Energetics and Metabolic Rate pdf - Metabolism and Temperature Regulation

as an enzyme to cause breakdown of ATP into adenosine diphosphate (ADP), thus

from ATP. Myosin, one of the important contractile proteins of the muscle fiber, acts

ATP Energizes Muscle Contraction.

tion, which keeps the ammonia concentration of the body fluids at a low level.

the extreme toxicity of ammonia in the body fluids, one can see the value of this reac-

energy to form urea, which is simply discarded by the body. However, remembering

formation from ammonia. One might wonder about the advisability of expending

of the body. Even the urea excreted by the kidneys requires ATP to cause its

synthesis of cholesterol, phospholipids, the hormones, and almost all other substances

thesis of fatty acids from acetyl coenzyme A. In addition, ATP energy is used for the

ATP energy is also used in the synthesis of glucose from lactic acid and in the syn-

tually stored in each of the peptide linkages.

of 48,000 calories of energy, which is far more than the 500 to 5000 calories even-

the cascade of reactions required to form each peptide linkage. This provides a total

5000 calories of energy per mole. It will be recalled from the discussion of protein

linkages, depending on which types of amino acids are linked, require from 500 to

ages between amino acids during the synthesis of proteins. The different peptide

tant intracellular processes that require ATP energy is the formation of peptide link-

ATP Energizes the Synthesis of Most Important Cellular Components.

citric acid cycle and then to acetyl coenzyme A and carbon dioxide.

Combustion of proteins,

beta-oxidation.

citric acid (Krebs) cycle.

other sugars such as fructose; this occurs in the cytoplasm of the cell through

—mainly glucose, but also smaller amounts of

we discussed the transfer of energy from various foods to ATP. To summarize, ATP

In previous chapters,

ATP Is Generated by Combustion of Carbohydrates, Fats, and Proteins.

only a few hundred of the available 12,000 calories, and the remainder of this energy

energy transfer is achieved. Some chemical reactions that require ATP energy use

energy in each bond, when liberated by decomposition of ATP, is enough to cause

conditions) vested in each of its two high-energy phosphate bonds. The amount of

mole under standard conditions, but as much as 12,000 calories under physiologic

large quantity of free energy (about 7300 calories, or 7.3 Calories [kilocalories], per

An attribute of ATP that makes it highly valuable as an energy currency is the

triphosphate, GTP). Many of the attributes of ATP are presented in Chapter 2.

can be done only through this medium of ATP (or the similar nucleotide guanosine

Indeed, the transfer of energy from foodstuffs to most functional systems of the cells

tions. For this reason, ATP has been called an energy “currency” in cell metabolism.

(ATP), which can be used as an energy source for almost all other cellular func-

bohydrates, fats, and proteins can all be used by cells to

In the past few chapters, we have pointed out that car-

an “Energy Currency”

(ATP) Functions as

Adenosine Triphosphate

881

Energetics and Metabolic Rate

in Metabolism

synthesize large quantities of adenosine triphosphate

almost any step of any chemical reaction in the body to take place if appropriate

is lost in the form of heat.

is produced from
1. Combustion of carbohydrates

the anaerobic process of glycolysis and in the cell mitochondria through the
aerobic

2. Combustion of fatty acids in the cell mitochondria by
3.

which requires hydrolysis to their component amino

acids and degradation of the amino acids to intermediate compounds of the

Among the most impor-

synthesis in Chapter 3 that four high-energy phosphate bonds are expended during

Muscle contraction will not occur without energy

hypoxia.When a person stops breathing, there is already

the stored glycogen of the cells.

Thus,

state, without the additional expenditure of ATP.

using 1 mole of ATP before it can begin to be split; this

moles of ATP. The reason for this difference is that free

into pyruvic acid, 2 moles of ATP are formed. However,

gen to pyruvic acid. For each mole of glucose that is split

without the utilization of oxygen;

only significant foods that can be used to provide energy

cause synthesis of ATP. However,

carbohydrates, fats, and proteins can all be oxidized to

discussions in Chapters 67 through 69, it is noted that

derived from foods only by oxidative metabolism. In the

Anaerobic Versus Aerobic Energy

reactions in the body depend on this constancy.

constant, because the rates of almost all the metabolic

importance of keeping the concentration of ATP nearly

ATP “buffer” system. One can readily understand the

reason, we can call the ATP-phosphocreatine system an

level as long as any phosphocreatine remains. For this

the concentration of ATP at an almost constant high

phocreatine to synthesize new ATP. This effect keeps

ATP by the cells calls forth the energy from the phos-

its energy elsewhere. Therefore, the slightest usage of

every time even the slightest amount of ATP expends

to proceed rapidly toward the formation of new ATP

causes the reaction between phosphocreatine and ADP

to 1500 calories per mole greater than that in ATP)

ATP + Creatine

Phosphocreatine + ADP

between ATP and phosphocreatine is demonstrated by

tional systems of the cells. This reversible interrelation

transferred rapidly back to ATP and then to the func-

to be used up, the energy in the phosphocreatine is

this storehouse of energy. Then, when the ATP begins

used to synthesize phosphocreatine, thus building up

of ATP are available in the cell, much of its energy is

energy interchangeably with ATP. When extra amounts

and the functional cellular systems, but it can transfer

Unlike ATP, phosphocreatine cannot act as a direct

energy phosphate bonds of ATP. The formula for crea-

centrations of the reactants). This is slightly greater than

abundant. Also, the high-energy bond (

energy phosphate bonds, is three to eight times as

Phosphocreatine,

the cells.

pling agent for energy transfer, this substance is not the

Despite the paramount importance of ATP as a cou-

Energy and as an “ATP Buffer”

positions.

transport systems energized by ATP then retransport

during each of the action potentials. However, active

this energy storage, with small amounts of potassium

of energy. The energy needed to pass each action

of energy storage. Likewise, a high concentration

differences of ions across the membranes. That is, a

The energy used during

ATP Energizes Nerve Conduction.

secreted by the glandular cells. In addition, energy is

substances against concentration gradients, because

The same principles

ATP Energizes Glandular Secretion.

requires energy, which is provided by ATP.

direction. To oppose the electrochemical gradient

an electrochemical gradient, even though the natural

glucose, amino acids, and acetoacetate can occur against

blood is discussed. In each instance, we noted that active

Chapters 4, 27, and 65, active transport of electrolytes

ATP Energizes Active Transport Across Membranes.

postulated mechanism by which ATP energy is used to

level during short bursts of maximal contraction. The

of ATP usage can rise to at least 150 times the resting

when muscle contraction is not occurring, but this rate

a small amount of ATP is normally degraded in muscles

releasing the energy required to cause contraction. Only

Metabolism and Temperature Regulation

882

Unit XIII

cause muscle contraction is discussed in Chapter 6.

and various nutrients across cell membranes and from
the renal tubules and gastrointestinal tract into the

transport of most electrolytes and substances such as

diffusion of the substances would be in the opposite

apply to glandular secretion as to the absorption of

energy is required to concentrate substances as they are

required to synthesize the organic compounds to be
secreted.

propagation of a nerve impulse is derived from
the potential energy stored in the form of concentration

high concentration of potassium inside the fiber and a
low concentration outside the fiber constitute a type

of sodium on the outside of the membrane and a low
concentration on the inside represent another store

potential along the fiber membrane is derived from

transferring out of the cell and sodium into the cell

the ions back through the membrane to their former

Phosphocreatine Functions as an
Accessory Storage Depot for

most abundant store of high-energy phosphate bonds in

which also contains high-

~) of phospho-

creatine contains about 8500 calories per mole under
standard conditions and as much as 13,000 calories per
mole under conditions in the body (37°C and low con-

the 12,000 calories per mole in each of the two high-

tinine phosphate is the following:

coupling agent for energy transfer between the foods

the following equation:

Ø≠

Note particularly that the higher energy level of the

high-energy phosphate bond in phosphocreatine (1000

Anaerobic energy means energy that can be derived
from foods without the simultaneous utilization of
oxygen; aerobic energy means energy that can be

carbohydrates are the

this energy release

occurs during glycolytic breakdown of glucose or glyco-

when stored glycogen in a cell is split to pyruvic acid,
each mole of glucose in the glycogen gives rise to 3

glucose entering the cell must be phosphorylated by

is not true of glucose derived from glycogen because it
comes from the glycogen already in the phosphorylated

the best source of energy under anaerobic conditions is

Anaerobic Energy Utilization During Hypoxia.

One of the prime

examples of anaerobic energy utilization occurs in acute

a small amount of oxygen stored in the lungs and an

HOOC

O
H

glycogen follows rapidly. Thus, oxidative metabolism

house is used first, and then anaerobic breakdown of

by oxidative metabolism, the phosphocreatine store-

cellular activities. If greater amounts of energy are

nerve impulse conduction, active absorption, and other

and growth, muscle contraction, glandular secretion,

Energy from ATP can be used by the different func-

energy is in this energy storehouse.

present in the cells than ATP, much of the cells’ stored

equilibrium with phosphocreatine in the cells, and

to form additional ATP. In turn, ATP is in reversible

from carbohydrates, fats, proteins, and other substances

ATP and the aerobic utilization of compounds derived

shown in Figure 72–1. This figure demonstrates the

ite picture of overall energy utilization by the cells, as

preceding discussion, we can now synthesize a compos-

With the background of the past few chapters and of the

important in many types of athletics.

Chapter 84 in relation to sports physiology; the ability

The principle of oxygen debt is discussed further in

oxygen debt.

its normal level. This extra consumption of oxygen after

of oxygen bound with hemoglobin and myoglobin, and

phocreatine, (4) to re-establish normal concentrations

ATP, (3) to reconvert creatine and phosphate to phos-

to reconvert adenosine monophosphate and ADP to

has accumulated during exercise back into glucose, (2)

sometimes for as long as 1 hour thereafter. This addi-

cise, a person continues to breathe hard and to consume

muscles, where it is stored once more in the form of

to glucose occurs principally in the liver cells, and

and oxidized in the citric acid cycle. The reconversion

exercise is over, oxidative metabolism is used to recon-

the lactic acid concentration of the blood rises. After the

during strenuous bouts of exercise is reduced, whereas

colysis. As a result, the glycogen content of muscles

quently, most of the extra energy required during stren-

rapidly than can oxidative release of energy. Conse-

can be maintained for only 5 to 10 seconds.

using all the phosphocreatine, maximum contraction

the cells is three to eight times this amount, but even by

than a second or so. The amount of phosphocreatine in

5 mmol/L of intracellular fluid, and this amount can

The maximum amount of ATP in muscle is only about

in the muscle cells, (2) phosphocreatine in the cells, and

comes from anaerobic sources: (1) ATP already present

they are too slow to respond. Instead, the extra energy

are much less capable during prolonged activity. Most

lactic acid, which diffuses out of the cells, as described

ting into pyruvic acid, and the pyruvic acid becoming

from glycolysis—that is, the glycogen of the cells split-

of energy. This can be derived for another minute or so

processes functioning for only about 2 minutes. Contin-

blood. This oxygen is sufficient to keep the metabolic

Chapter 72

Energetics and Metabolic Rate

883

additional amount stored in the hemoglobin of the

ued life beyond this time requires an additional source

in Chapter 67.

Anaerobic Energy Utilization During Strenuous Bursts of Activity
Is Derived Mainly from Glycolysis.

Skeletal muscles can

perform extreme feats of strength for a few seconds but

of the extra energy required during these bursts of activ-
ity cannot come from the oxidative processes because

(3) anaerobic energy released by glycolytic breakdown
of glycogen to lactic acid.

maintain maximum muscle contraction for no more

Release of energy by glycolysis can occur much more

uous activity that lasts for more than 5 to 10 seconds but
less than 1 to 2 minutes is derived from anaerobic gly-

vert about four fifths of the lactic acid into glucose;
the remainder becomes pyruvic acid and is degraded

the glucose is then transported in the blood back to the

glycogen.

Oxygen Debt Is the Extra Consumption of Oxygen After Completion
of Strenuous Exercise.

After a period of strenuous exer-

large amounts of oxygen for at least a few minutes and

tional oxygen is used (1) to reconvert the lactic acid that

(5) to raise the concentration of oxygen in the lungs to

exercise is over is called repaying the

of a person to build up an oxygen debt is especially

Summary of Energy Utilization
by the Cells

anaerobic utilization of glycogen and glucose to form

because larger quantities of phosphocreatine are

tioning systems of the cells to provide for synthesis

demanded for cellular activities than can be provided

Creatine + PO

5. Active absorption

ATP

Acetyl-CoA

Deaminated
amino acids

Other substrates

Glucose

Glycogen

AMP

Phosphocreatine

(Anaerobic)

Energy for
1. Synthesis and growth
2. Muscular contraction
3. Glandular secretion
4. Nerve conduction

6. Etc.

Lactic acid Pyruvic acid

Press, 1946, 1952.)

(Modified from Soskin S, Levine

fer from foods to the adenylic

Figure 72–1

Overall schema of energy trans-

acid system and then to the
functional elements of the cells.

R: Carbohydrate Metabolism.
Chicago: University of Chicago

person with diabetes mellitus—that is, the substrate

by the rising levels of the curves. As an example, when

the reaction increases proportionately, as demonstrated

from an arbitrary value of 1 up to 2, 4, or 8, the rate of

enzyme. Thus, as the enzyme concentration increases

of the figure, the rate of a chemical reaction is deter-

Figure 72–2 shows that

Role of Enzyme Concentration in Regulation of Metabolic

Figure

Michaelis-Menten equation.

This is called the

tration of the substrate that binds with the enzyme. The

that it can react with other substances. Therefore, the

loosely with one of the substrates of the reaction. This

The mechanism by which an enzyme catalyzes a

throughout the body.

enzymatically catalyzed chemical reactions, which are

cussing the control of energy release in the cell, it is nec-

in the Cell

Control of Energy Release

slower rates of usage, the oxidative processes can con-

nearly as rapidly as the anaerobic processes can, but at

Metabolism and Temperature Regulation

884

Unit XIII

cannot deliver bursts of extreme energy to the cells

tinue as long as energy stores (mainly fat) exist.

Rate Control of Enzyme-Catalyzed Reactions.

Before dis-

essary to consider the basic principles of rate control of

the types of reactions that occur almost universally

chemical reaction is for the enzyme first to combine

alters the bonding forces on the substrate sufficiently so

rate of the overall chemical reaction is determined by
both the concentration of the enzyme and the concen-

basic equation expressing this concept is as follows:

72–2 shows the application of this equation.

Reactions.

when the substrate is

present in high concentration, as shown in the right half

mined almost entirely by the concentration of the

large quantities of glucose enter the renal tubules in a

glucose is in great excess in the tubules—further
increases in tubular glucose have little effect on glucose

Substrate

Enzyme

Substrate

Rate of reaction

[

]

[

]

[

]

peptide linkages, and this stores energy in these link-

thesized, large portions of ATP are used to form the

becomes heat. For example, when proteins are syn-

tional systems of the cells, most of this eventually

the functional systems.

so that even under optimal conditions, no more than 27

ferred from ATP to the functional systems of the cells,

tion. Then, still more energy becomes heat as it is trans-

the energy in foods becomes heat during ATP forma-

of this energy becomes heat. On average, 35 per cent of

in foods is transferred to ATP; instead, a large portion

the preceding chapters, we noted that not all the energy

Body.

heat liberation during chemical reactions.

ical reactions in all the cells of the body, and the

The

because all the ADP soon becomes ATP.

absence of cellular activity, the release of energy stops

trolled by the degree of activity of the cell. In the

process, the amount of energy released in the cell is con-

release of energy from food. Thus, by this simple

of activity of the cell. This ADP then automatically

concentration of ADP in direct proportion to the degree

of activity, ATP is converted into ADP, increasing the

When the cells become active, regardless of the type

for almost all energy metabolism of the body.

energy in the body. Thus,

metabolic pathways that release energy from food, as

strates are quite slow. They include all the oxidative

cal reactions that depend on ADP as one of the sub-

ADP in the cells is extremely slight, so that the chemi-

conditions, the concentration of

in the entire series.

rate of reaction of the slowest step in the series. This is

tion, and so on. Therefore, the overall rate of a complex

reactions of the body occur in series, with the product

and renal tubules when their concentrations are low.

enzyme concentration. This is the relationship seen in

tion, the rate of the reaction becomes directly propor-

Note also in Figure 72

Role of Substrate Concentration in Regulation of Meta-

the concentration of the glucose itself.

transport enzymes in the proximal tubular cells, not by

rated. Under these conditions, the rate of reabsorption

reabsorption, because the transport enzymes are satu-

of the glucose is limited by the concentration of the

bolic Reactions.

–2 that when the

substrate concentration becomes low enough that only
a small portion of the enzyme is required in the reac-

tional to the substrate concentration as well as the

the absorption of substances from the intestinal tract

Rate Limitation in a Series of Reactions.

Almost all chemical

of one reaction acting as a substrate for the next reac-

series of chemical reactions is determined mainly by the

called the rate-limiting step

ADP Concentration as a Rate-Controlling Factor in Energy
Release.

Under resting

well as essentially all other pathways for the release of

ADP is a major rate-limiting

factor

increases the rates of all the reactions for the metabolic

Metabolic Rate

metabolism of the body simply means all the chem-

meta-

bolic rate is normally expressed in terms of the rate of

Heat Is the End Product of Almost All the Energy Released in the

In discussing many of the metabolic reactions in

per cent of all the energy from food is finally used by

Even when 27 per cent of the energy reaches the func-

Enzyme concentration

Rate of reaction

Substrate concentration

enzyme-catalyzed reaction.

Effect of substrate and enzyme concentrations on the rate of

Figure 72–2

In general, over a 24-hour period, a person performing

about 17 times as much energy as lying in bed asleep.

physical activity. For example, walking up stairs requires

ent individuals, depending on the type and amount of

energy expenditure, but it can vary markedly in differ-

The amount of energy used to perform daily physical

ing only essential functions is 2000 Calories.

2000 to 2250 Calories. Therefore, the approximate daily

without exercising, his total energy requirement reaches

about 1850 Calories per day. If he sits in a chair all day

or more Calories, so that the same man lying in bed and

amount of energy used each day by an additional 200

bed all day uses about 1650 Calories of energy. The

for Daily Activities

processing food; and (4) maintaining body temperature.

various physical activities; (3) digesting, absorbing, and

metabolic rate); (2) performing

into several measurable components, including energy

American diet. Energy output can also be partitioned

from fats, and 15 per cent from proteins in the average

intake is derived from carbohydrates, 40 per cent

stable body weight. About 45 per cent of daily energy

As discussed in Chapter 71, energy intake is balanced

Energy Output

Energy Metabolism—

versely, if the person obtains most energy from fat, the

Calories/L), would be about 4 per cent too little. Con-

for the average energy equivalent of oxygen (4.825

culated quantity of energy liberated, based on the value

the period of the metabolic rate determination, the cal-

given period of time.

oxygen; using this energy equivalent, one can calculate

This is called the

4.825 Calories.

ated per liter of oxygen used in the body averages about

lized. For the average diet, the

oxygen, regardless of the type of food being metabo-

gures, it is striking how nearly equiva-

4.60 Calories.

are released; with fat, 4.70 Calories; and with protein,

released; when metabolized with starches, 5.06 Calories

metabolized with glucose, 5.01 Calories of energy are

rate of oxygen utilization. When 1 liter of oxygen is

be calculated with a high degree of accuracy from the

different foods, the whole-body metabolic rate can also

Indirect Calorimetry—The “Energy Equivalent” of Oxygen.

and is used only for research purposes.

s body.

an accurate thermometer, is equal to the rate at which

heat gain by the water bath, which can be measured with

the air through pipes in a cool water bath. The rate of

air of the chamber. However, the air temperature within

chamber. Heat formed by the subject

The

calorimeter.

body in a large, specially constructed

try, one measures the quantity of heat liberated from the

heat liberated from the body in a given time.

external work, the whole-body metabolic rate can be

Direct Calorimetry Measures Heat Liberated from the Body.

Metabolic Rate

Measurement of the Whole-Body

kilocalorie,

body. Consequently, the Calorie

C. The calorie is

is the unit used for this purpose. It will be recalled that

functional processes of the body. Most often, the

and related subjects intelligently, it is necessary to use

To discuss the metabolic rate of the body

diture of energy is not taking place, all the energy

raising a mass against gravity. But when external expen-

body up steps, a type of potential energy is created by

some form of work outside the body. For instance, when

eventually converted into heat. The only signi

vessels, the friction of the different layers of blood

tial energy. As the blood

pumping blood. The blood distends the arterial system,

within the tissues, which generates heat.

limbs can move. This viscous movement causes friction

ity. Much of this energy simply overcomes the viscosity

body.

When proteins are degraded, the energy stored in the

ages. But there is also continuous turnover of proteins

Chapter 72

Energetics and Metabolic Rate

885

—

some being degraded while others are being formed.

peptide linkages is released in the form of heat into the

Another example is the energy used for muscle activ-

of the muscles themselves or of the tissues so that the

Consider also the energy expended by the heart in

and this distention itself represents a reservoir of poten-

flows through the peripheral

flowing over one another and the friction of the blood
against the walls of the vessels turn all this energy into
heat.

Essentially all the energy expended by the body is

ficant

exception occurs when the muscles are used to perform

the muscles elevate an object to a height or propel the

released by the metabolic processes eventually becomes
body heat.

The Calorie.

some unit for expressing the quantity of energy released
from the different foods or expended by the different

Calorie

1 calorie—spelled with a small “c” and often called a
gram calorie—is the quantity of heat required to raise
the temperature of 1 gram of water 1°
much too small a unit when referring to energy in the

—sometimes spelled

with a capital “C” and often called a

which

is equivalent to 1000 calories—is the unit ordinarily
used in discussing energy metabolism.

Because a person ordinarily is not performing any

determined by simply measuring the total quantity of

In determining the metabolic rate by direct calorime-

subject is placed in an air chamber that is so well insu-
lated that no heat can leak through the walls of the

’s body warms the

the chamber is maintained at a constant level by forcing

heat is liberated by the subject’

Direct calorimetry is physically difficult to perform

Because more than 95 per cent of the energy expended
in the body is derived from reactions of oxygen with the

Using these fi

lent are the quantities of energy liberated per liter of

quantity of energy liber-

energy equivalent of

with a high degree of precision the rate of heat libera-
tion in the body from the quantity of oxygen used in a

If a person metabolizes only carbohydrates during

calculated value would be about 4 per cent too great.

Factors That Influence

with energy output in healthy adults who maintain a

used for (1) performing essential metabolic functions of
the body (the “basal”

Overall Energy Requirements

An average man who weighs 70 kilograms and lies in

process of eating and digesting food increases the

eating a reasonable diet requires a dietary intake of

energy requirement for a very sedentary man perform-

activities is normally about 25 per cent of the total

heavy labor can achieve a maximal rate of energy uti-

skeletal muscle mass.

cant. Much of this effect of the male

increase the BMR a small amount, but usually not

about 10 to 15 per cent. The female sex hormones may

The male sex

regions.

living in different geographical zones; for example,

body and therefore increases metabolic rate. Adapta-

normal. As discussed in Chapter 76, thyroxine increases

normal. Conversely, total loss of thyroid secretion

gland secretes maximal amounts of thyroxine, the meta-

When the thyroid

uence the BMR, as discussed next.

of adipose tissue. However, there are other factors that

women, compared with men, are due partly to their

rate of metabolism. Likewise, slightly lower BMRs in

ment of muscle with adipose tissue, which has a lower

females of different ages are shown in Figure 72

height and weight. The average values for males and

square meter of body surface area, calculated from

reason, BMR is usually corrected for differences in

accounts for 20 to 30 per cent of the BMR. For this

Skeletal muscle, even under resting conditions,

amount of skeletal muscle and body size.

other organs, the

ties of the central nervous system, heart, kidneys, and

per hour in an average 70-kilogram man. Although

The BMR normally averages about 65 to 70 Calories

6. No physical activity is permitted during the test.

5. The temperature of the air must be comfortable

4. All psychic and physical factors that cause

3. No strenuous activity is performed for at least 1

sleep.

2. The BMR is determined after a night of restful

hours.

1. The person must not have eaten food for at least 12

s metabolic rate with that of another. The usual

able among different individuals, measurement of

expenditure in most sedentary individuals (Figure 72

of the body. This minimum level of energy required

Even when a person is at complete rest, considerable

for the Body to Exist

The Minimum Energy Expenditure

ical activity.

lization as great as 6000 to 7000 Calories, or as much as

Metabolism and Temperature Regulation

886

Unit XIII

3.5 times the energy used under conditions of no phys-

Basal Metabolic Rate (BMR)—

energy is required to perform all the chemical reactions

to exist is called the basal metabolic rate (BMR) and
accounts for about 50 to 70 per cent of the daily energy

–3).

Because the level of physical activity is highly vari-

the BMR provides a useful means of comparing one
person’
method for determining BMR is to measure the rate of
oxygen utilization over a given period of time under the
following conditions:

hour before the test.

excitement must be eliminated.

and between 68° and 80°F.

much of the BMR is accounted for by essential activi-

variations in BMR among different

individuals are related mainly to differences in the

body size by expressing it as Calories per hour per

–4.

Much of the decline in BMR with increasing age is

probably related to loss of muscle mass and replace-

lower percentage of muscle mass and higher percentage

can infl

Thyroid Hormone Increases Metabolic Rate.

bolic rate sometimes rises 50 to 100 per cent above

decreases the metabolic rate to 40 to 60 per cent of

the rates of the chemical reactions of many cells in the

tion of the thyroid gland—with increased secretion in
cold climates and decreased secretion in hot climates—
contributes to the differences in BMRs among people

people living in arctic regions have BMRs 10 to 20 per
cent higher than those of persons living in tropical

Male Sex Hormone Increases Metabolic Rate.

hormone testosterone can increase the metabolic rate

enough to be signifi
sex hormone is related to its anabolic effect to increase

Growth Hormone Increases Metabolic Rate.

Growth hormone

can increase the metabolic rate 15 to 20 per cent as a
result of direct stimulation of cellular metabolism.

Thermic effect of food (8%)

100

Purposeful physical activity (25%)

Nonexercise activity (7%)

Arousal

Sleeping

metabolic

rate

Basal

metabolic

rate (60%)

% Daily energy usage

Components of energy expenditure.

Figure 72–3

54
52
50
48
46
44
42
40
38

Males

Females

34
32
30

Age (years)

Basal metabolism (Calories/m

/hour)

Normal basal metabolic rates at different ages for each sex.

Figure 72–4

intake. The mechanism responsible for sympathetic acti-

buffer against obesity. Recent studies indicate that

per cent, although this might increase signi

who has virtually no brown fat, is probably less than 15

s metabolism more than 100 per cent.The mag-

cells, and maximal sympathetic stimulation can increase

large amount of heat but almost no ATP, so that almost

the sympathetic nerves, the mitochondria produce a

That is, when the cells are stimulated by

of one large fat globule. In these cells, the process of

amounts of heat. This type of fat contains large numbers

brown fat,

In certain types of fat tissue, called

which releases norepinephrine and epinephrine, which

response to cold stress. This type of thermogenesis is

nonshivering thermogenesis,

as discussed in Chapter 73. Another mechanism,

by increasing muscle activity in response to cold stress,

aimed primarily at regulation of body temperature.

food cause liberation of heat, these mechanisms are not

in many persons.

The thermogenic effect of food accounts for

and this lasts for 3 to 12 hours. This effect of protein on

the metabolic rate usually begins rising within an hour,

about 4 per cent. However, after a high-protein meal,

hydrates or fats, the metabolic rate usually increases

body. This is called the

with digestion, absorption, and storage of food in the

After a meal is ingested, the metabolic rate increases as

Thermogenic Effect of Food

s daily energy usage.

Together,

dgeting.

no daily exercise or physical work, signi

physical activity to prevent excess fat stores and obesity.

and the excess energy is stored mainly as fat.This under-

caloric intake periodically exceeds energy expenditure,

food supplies are plentiful, such as the United States,

balance. However, in industrialized countries where

cal activity among individuals, this component of energy

Table 72

body for a few seconds to about 50 times normal, or to

a few seconds. For the entire body, maximal muscle

rate is strenuous exercise. Short bursts of maximal

The factor that most dramatically increases metabolic

Energy Used for Physical Activities

decrease in metabolic rate, to the extent that the body

nal stages of many disease conditions, the

cells. In the

This fall is due to two principal factors: (1) decreased

decreases 10 to 15 per cent below normal during sleep.

The metabolic rate

in temperature. This is discussed in more detail in

an average of about 120 per cent for every 10

cause, increases the chemical reactions of the body by

Fever, regardless of its

Chapter 72

Energetics and Metabolic Rate

887

Fever Increases Metabolic Rate.

°C rise

Chapter 73.

Sleep Decreases Metabolic Rate.

tone of the skeletal musculature during sleep and (2)
decreased activity of the central nervous system.

Malnutrition Decreases Metabolic Rate.

Prolonged malnutri-

tion can decrease the metabolic rate 20 to 30 per cent,
presumably due to the paucity of food substances in the

inanition that accompanies the disease causes a marked

temperature may fall several degrees shortly before
death.

muscle contraction in a single muscle can liberate as
much as 100 times its normal resting amount of heat for

exercise can increase the overall heat production of the

about 20 times normal for more sustained exercise in a
well-trained individual.

–1 shows the energy expenditure during dif-

ferent types of physical activity for a 70-kilogram man.
Because of the great variation in the amount of physi-

expenditure is the most important reason for the dif-
ferences in caloric intake required to maintain energy

scores the importance of maintaining a proper level of

Even in sedentary individuals who perform little or

ficant energy

is spent on spontaneous physical activity required
to maintain muscle tone and body posture and on other
nonexercise activities such as “fi

”

these nonexercise activities account for about 7 per cent
of a person’

Energy Used for Processing Food—

a result of the different chemical reactions associated

thermogenic effect of food,

because these processes require energy and generate
heat.

After a meal that contains a large quantity of carbo-

reaching a maximum of about 30 per cent above normal,

the metabolic rate is called the specific dynamic action
of protein.
about 8 per cent of the total daily energy expenditure

Energy Used for Nonshivering
Thermogenesis—Role of
Sympathetic Stimulation

Although physical work and the thermogenic effect of

Shivering provides a regulated means of producing heat

can also produce heat in

stimulated by sympathetic nervous system activation,

in turn increase metabolic activity and heat generation.

sym-

pathetic nervous stimulation causes liberation of large

of mitochondria and many small globules of fat instead

oxidative phosphorylation in the mitochondria is mainly
“uncoupled.”

all the released oxidative energy immediately becomes
heat.

A neonate has a considerable number of brown fat

the child’
nitude of this type of thermogenesis in an adult human,

ficantly after

cold adaptation.

Nonshivering thermogenesis may also serve as a

sympathetic nervous system activity is increased in
obese persons who have a persistent excess caloric

Energy Expenditure During Different Types of Activity for a

Table 72–1

Walking up stairs rapidly

1100

Running (5.3 miles per hour)

570

Swimming

500

Sawing wood

480

Carpentry, metalworking, industrial painting

240

Walking slowly (2.6 miles per hour)

200

Typewriting rapidly

140

Dressing and undressing

118

Standing relaxed

105

Sitting at rest

100

Awake lying still

Sleeping

Form of Activity

Calories per Hour

70-Kilogram Man

Extracted from data compiled by Professor M. S. Rose.

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Winder WW: Energy-sensing and signaling by AMP-acti-

Physiol 95:1728, 2003.

Wilson MM, Morley JE: Aging and energy balance. J Appl

ity on daily energy expenditure. Proc Nutr Soc 62:645,

Westerterp KR: Impacts of vigorous and non-vigorous activ-

J Exp Biol 204:3183, 2001.

Westerterp KR: Limits to sustainable human metabolic rate.

metabolism. Curr Opin Clin Nutr Metab Care 6:469, 2003.

van Marken Lichtenbelt WD, Daanen HA: Cold-induced

clinical implications. Ann Intern Med 139:205, 2003.

Silva JE: The thermogenic effect of thyroid hormone and its

53:276, 2004.

Seals DR, Bell C: Chronic sympathetic activation: conse-

1):S130, 2004.

mitochondrial uncoupling proteins. Diabetes 53(Suppl

Rousset S, Alves-Guerra MC, Mozo J, et al: The biology of

Annu Rev Pharmacol Toxicol 44:297, 2004.

and regulation of energy expenditure: a family affair.

Robidoux J, Martin TL, Collins S: Beta-adrenergic receptors

Diseases. 1998. Available at: http://www.nhlbi.nih.gov/

MD: National Heart, Lung, and Blood Institute and

and Obesity in Adults: The Evidence Report. Bethesda,

cation, Evaluation, and Treatment of Overweight

National Institutes of Health: Clinical Guidelines on the

tissue thermogenesis. News Physiol Sci 19:67, 2004.

Morrison SF: Central pathways controlling brown adipose

278:29385, 2003.

induced thermogenesis, and obesity. J Biol Chem

Lowell BB, Bachman ES: Beta-adrenergic receptors, diet-

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journey to obesity. Nat Med 10:355, 2004.

Evans RM, Barish GD, Wang YX: PPARs and the complex

96:3, 2004.

understanding of modern chronic diseases. J Appl Physiol

genotypes: connecting the dots toward an evolutionary

Chakravarthy MV, Booth FW: Eating, exercise, and

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Cannon B, Nedergaard J: Brown adipose tissue: function and

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Argyropoulos G, Harper ME: Uncoupling proteins and ther-

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thalamus. Sympathetic stimulation, by increasing ther-

ated partly through the effects of increased leptin, which

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Metabolism and Temperature Regulation

888

Unit XIII

activates pro-opiomelanocortin neurons in the hypo-

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