thermic effect of food (TEF),which represents the energy cost of digesting foodand absorbing nutrients This factor is small,approximately 5 percent to 10 percent of totalenergy expenditure

Energy expended for physical activity representsthe third category of energy output This is the onlycomponent readily modified by lifestyle Activityfactors have been developed that correlate with dif-ferent activity levels The representative activityfactor for resting (asleep) is 1.0; for very light work,1.5; for light work, 2.5; for moderately heavy work,5.0; and for heavy labor, basketball, football, soccer,and the like, 7.0

The RDA (RECOMMENDED DIETARY ALLOWANCE)for energy takes into account differences amongpeople based on their sex, body size, age, preg-nancy, lactation, and activity level For example,

an average energy allowance for a healthy malebetween the ages of 25 and 50 and weighing

174 lb is 2,900 calories per day For females ing 143 lb., the average energy allowance is 1,900calories per day (See also DIETING; EXERCISE; FAST-

weigh-ING; WEIGHT MANAGEMENT.)

energy 229

enrichment The addition of nutrients to foods as

established by federal standards In 1980 the U.S

FDAestablished guidelines for the addition of

nutri-ents to food: decrease a nutritional deficiency for a

given population; restore nutrients lost in

process-ing; and improve the nutritional quality of food

substitutes and imitations Though enrichment is

not required, the FDA has established standards for

those enriched foods that include VITAMIN A, VITA

-MIN D, iodine, THIAMIN, RIBOFLAVIN, NIACIN, IRON,

and calcium The term FORTIFICATIONis often used

synonymously with enrichment, although

fortifi-cation originally meant the addition of specific

nutrients to foods, in excess of those usually found

in those foods These additions are usually in

mod-erate amounts

England and Canada have enrichment programs

similar to that of the United States Nutrients are

commonly added to salt, milk, margarine, and

CEREAL GRAIN products, including bleached flour

and pasta, white rice, cornmeal, and bread and

buns FDA standards of enrichment include iodine

in salt at a level of 7.6 mg per 100 g; vitamin D is

added to milk at a level of 400 IU per quart; and

vitamin A is added to margarine at a level of 15,000

IU per pound (the average vitamin A content of

butter throughout the year) Bread may also be

enriched by the addition of enriched flour, pure

nutrients, or nutritional yeast with iron Fortified

beverages include vitamin A- and vitamin

D-forti-fied milk; calcium- and vitamin B12-fortified soy

milk Calcium is added to some orange juices and

VITAMIN C is added to some synthetic beverages

(See also ERGOCALCIFEROL; FOOD ADDITIVES; FOOD

FORTIFICATION.)

enteritis Inflammation of the intestine,

particu-larly the layer of cells lining the intestine and also

the underlying connective tissue (See also INFLAM

-MATORY BOWEL DISEASE.)

enterohepatic circulation The absorption of

nutrients and other materials by the INTESTINE,

their transport to the LIVER and reutilization A

variety of substances released into the intestine can

be reabsorbed by the intestine Thus BILE salts

released from the GALLBLADDERare absorbed by the

small intestine, where they pass into the capillaries

and then are transported to the liver via the portalvein Bile salts can be taken up by the liver andreleased again in bile Less than 1 percent of bilesalts escapes this recycling process daily This smallloss nonetheless represents the major route bywhich CHOLESTEROLis altered and lost by the body.(See also FAT DIGESTION.)

enteropathy A general medical term referring tothe broad class of intestinal disease Typical exam-ples are CROHN’S DISEASE, CELIAC DISEASE, INFLAM-

MATORY BOWEL DISEASE, and infectious DIARRHEA.Enteropathies can affect the small intestine, thecolon or both portions of the intestine

enteropeptidase An intestinal enzyme ble for activating protein-degrading enzymes fromthe pancreas The pancreas secretes enzymes thatdigest proteins as inactive precursors in order toprevent the pancreas from digesting itself Theseinactive enzymes encounter enteropeptidase,which triggers the following cascade of events.Enteropeptidase activates trypsin, a powerful di-gestive enzyme Trypsin in turn activates all otherprotein-digesting enzymes of the pancreas Severemilk intolerance is linked to enteropeptidase deficiency, inadequate pancreatic enzymes, and,ultimately, to maldigestion (See also DIGESTIVE ENZYMES.)

responsi-enterotoxin Bacterial products released or duced in the intestine that can cause illness Cer-tain bacterial species produce chemicals that causefood poisoning, including enterotoxin-producingstrains of ESCHERICHIA COLI that can contaminateundercooked or raw meat Staphyloccocal FOOD POISONINGis caused by ingestion of bacterial toxins

pro-in contampro-inated food, usually pro-in custards, milk

products, and unrefrigerated meats, while Shigella

species cause dysentery (very severe DIARRHEA) byreleasing toxins at sites of infection In contrast, thebacterium SALMONELLA causes food poisoningthrough rapid intestinal growth without the pro-duction of enterotoxins (See also ACIDOPHILUS.)

environmental medicine A branch of medicinethat focuses on the role of environmental factors in

230 enrichment

illness Allergies or sensitivities to chemicals in

food, water, and air, at home and at work, are

rel-evant topics Exposure to substances that cause

symptoms can be physical or chemical, and usually

symptoms are chronic and cyclic Environmental

ecologists believe that environmentally induced

ill-ness is the most common cause of chronic

symp-toms Multiple organ systems are often involved

Treatment involves avoidance, control of air and

water quality, and immunization therapy to control

adverse responses and rotation diets (See also

CHEMICAL SENSITIVITY.)

enzymes A class of proteins that speed up

(cat-alyze) the chemical reactions of cells Enzymes

are extremely important and dynamic proteins

The body carries out more than 100,000 different

chemical reactions, and each of these requires

a specific protein catalyst to speed up and control

reactions Because catalysts are not used up as

they do their work, tiny amounts of enzymes

can form a huge amount of products from

sub-strates

Enzymes play major roles in METABOLISM They

are required for DIGESTION, energy production,

nerve function, muscle contraction, and regulation

of blood pH They help form all cell structures such

as RNA and DNA—and much, much more

Chem-ical compounds that react in enzyme-catalyzed

reactions are called substrates A classic model is

helpful to visualize how enzymes work: A key fits

into a carefully constructed keyhole where

chemi-cal bonds are made or broken The names of

enzymes often use the suffix “-ase,” for example,

CATALASE, dehydrogenase, AMYLASE, and oxidase

Like all PROTEINS, enzymes are long chains of

amino acids (POLYPEPTIDES) composed of the 20

AMINO ACID building blocks Each type of enzyme

possesses a unique amino acid sequence, which is

encoded in genes The amino acid sequence

deter-mines both the structure and the function of a

given enzyme Mutations in genes are due to

chemical modifications of DNA that lead to the

production of proteins with altered amino acid

sequences Mutant proteins may not function as

well as the unaltered protein; in the most severe

cases, mutations can lead to completely

nonfunc-tional enzymes and genetic diseases PHENYLKE

-TONURIA, an inherited inability to break down theamino acid phenylalanine, is an example

The thousands of different enzyme reactions fallinto just six categories “Hydrolases” break chemicallinkages in molecules by adding water (hydrolysis).Amylase (STARCH digestion), LIPASE (FAT digestion),and proteolytic enzymes like trypsin (protein diges-tion) are examples “Transferases” move a group ofatoms from one compound to another, for example,transaminases transfer amino (–NH2) groups to car-boxylic acids in order to form new amino acids

“Lyases” break bonds between carbon and carbonatoms, between carbon and nitrogen atoms (andcarbon and oxygen atoms) of compounds withoutadding water molecules or oxygen “Isomerases”catalyze rearrangements of atoms within molecules.Several reactions in glucose breakdown require iso-merizations “Ligases” utilize the energy of ATP tocreate new carbon compounds Energy-requiringsteps play key roles in the synthesis of fat from FATTY ACIDS, of proteins from amino acids, of complex car-bohydrates from simple sugars And finally, “oxido-reductases” carry out oxidation-reduction reactions.These enzymes are called “dehydrogenases” or “oxi-dases,” for example, lactate dehydrogenase, whichproduces LACTIC ACIDin the muscle during strenuousexertion

Metabolic Pathways

Enzymes often participate in functional sequences

of reactions called metabolic pathways In metabolicpathways, the product of one enzyme becomes thesubstrate for the following enzyme, which modifiesthe intermediate product before passing it on to thenext enzyme, much like a bucket brigade A typicalsequence can be illustrated as A➞ B➞ C➞ D, inwhich enzyme 1 catalyzes the reaction A➞ B;enzyme 2 catalyzes B➞ C; enzyme 3 catalyzes

C➞ D; and so on

Regulation

Enzyme pathways are often carefully regulated.The first enzyme committed to the pathway oftencontrols a bottleneck step, limiting the flow ofmaterial through the pathway Such rate-limitingenzymes can be activated to speed up the pathway

or they can be inhibited, to slow it down Thus,entire metabolic pathways can be turned on or off

by regulating such key enzymes

enzymes 231