Preview Marks’ Basic Medical Biochemistry A Clinical Approach (5th Edition) by Michael A. Lieberman Alisa Peet (2017)

Preview Marks’ Basic Medical Biochemistry A Clinical Approach (5th Edition) by Michael A. Lieberman Alisa Peet (2017) Preview Marks’ Basic Medical Biochemistry A Clinical Approach (5th Edition) by Michael A. Lieberman Alisa Peet (2017) Preview Marks’ Basic Medical Biochemistry A Clinical Approach (5th Edition) by Michael A. Lieberman Alisa Peet (2017) Preview Marks’ Basic Medical Biochemistry A Clinical Approach (5th Edition) by Michael A. Lieberman Alisa Peet (2017) Preview Marks’ Basic Medical Biochemistry A Clinical Approach (5th Edition) by Michael A. Lieberman Alisa Peet (2017)

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It has been 5 years since the fourth edition was completed The fifth edition hassome significant organizational changes, as suggested by extensive surveys offaculty and students who used the fourth edition in their classes and studies Themajor pedagogic features of the text remain They have been enhanced by thefollowing changes for the fifth edition:

1 Every patient history has been reviewed and revised to reflect currentstandards of care (as of 2016) The patient names have also been changed

to a first name and last initial A key indicating the “old” names and

“new” names is available in the online supplement associated with thetext

2 The Biochemical Comments associated with each chapter have beenupdated, where appropriate, to allow students to experience where

current research efforts are headed

3 The presentation of metabolism has been altered such that glycolysis isnow the first topic discussed, followed by the tricarboxylic acid cycle,and then oxidative phosphorylation The correlation between fourth

is based on Chapter 26 of the fourth edition

ii Chapter 20 of the fifth edition (Cellular Bioenergetics:

Adenosine Triphosphate and O2) is based on Chapter 19 of thefourth edition

iii Chapter 21 of the fifth edition (Digestion, Absorption, and

Transport of Carbohydrates) is based on Chapter 27 of the fourthedition

iv Chapter 22 of the fifth edition (Generation of Adenosine

Triphosphate from Glucose, Fructose, and Galactose:

Glycolysis) is based on Chapter 22 of the fourth edition and alsocontains parts of Chapter 29 of the fourth edition (Pathways ofSugar Metabolism: Pentose Phosphate Pathway, Fructose, andGalactose Metabolism)

v Chapter 23 of the fifth edition (Tricarboxylic Acid Cycle) isbased on Chapter 20 of the fourth edition

vi Chapter 24 of the fifth edition (Oxidative Phosphorylation andMitochondrial Function) is based on Chapter 21 of the fourthedition

vii Chapter 25 of the fifth edition (Oxygen Toxicity and Free-Radical Injury) is based on Chapter 24 of the fourth edition.viii Chapter 26 of the fifth edition (Formation and Degradation ofGlycogen) is based on Chapter 28 of the fourth edition

ix Chapter 27 of the fifth edition (Pentose Phosphate Pathway andthe Synthesis of Glycosides, Lactose, Glycoproteins, and

Glycolipids) is based on Chapter 30 of the fourth edition, alongwith a section (The Pentose Phosphate Pathway) of Chapter 29

of the fourth edition This led to the deletion of old Chapter 29from the Table of Contents of the fifth edition

Dietary Lipids) is based on Chapter 32 of the fourth edition

ii Chapter 30 of the fifth edition (Oxidation of Fatty Acids andKetone Bodies) is based on Chapter 23 of the fourth edition iii Chapter 31 of the fifth edition (Synthesis of Fatty Acids,

Triacylglycerols, and the Major Membrane Lipids) is based onChapter 33 of the fourth edition and also contains basic

information concerning the eicosanoids from Chapter 35 of thefourth edition Material from Chapter 35 of the fourth editionthat was not incorporated into Chapter 31 of the fifth edition isavailable as an online supplement A separate chapter on

eicosanoid metabolism is not present in the fifth edition

iv Chapter 32 of the fifth edition (Cholesterol Absorption,

Synthesis, Metabolism, and Fate) is based on Chapter 34 of thefourth edition

v Chapter 33 of the fifth edition (Metabolism of Ethanol) is based

on Chapter 25 of the fourth edition

vi Chapter 34 of the fifth edition (Integration of Carbohydrate andLipid Metabolism) is based on Chapter 36 of the fourth edition

d Section VI (Nitrogen Metabolism) has the same chapter order as inthe fourth edition, but because two chapters have been deleted

previously from the text, the chapter numbers in the fifth edition aretwo less than in the fourth edition Section VI in the fifth editioncomprises Chapters 35 through 40, whereas in the fourth edition, it isChapters 37 through 42

e Section VII (Tissue Metabolism) has the same chapter order as in thefourth edition, but the chapter numbers in the fifth edition are two lessthan in the fourth edition Section VII in the fifth edition comprisesChapters 41 through 47, whereas in the fourth edition, it is Chapters

43 through 49

4 The number of printed review questions at the end of each chapter hasbeen increased to 10, up from 5 questions per chapter in the fourth

edition (470 total questions) The online question bank associated withthe text has also been increased to 560 questions, as compared to 468

As stated in previous editions, in revising a text geared primarily towardmedical students, the authors always struggle with new advances in biochemistryand whether such advances should be included in the text We have taken theapproach of only including advances that will enable the student to better relatebiochemistry to medicine and future diagnostic tools Although providing

incomplete, but exciting, advances to graduate students is best for their

education, medical students benefit more from a more directed approach—onethat emphasizes how biochemistry is useful for the practice of medicine This is

a major goal of this text

Any errors are the responsibility of the authors, and we would appreciatebeing notified when such errors are found

The accompanying website for this edition of Marks’ Basic Medical

Biochemistry: A Clinical Approach contains the aforementioned additional

multiple-choice questions for review, a table listing patient names for the fifthedition and how they correspond to those of the fourth edition, summaries of allpatients described in the text (patient cases), all chapter references and additionalreading (with links to the article in PubMed, where applicable), a listing of

diseases discussed in the book (with links to appropriate websites for more

information), and a summary of all of the methods described throughout the text

indicates a female patient

indicates a male patient

indicates a patient who is an infant or young child

As each chapter unfolds, icons appear in the margin, identifying informationrelated to the material presented in the text:

indicates a clinical note, usually related to the patients in The WaitingRoom for that chapter These notes explain signs or symptoms of a patient

or give some other clinical information relevant to the text

required to perform, and interpret, common laboratory tests

Questions and answers also appear in the margin and should help to keepstudents thinking as they read the text:

indicates a question

indicates the answer to the question The answer to a question is alwayslocated on the next page If two questions appear on one page, the answersare given in order on the next page

Each chapter ends with these three sections: Key Concepts, Clinical

Comments, and Biochemical Comments:

The Key Concepts summarize the important take-home messages from thechapter

The Clinical Comments give additional clinical information, often

describing the treatment plan and the outcome

The Biochemical Comments add biochemical information that is not

covered in the text or explore some facet of biochemistry in more detail orfrom another angle

Finally, Review Questions are presented These questions are written in aUnited States Medical Licensing Examination–like format, and many of themhave a clinical slant Answers to the review questions, along with detailed

explanations, are provided at the end of every chapter

The authors would like to thank Professor Kent Littleton of Bastyr University,for his careful reading of the fourth edition and pointing out mistakes and errorsthat required correcting for the fifth edition We greatly appreciate his efforts inimproving the text Dr Bonnie Brehm was instrumental in helping with thenutrition aspects of the text, and Dr Rick Ricer was invaluable in writing

questions, both for the text and the online supplement We would also like toacknowledge the initial contributions of Dawn Marks, whose vision of a

textbook geared toward medical students led to the first edition of this book Hervision is still applicable today

21 Digestion, Absorption, and Transport of Carbohydrates

22 Generation of Adenosine Triphosphate from Glucose, Fructose, andGalactose: Glycolysis

28 Gluconeogenesis and Maintenance of Blood Glucose Levels

SECTION V

Lipid Metabolism

29 Digestion and Transport of Dietary Lipids

31 Synthesis of Fatty Acids, Triacylglycerols, and the Major MembraneLipids

Subject Index

n order to survive, humans must meet two basic metabolic requirements: Wemust be able to synthesize everything our cells need that is not supplied byour diet, and we must be able to protect our internal environment from toxinsand changing conditions in our external environment To meet these

requirements, we metabolize our dietary components through four basic types ofpathways: fuel oxidative pathways, fuel storage and mobilization pathways,biosynthetic pathways, and detoxification or waste disposal pathways

Cooperation between tissues and responses to changes in our external

environment are communicated through transport pathways and intercellularsignaling pathways (Fig I.1)

thinking, and reproducing Thus, several of our metabolic pathways are fuel

oxidative pathways that convert fuels into energy that can be used for

biosynthetic and mechanical work But what is the source of energy when we arenot eating, such as between meals, and while we sleep? How does a person on ahunger strike that you read about in the morning headlines survive so long? We

have other metabolic pathways that are fuel storage pathways The fuels that we

store can be mobilized during periods when we are not eating or when we needincreased energy for exercise

pathways devoted to removing toxins that can be present in our diets or in the air

we breathe, introduced into our bodies as drugs, or generated internally from themetabolism of dietary components Dietary components that have no value to the

pathways that allow it to carry out this function However, adipose tissue is

lacking many of the pathways that synthesize required compounds from dietaryprecursors To enable our cells to cooperate in meeting our metabolic needs

during changing conditions of diet, sleep, activity, and health, we need transport

pathways into the blood and between tissues and intercellular signaling

pathways One means of communication is for hormones to carry signals to

tissues about our dietary state For example, a message that we have just had ameal, carried by the hormone insulin, signals adipose tissue to store fat

In the following section, we will provide an overview of various types ofdietary components and examples of the pathways involved in using these

components We will describe the fuels in our diet, the compounds produced bytheir digestion, and the basic patterns of fuel metabolism in the tissues of ourbodies We will describe how these patterns change when we eat, when we fastfor a short time, and when we starve for prolonged periods Patients with

medical problems that involve an inability to deal normally with fuels will beintroduced These patients will appear repeatedly throughout the book and will

be joined by other patients as we delve deeper into biochemistry

It is important to note that this section of the book contains an overview of

basic metabolism, which allows patients to be presented at an elementary leveland to whet students’ appetites for the biochemistry to come The goal is to

to processes that occur in all living cells

For additional ancillary materials related to this chapter, pleasevisit thePoint

Fuel Metabolism We obtain our fuel primarily from the macronutrients (i.e., carbohydrates, fats, and proteins) in our diet As we eat, our foodstuffs are digested and absorbed The products of digestion circulate in the blood, enter various tissues, and are eventually taken up by cells and oxidized to produce energy To completely convert our fuels to carbon dioxide (CO2) and water(H2O), molecular oxygen (O2) is required We breathe to obtain this oxygen and

to eliminate the CO2 that is produced by the oxidation of our foodstuffs

Fuel Stores Any dietary fuel that exceeds the body’s immediate energy needs is stored, mainly as triacylglycerol (fat) in adipose tissue, as glycogen (a

carbohydrate) in muscle, liver, and other cells, and, to some extent, as protein in

muscle When we are fasting, between meals and overnight while we sleep, fuel

is drawn from these stores and is oxidized to provide energy (Fig 1.1)

Fuel Requirements We require enough energy each day to drive the basic functions of our bodies and to support our physical activity If we do not

consume enough food each day to supply that much energy, the body’s fuelstores supply the remainder and we lose weight Conversely, if we consumemore food than required for the energy we expend, our body’s fuel stores enlargeand we gain weight

Other Dietary Requirements In addition to providing energy, the diet provides precursors for the biosynthesis of compounds necessary for cellular and tissue structure, function, and survival Among these precursors are the essential fatty acids and essential amino acids (those that the body needs but cannot

synthesize) The diet must also supply vitamins, minerals, and water.

Waste Disposal Dietary components that we can use are referred to as nutrients However, both the diet and the air we breathe contain xenobiotic compounds,

compounds that have no use or value in the human body and may be toxic

These compounds are excreted in the urine and feces together with metabolicwaste products

T H E WA I T I N G

R O O M

Percy V is a 59-year-old school teacher who was in good health until his

wife died suddenly Since that time, he has experienced an increasingdegree of fatigue and has lost interest in many of the activities he previouslyenjoyed Shortly after his wife’s death, one of his married children moved far

called an ambulance Mr V was admitted to the hospital psychiatry unit with adiagnosis of mental depression associated with dehydration and malnutrition

Otto S is a 25-year-old medical student who was very athletic during high

school and college but is now out of shape Since he started medical

school, he has been gaining weight He is 5 ft 10 in tall and began medical

school weighing 154 lb, within his ideal weight range By the time he finishedhis last examination in his first year, he weighed 187 lb He has decided to

consult a physician at the student health service before the problem gets worse,

as he would like to reduce his weight (at 187 lb, his body mass index [BMI] is27] to his previous level of 154 lb (which would reduce his BMI to 23, in themiddle of the healthy range of BMI values)

Ivan A is a 56-year-old accountant who has been obese for a number of

years He exhibits a pattern of central obesity, called an “apple shape,”which is caused by excess adipose tissue being disproportionally deposited in theabdominal area His major recreational activities are watching TV while drinkingscotch and soda and doing occasional gardening At a company picnic, he

became very “winded” while playing softball and decided it was time for a

general physical examination At the examination, he weighed 264 lb at 5 ft 10

in tall His blood pressure was elevated, 155 mm Hg systolic and 95 mm Hgdiastolic (for Ivan’s age, hypertension is defined as >140 mm Hg systolic and

>90 mm Hg diastolic) For a male of these proportions, a BMI of 18.5 to 24.9would correspond to a weight between 129 and 173 lb Mr A is almost 100 lboverweight, and his BMI of 37.9 is in the range defined as obesity

Ann R is a 23-year-old buyer for a woman’s clothing store Despite the

fact that she is 5 ft 7 in tall and weighs 99 lb, she is convinced she is

overweight Two months ago, she started a daily physical activity program thatconsists of 1 hour of jogging every morning and 1 hour of walking every

evening She also decided to consult a physician about weight loss If patientsare above (like Ivan A.) or below (like Ann R.) their ideal weight, the physician,often in consultation with a registered dietitian, prescribes a diet designed tobring the weight into the ideal range

fuels to acetyl coenzyme A (acetyl-CoA), a precursor of the tricarboxylic acid (TCA) cycle The TCA cycle is a series of reactions that completes the oxidation

of fuels to CO2 (see Chapter 23) Electrons lost from the fuels during oxidative

reactions are transferred to O2 by a series of proteins in the electron transportchain (see Chapter 24) The energy of electron transfer is used to convert ADPand Pi to ATP by a process known as oxidative phosphorylation.

Oxidation of carbohydrates to CO2 and H2O in the body produces

approximately 4 kcal/g (Table 1.1) In other words, every gram of carbohydrate

we eat yields approximately 4 kcal of energy Note that carbohydrate moleculescontain a significant amount of oxygen and are already partially oxidized beforethey enter our bodies (see Fig 1.4)

B Proteins

Proteins are composed of amino acids that are joined to form linear chains (Fig.

1.5) In addition to carbon, hydrogen, and oxygen, proteins contain

approximately 16% nitrogen by weight The digestive process breaks downproteins to their constituent amino acids, which enter the blood The complete

C Fats

Fats are lipids composed of triacylglycerols (also called triglycerides) A

triacylglycerol molecule contains three fatty acids esterified to one glycerolmoiety (Fig 1.6)

Fats contain much less oxygen than is contained in carbohydrates or proteins.Therefore, fats are more reduced and yield more energy when oxidized Thecomplete oxidation of triacylglycerols to CO2 and H2O in the body releasesapproximately 9 kcal/g, more than twice the energy yield from an equivalentamount of carbohydrate or protein (see Table 1.1).

for a total of 615 calories/day

D Alcohol

Alcohol (ethanol, in the context of the diet) has considerable caloric content.Ethanol (CH3–CH2–OH) is oxidized to CO2 and H2O in the body and yieldsapproximately 7 kcal/g; that is, more than carbohydrate or protein but less thanfat

II Body Fuel Stores

Humans carry supplies of fuel within their bodies (Table 1.2), which are similar

to the fuel supplies in the plants and animals we eat These fuel stores are light inweight, large in quantity, and readily converted into oxidizable substances Most

of us are familiar with fat, our major fuel store, which is located in adipose

tissue Although fat is distributed throughout our bodies, it tends to increase inquantity in our hips, thighs, and abdomens as we advance into middle age Inaddition to our fat stores, we also have important, although much smaller, stores

of carbohydrate in the form of glycogen located primarily in our liver and

muscles Glycogen consists of glucose molecules joined together to form a large,branched polysaccharide (see Fig 1.4) Body protein, particularly the protein ofour large muscle masses, also serves to some extent as a fuel store, and we draw

on it for energy when we fast

stored triacylglycerol has only about 18 kg of adipose tissue

B Glycogen

Our stores of glycogen in liver, muscle, and other cells are relatively small inquantity but are nevertheless important Liver glycogen is used to maintain bloodglucose levels between meals, which is necessary for optimal functioning of thenervous system Thus, the size of this glycogen store fluctuates during the day;

an average 70-kg man might have 200 g or more of liver glycogen after a mealbut only 80 g after an overnight fast Muscle glycogen supplies energy for

muscle contraction during exercise At rest, the 70-kg man has approximately

150 g of muscle glycogen Almost all cells, including neurons, maintain a smallemergency supply of glucose as glycogen

approximately 4 times its weight as water Storage of energy as triacylglycerolcontains much less water weight

C Protein

Protein serves many important roles in the body; unlike fat and glycogen, it isnot solely a fuel store like fat or glycogen Muscle protein is essential for body

movement Other proteins serve as enzymes (catalysts of biochemical reactions)

or as structural components of cells and tissues Only a limited amount of body

protein can be degraded, approximately 6 kg in the average 70-kg man, beforeour body functions are compromised

induced thermogenesis [DIT]) Thus, the DEE, in kilocalories per day, = BMR

(or RMR) + the energy needed for physical activity + DIT

A Basal Metabolic Rate

Two terms have been used to define the energy required by the body, the BMRand the RMR The BMR is a measure of the energy required to maintain life: thefunctioning of the lungs, kidneys, and brain; the pumping of the heart; the

maintenance of ionic gradients across membranes; the reactions of biochemicalpathways; and so forth The BMR was originally defined as the energy

expenditure of a person mentally and bodily at rest in a thermoneutral

environment 12 to 18 hours after a meal However, when a person is awakenedand their heat production or oxygen consumption is measured, they are no longer

sleeping or totally at mental rest, and their metabolic rate is called the resting

(REE) The RMR and BMR differ very little in value, and for the purposes ofthis text, we will focus on the BMR

The BMR, which is usually expressed in kilocalories per day, is affected bybody size, age, sex, and other factors (Table 1.3) It is proportional to the amount

free) body mass Obviously, the amount of energy required for basal functions in

of metabolically active tissue (including the major organs) and to the lean (or fat-a large person is greater than the amount required in a small person However,the BMR is usually lower for women than for men of the same weight becausewomen usually have more metabolically inactive adipose tissue Body

temperature also affects the BMR, which increases by 12% with each degreecentigrade (7% with each degree Fahrenheit) increase in body temperature (i.e.,

“feed a fever; starve a cold”) The ambient temperature affects the BMR, whichincreases slightly in colder climates as thermogenesis is activated Excessivesecretion of thyroid hormone (hyperthyroidism) causes the BMR to increase,whereas diminished secretion (hypothyroidism) causes it to decrease The BMRincreases during pregnancy and lactation Growing children have a higher BMRper kilogram body weight than adults because a greater proportion of their

bodies is composed of brain, muscle, and other more metabolically active

tissues The BMR declines in aging individuals because their metabolically

active tissue is shrinking and body fat is increasing In addition, large variationsexist in BMR from one adult to another, determined by genetic factors

A rough estimate of the BMR may be obtained by assuming it is either 24 or21.6 kcal/day/kg body weight (for men or for women, respectively) and

multiplying by the body weight An easy way to remember this is 1 kcal/kg/hrfor men and 0.9 kcal/kg/hr for women This estimate works best for young

calculating the BMR use empirically derived equations for different gender andage groups (Table 1.4) Even these calculations do not take into account

in centimeters, and the age in years) Ms R weighs 99 lb or 45 kg (99

divided by 2.2 lb/kg) Her estimated BMR = (21.6 calories/kg/day) × (45kg) = 972 calories/day Her BMR from Table 1.4 is above this value (10

Registered dietitians often use extensive tables for calculatingenergy requirements, which are based on height, weight, age,

gender, and activity level A more accurate calculation is based on thefat-free mass (FFM), which is equal to the total body mass minus themass of the person’s adipose tissue With FFM, the BMR is calculatedusing the equation BMR = 186 + FFM × 23.6 kcal/kg/day This formulaeliminates differences between sexes and between elderly versus youngindividuals that are attributable to differences in relative adiposity

However, determining FFM is relatively cumbersome—one techniquerequires weighing the patient underwater and measuring the residual lungvolume More recently, dual-energy X-ray absorptiometry (DXA) is anequally accurate, but simpler, technique that is used to determine thepatient’s total amount of fat and FFM

Indirect calorimetry, a technique that measures O2 consumption and

CO2 production, can be used when more accurate determinations ofenergy need are required for hospitalized patients A portable indirect

quotient (RQ), which is the ratio of O2 consumed to CO2 produced The

RQ is 1.00 for individuals oxidizing carbohydrates, 0.83 for protein, and0.71 for fat From these values, the DEE can be determined

oxygen and CO2 consumption and is easier to use, but it is less accurate

A rough estimate of the energy required per day for physical activity can bemade by using a value of 30% of the BMR (per day) for a very sedentary person(such as a medical student who does little but study) and a value of 60% to 70%

of the BMR (per day) for a person who engages in about 2 hours of moderatephysical activity per day (see Table 1.5) A value of 100% or more of the BMR

The energy required to process the types and quantities of food in the typicalAmerican diet is probably equal to approximately 10% of the kilocalories

ingested This amount is roughly equivalent to the error involved in rounding offthe caloric content of carbohydrate, fat, and protein to 4, 9, and 4, respectively.Therefore, DIT is often ignored and calculations are based simply on the BMRand the energy required for physical activity

cardiovascular disease, so it is not surprising that cardiovascular disease

is the major cause of death in this country

D Calculations of Daily Energy Expenditure

The total DEE is usually calculated as the sum of the BMR (in kilocalories perday) plus the energy required for the amount of time spent in each of the varioustypes of physical activity (see Table 1.5) An approximate value for the DEE can

be determined from the BMR and the appropriate percentage of the BMR

required for physical activity (given earlier) For example, a very sedentarymedical student would have a DEE equal to the RMR plus 30% of the BMR (or1.3 × BMR), and an active person’s daily expenditure could be two times theBMR

What are reasonable estimates for Ivan A.’s and Ann R.’s DEE?

Mr A.’s BMR is 1,992 calories/day He is sedentary, so he only

requires approximately 30% more calories for his physical activity.Therefore, his daily expenditure is approximately 1,992 + (0.3 × 1,992)

or 1.3 × 1,992 or 2,590 kcal/day Ms R.’s BMR is 1,238 calories/day.

She performs 2 hours of moderate physical activity per day (jogging andwalking), so she requires approximately 65% more calories for her

physical activity Therefore, her daily expenditure is approximately 1,238+ (0.65 × 1,238) or 1.65 × 1,238 or 2,043 calories/day

E Healthy Body Weight

Ideally, we should strive to maintain a weight consistent with good health TheBMI, calculated as weight divided by height2 (kg/m2), is currently the preferredmethod for determining whether a person’s weight is in the healthy range Thisformula, in the English system, is (weight [in pounds] × 704)/height2 (with

In general, adults with BMI values <18.5 are considered underweight Thosewith BMIs between 18.5 and 24.9 are considered to be in the healthy weightrange, between 25 and 29.9 are in the overweight or preobese range, and 30 andabove are in the obese range Class I obesity is defined as a BMI of 30 to 34.9,class II as a BMI of 35 to 39.9, and class III (or extreme obesity) as a BMI of 40

or greater Degrees of protein-calorie malnutrition (marasmus) are classifiedaccording to the BMI A BMI of 17.0 to 18.4 is degree I; values of 16.0 to 16.9

is degree II; and any value less than 16.0 is degree III, the most severe form ofprotein-energy malnutrition

F Weight Gain and Loss

To maintain our body weight, we must stay in caloric balance We are in caloricbalance if the calories in the food we eat equal our DEE If we eat less food than

we require for our DEE, our body fuel stores supply the additional calories and

we lose weight Conversely, if we eat more food than we require for our energyneeds, the excess fuel is stored (mainly in our adipose tissue) and we gain

is only about 2,000 to 3,000 calories/day, eating one-third to one-half the normalamount will cause a person to lose weight rather slowly Fad diets that promise aloss of weight much more rapid than this have no scientific merit In fact, the

to loss of body water This loss of water occurs in part because muscle tissueprotein and liver glycogen are degraded rapidly to supply energy during the earlyphase of the diet When muscle tissue (which is approximately 80% water) andglycogen (approximately 70% water) are broken down, this water is excretedfrom the body

IV Dietary Requirements

In addition to supplying us with fuel and with general purpose building blocksfor biosynthesis, our diet also provides us with specific nutrients that we need toremain healthy We must have a regular supply of vitamins and minerals and of

the essential fatty acids and essential amino acids Essential means that they are

essential in the diet; the body cannot synthesize these compounds from othermolecules and therefore must obtain them from the diet Nutrients that the body

requires in the diet only under certain conditions are called conditionally

essential.

The Recommended Dietary Allowance (RDA) and the Adequate Intake (AI) provide quantitative estimates of nutrient requirements The RDA for a

A Carbohydrates

The RDA for carbohydrate is 130 g/day for children and adults and is based onthe amount of carbohydrate needed to provide the brain with an adequate supply

of glucose Another value, the acceptable macronutrient distribution range

(AMDR) is the recommended range of intake for a macronutrient that is

associated with a reduced risk of disease while providing adequate intake ofessential nutrients The AMDR is expressed as a percentage of caloric intake.For example, according to its AMDR, carbohydrate should provide 45% to 65%

Carbohydrates can be synthesized from amino acids, and we can convert onetype of carbohydrate to another However, health problems are associated withthe complete elimination of carbohydrate from the diet, partly because a low-carbohydrate diet must contain higher amounts of fat to provide us with theenergy we need High-fat diets are associated with obesity, atherosclerosis, andother health problems

B Essential Fatty Acids

The recommended range (AMDR) for dietary fat is 20% to 35% of total calories.Although most lipids required for cell structure, fuel storage, or hormone

on gender, height, and body frame size, that are associated with the

greatest longevity However, these tables are considered inadequate for anumber of reasons (e.g., they reflect data from upper-middle-class whitegroups) The BMI is the classification that is currently used clinically It

is based on two simple measurements, height without shoes and weight

with minimal clothing

correlation with independent measures of body fat The major weakness

of the use of the BMI is that some very muscular individuals may beclassified as obese when they are not There are also some differences inBMI and risk of certain diseases that vary by race or ethnicity Othermeasurements to estimate body fat and other body compartments, such asweighing individuals underwater, are more difficult, expensive, and time-consuming and have generally been confined to research purposes

If patients are above or below the healthy BMI range (such as Ivan

A or Ann R.), the physician, often in consultation with a registered

dietitian, prescribes a diet designed to bring the weight into the healthyrange

Are Ivan A and Ann R gaining or losing weight?

Mr A expends about 2,590 calories/day and consumes 4,110

calories/day By this calculation, he consumes 1,520 kcal more

than he expends each day and is gaining weight Ms R expends 2,043