Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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ENERGY

Contents

Measurement of Food Energy

Intake and Energy Requirements

Measurement of Energy Expenditure

Energy Expenditure and Energy Balance

Measurement of Food Energy

J C Brown, AFRC Institute of Food Research,

Norwich, UK

This article is reproduced from Encyclopaedia of Food Science,

Background

0001 The accurate assessment of the energy value of foods

and diets is essential for the study of energy metabol-

ism and energy balance, and for dealing with prob-

lems of malnutrition and obesity. Whereas the body

oxidizes the constituents of food in the process of

metabolism, the bomb calorimeter determines the

energy released as a result of direct oxidation of the

food constituents, and so enables measurement of an

energy value. The energy value obtained is the heat of

combustion, which is the maximum potential energy

present in the food and is called its gross energy. The

gross energy makes no allowance for losses of energy,

owing to incomplete digestion or during metabolism.

0002 In nutritional studies, the energy values used are for

metabolizable energy, which is the energy available to

the body from food. These values are usually calcu-

lated using energy conversion factors applied to the

fat, protein, and carbohydrate content of a food, and

take into account the average energy losses in feces

and urine. For greater accuracy, normal losses of

energy to feces and urine can be deducted from the

gross energy, which is determined by bomb calorim-

etry, to give a metabolizable energy. In more precise

studies the metabolizable energy value is measured,

rather than calculated, by determining these energy

losses directly.

Principles of Bomb Calorimetry for the

Determination of Energy Values of Foods

0003 The principle that allows the determination of energy

value using bomb calorimetry is based on the first law

of thermodynamics. Implicit in this law is the finding

termed ‘the law of constant heat summation,’ which

appeared in a paper published by G.H. Hess in 1838.

Hess showed that the heat produced in a given

chemical reaction was always the same, regardless

of the intermediate steps.

0004The major sources of energy in foods are the

organic constituents fat, carbohydrates, and protein,

with smaller contributions from alcohol, organic

acids, and traces of other organic materials such as

mineral oils. The foods are completely oxidized in the

bomb calorimeter, to yield the end products: carbon

dioxide, water, oxides of nitrogen and sulfur, and

heat. In the process of bomb calorimetry, the sample

is rapidly combusted in oxygen at increased pressure,

and the heat production is measured. The apparatus

is calibrated with benzoic acid, a thermochemical

standard, and a heat of combustion is derived for

the food.

0005Occasionally, when an absolute measurement of

heat of combustion is required, corrections are ap-

plied to the result obtained, e.g., for (1) sulfuric acid

production from sulfur in the sample, (2) nitric acid

production present in the bomb during combustion,

and (3) departures from the standard state, pressure,

or temperature of reactants or products. For the

purposes of determining the energy values of foods

or biological samples, the above corrections are

normally sufficiently small to be ignored.

Types of Equipment Used

0006There are two different types of bomb calorimeter

currently in use: the adiabatic (without gain or loss

of heat) and the ballistic. The adiabatic bomb is

favored for accurate nutritional studies. The ballistic

bomb gives more rapid determinations but with a

lower accuracy.

Development of the Adiabatic Bomb Calorimeter

0007The structure and operation of all bomb calorimeters,

from the earliest (dated around 1900) to those cur-

rently in use, are similar. The bomb chamber in which

combustion of the sample takes place is typically

made from noncorroding steel and submerged in a

water-filled calorimeter vessel. The water is continu-

ously stirred and its temperature recorded, before and

ENERGY/Measurement of Food Energy 2087

after it rises as a result of heat produced by the com-

bustion of the fuel. With the early designs of bomb

calorimeter, it was necessary to determine a cooling

correction, since some of the heat generated during

the combustion was lost to surrounding air. Assess-

ment of cooling correction involved the recording of

accurate temperature observations made at precise

time intervals, commencing at least 5 min before

firing and continuing until at least 5 min after a con-

stant rate of temperature change is resumed.

0008 Major developments in bomb calorimeter design

were concerned with providing heated outer water

jackets capable of minizing heat loss from the calorim-

eter vessel surrounding the bomb. In 1900, Richards

had used a closely controlled exothermic reaction

within the water jacket to control the temperature.

Benedict and Higgins, in 1910, introduced the use of

an electrical heating coil to control the temperature of

the water jacket, but rapid and close matching of the

temperatures of the jacket and calorimeter vessel

containing the bomb was still impossible. In 1916,

Daniels achieved this by using heating elements,

making the whole jacket a conductor. This system

was automated by Raymond, Canaway, and Harris,

in 1957, to produce an adiabatic bomb calori-

meter, which was the basis for current systems. The

Berthold–Maher and Maher–Cook bomb calorim-

eters, produced by Gallenkamp in the early 1950s,

had a simple copper water jacket to provide some

insulation from the surroundings, but around the

late 1950s, Gallenkamp were producing an adiabatic

bomb calorimeter.

0009 The current adiabatic bomb, produced by Sanyo

Gallenkamp, is shown in Figure 1 and is an improved

version of the early ones described above. It is

equipped with heaters, which, using a sensitive elec-

trical system, maintain the outer water jacket at

the same temperature as that of the water in the

calorimeter vessel throughout the period of oper-

ation. Employment of the adiabatic concept elimin-

ates cooling from the calorimeter vessel and the need

for a cooling correction. This gives the system advan-

tages over previous designs: (1) determinations are

more rapid and lead to less operator fatigue, since

continuous readings of temperature are not neces-

sary; (2) readings only need to be taken once the

equipment has stabilized, before and after the tem-

perature rise resulting from combustion. Further im-

provement in the control of the temperature of the

outer water jacket has been achieved by employing a

heated reservoir of water (shown in Figure 1), which

injects warm water into that already circulating in

the outer water jacket.

Operation of the Adiabatic Bomb Calorimeter

0010The energy value is determined on a homogeneous

dry sample that is compacted into a preweighed

crucible. The electrodes are connected by a piece of

nickel/chromium fuse wire. To facilitate combustion,

a piece of cotton is tied to the wire and is placed in

contact with the sample below. The crucible and elec-

trodes are sealed in the bomb chamber, and oxygen

is introduced to a pressure of 3030 kPa. The bomb

chamber is placed inside the calorimeter vessel, which

contains a constant mass of water, and the lid of the

calorimeter is closed. The temperature of the water in

the calorimeter vessel is recorded, and, once stabil-

ized, the sample is ignited. The heat produced by the

combustion is liberated into the bomb, water, and

calorimeter vessel. The temperature of the outer

water jacket increases in parallel with that of the

water in the calorimeter vessel as heating elements

are activated via balanced thermistors located in the

water jacket and calorimeter vessel. A new steady

temperature is achieved in 5–8 min and the final

temperature reading taken. The heat capacity (heat

Matched

thermisters

Digital

thermometer

Oxygen

inlet valve

Electrode

assembly with

nickel/chromium

fuse and cotton

wick

Crucible

containing

sample

Bomb

Water

circulator

Control of

solenoid

valve

Solenoid

valve

Heaters

Heated

reservoir

Stirrer

Firing

plug

Calorimeter

Outer water jacket

fig0001 Figure 1 Adiabatic bomb calorimeter. Reproduced from Energy: Measurement of Food Energy, Encyclopaedia of Food Science, Food

Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

2088 ENERGY/Measurement of Food Energy

released per unit temperature rise) of the apparatus is

determined using benzoic acid, and the temperature

rise obtained from samples is translated into the

energy released. This type of bomb allows corrections

for sulfuric acid and nitric acid production to be

made, if required, by collection and dual titration of

the rinsings from the inside of the bomb chamber.

This process is extremely tedious and is unnecessary

for most purposes (unless the amount of sulfur pre-

sent in the fuel leads to a significant quantity of extra

heat release when it is converted to sulfuric acid

rather than sulfur dioxide). Control of the adiabatic

bomb calorimeter by microcomputer is also now pos-

sible, but this only controls recording measurements

of temperature, and firing. The more labor-intensive

parts of the operation – preparation of samples, intro-

duction of oxygen, and cleaning and reloading of the

bomb – must still be performed by the operator.

Development of the Ballistic Bomb Calorimeter

0011 In 1959, Fox, Miller, and Payne described an alterna-

tive to the adiabatic bomb, giving quicker determin-

ations of energy values. Their design made use of

a light bomb casing and eliminated the water-filled

calorimeter vessel. On combustion, most of the heat

produced is transmitted to the upper parts of the bomb

casing and measured using a thermocouple linked to a

meter. No attempt is made to measure the equilibrium

temperature of the bomb. In the modified version, a

thermocouple is attached to the lid and connected to a

galvanometer via a series resistance. This bomb,

known as the ballistic bomb, was subsequently

improved by reducing the thermal capacity of the

upper casing.

0012The current ballistic bomb, shown in Figure 2,

is directly descended from that described above.

Possible sources of error involved variation in (1)

the oxygen pressure, (2) the initial temperature, and

(3) the crucible position in the bomb. These were

standardized at the optimum level for operation.

Operation of the Ballistic Bomb Calorimeter

0013The sample, crucible, and electrodes are assembled in

the bomb chamber, as in the adiabatic bomb. Oxygen

is introduced, and, once stabilized (30 s), the sample is

ignited, with maximum deflection on the galvano-

meter occurring after about 40 s. Deflection of the

galvanometer scale is translated into the energy

released by calibration with benzoic acid.

Comparison of Adiabatic and Ballistic Bomb

Calorimeters

0014The ballistic bomb has an accuracy of +1.1% on six

determinations, compared with +0.1% on two de-

terminations with the adiabatic bomb. It was argued

that a standard error of less than +0.5% of the

energy value is essential in accurate energy-balance

studies. This level of accuracy would not normally be

obtained with the ballistic bomb unless about 15

replicates were carried out. It was confirmed that to

obtain the same precision on the ballistic bomb as

that of the adiabatic, six times as many replicates

would have to be carried out. Since the ballistic

bomb is only two to three times faster, it is not a

Galvanometer

connector

Nickel/chromium fuse

and cotton wick

Crucible containing

sample

Rubber 'O' ring

Firing

plugs

Stainless steel

bomb

Electrodes

Sealing ring

Stainless steel

base

fig0002 Figure 2 Ballistic bomb calorimeter. Reproduced from Energy: Measurement of Food Energy, Encyclopaedia of Food Science, Food

Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

ENERGY/Measurement of Food Energy 2089

serious alternative to the adiabatic system for accur-

ate energy balance studies. Table 1 gives average

heats of combustion or gross energies of some whole

foods and protein, fat, and carbohydrate, determined

using bomb calorimetry.

Limitations of Gross Energy Values

Obtained for Diets

001 5 The energy value determined for a diet using bomb

calorimetry isthe energy thatisreleasedwhen the fuels

are fully oxidized. This complete oxidation does not

occur in the body. The energy value obtained for the

food (gross energy) takes no account of the digestibil-

ity or metabolizability of the energy in the food by the

body. Equations (1) and (2) show the relationship that

exists between gross, digestible, metabolizable, fecal

and urinary energy (GE, DE, ME, FE, and UE,

respectively):

GE  FE ¼ DE :ð1Þ

The digestibility of the energy of protein, fat, and

carbohydrate is reckoned to be about 97%, 95%,

and 98%, respectively.

DE  UE ¼ ME :ð2Þ

This step sometimes involves additional energy losses

owing to combustible gases produced during anaer-

obic fermentation.

001 6 The energy in protein is not fully metabolized,

some energy-yielding nitrogenous components being

present in urine. Atwater determined the heat com-

bustion of food components and found that urine

contained 5.23kJ g

1

(1.25 cal g

1

) of absorbable

protein, if the subject was in nitrogen equilibrium.

(See Protein: Digestion and Absorption of Protein

and Nitrogen Balance.)

0017 The usual purpose of determining the energy value

is to obtain a measure of the energy that the body

gains from a food, its metabolizable energy value.

Metabolizable energy can be measured by determin-

ation of gross energy content of diet, feces and urine

(eqns (1) and (2)). Collection and analysis of these

materials, however, are time-consuming and difficult,

especially in humans. The alternative that is most

commonly employed, but less accurate, involves the

use of appropriate energy conversion factors. These

can be used to calculate the metabolizable energy

from the composition of the food or diet, or to correct

the gross energy obtained by bomb calorimetry for

the usual energy losses to feces and urine.

0018 In 1896, Atwater put forward energy conversion

factors for calculating the metabolizable energy

obtained from protein, fat and carbohydrate of 17,

37 and 17 kJ g

1

(4, 9, 4 kcal g

1

), respectively, which

accounted for losses to feces and urine. These factors

vary between foods, and, using the more precise

system of Merrill and Watt, it is possible to use the

compositional data and energy conversion factors

derived for each individual food to calculate the

metabolizable energy value. The British system of

average factors used for most purposes are 17 kJ g

1

(4.0 kcal g

1

) for total protein, 37 kJ g

1

(9 kcal g

1

)

for total fat, and 16 kJ g

1

(3.75 kcal g

1

) for total

available carbohydrate (measured as monosacch-

arides).

0019An alternative to the use of compositional data for

estimating metabolizable energy is to correct the

gross energy, determined by bomb calorimetry, for

usual energy losses in feces and urine. Miller and

Payne assumed 95% digestibility of diets and put

forward a system for estimating the metabolizable

energy from determined gross energy (kJ g

1

) and

tbl0001Table 1 Average heats of combustions of whole foods and of

protein, fat, and carbohydrate from different types of foods

Material Heat of combustion

(kJg

1

, dry-weight basis)

Food type

Beef 27.2

Mutton 30.0

Pork 32.8

Poultry, fish 26.4

Eggs 29.7

Butter 37.0

Milk 22.8

Bread 19.0

Rice 18.3

Oats, rolled 20.2

Legumes 18.2

Vegetables, fresh 17.6

Fruit, fresh 18.4

Proteins

Lean-meat protein 23.6

Milk protein 23.7

Egg protein 24.1

Wheat, gluten 24.9

Legumin 24.2

Fats

Meat fat 39.8

Lard 40.1

Butter fat 38.8

Cereal oil 39.0

Nut, olive oil 39.7

Carbohydrates

Glucose 15.6

Cane sugar 16.6

Milk sugar 16.2

Starch 17.5

Cellulose 17.5

Glycogen 17.5

Nonstarch polysaccharide

Cereal 17.5 (16.7–18.5)

Vegetable 16.8 (16.2–17.9)

Fruit 16.5 (14.9–17.3)

2090 ENERGY/Measurement of Food Energy

the nitrogen content of the diet (N;gkg

1

), which is

shown in eqn (3). This equation is appropriate for

determination of ME (kgg

1

) in Western diets that

are low in dietary fiber:

ME ¼ 0:95GE  31 :88N :ð3 Þ

However, 95% digestibility is not always true, espe-

cially of diets high in dietary fiber. An alternative

system, based on a regression equation that deducts

a value of 16.7 kJ g

1

(4 kcal g

1

) for unavailable

carbohydrate (UC), or dietary fiber was proposed.

This equation (eqn (4)) was derived using diets of up

to 30 g of dietary fiber per day; thus, it has this upper

limit.

ME ¼ 0:977GE  27:6N  16 :7UC :ð4 Þ

A further equation (eqn (5)) takes account of the

lowering of digestibility of gross energy diets as the

level of dietary fiber (F; %) increases. This equation is

used where dietary fiber intake is above 30 g per day,

and especially where the diet is high in cereals.

ME ¼ 0:95  FðÞGE  31 :4N :ð5 Þ

An equation, based on a regression analysis of many

diets, including those of more than 30 g of dietary

fiber per day, was derived. This equation (eqn (6))

accounts for the effect of unavailable carbohydrate

on the digestible energy content of the diet and can be

used in all cases, except with high-cereal diets. Equa-

tions (5) and (6) therefore complement each other in

terms of application.

ME ¼ 0:96GE  9UC  30N :ð6 Þ

The coefficient of individual variation on these equa-

tions is around 1% of GE; however, there is uncer-

tainty about how appropriate they are for predicting

ME of diets of the elderly, the very young (under 8

years), individuals in the diseased state or those of

differentraces.Sincetheenergyequationsareintended

forcalculatingavailableenergyfromwholefoods,they

are most likely to be inappropriate when used for

diets including isolates, or novel ingredients, which

may fall outside of the scope of the derived equations.

0020 To obtain an accurate value of DE or ME, the heat

of combustion of diet, feces, and urine must be deter-

mined by bomb calorimetry. Most of the considered

variation in metabolizability of a diet or ingredient

comes about as a result of a difference in digestibility

of energy rather than metabolizability (except for

very poorly metabolized materials such as novel

sugars that are excreted in the urine). It is therefore

sometimes sufficient and more practical in accurate

biological studies to make a measurement of DE –

which does not require collection and analysis of

urine – rather than ME.

Seealso: Protein:DigestionandAbsorptionofProteinand

NitrogenBalance

Further Reading

Livesey G (1990) Energy values of unavailable carbo-

hydrate and diets: an inquiry and analysis. American

Journal of Clinical Nutrition 51: 617–637.

McLean JA and Tobin G (1987) Animal and Human Calor-

imetry. Cambridge: Cambridge University Press.

Merril AL and Watt BK (1955) Energy Value of Foods:

Basis and Derivation. Washington, DC: US Government

Printing Office.

Miller DS and Payne PR (1959) A ballistic bomb calorim-

eter. British Journal of Nutrition 13: 501–508.

Intake and Energy

Requirements

G R Goldberg,BritishNutritionFoundation,

London,UK

Introduction

0001This chapter outlines the progress that has been made

in the last 10 years in the evaluation and interpret-

ation of energy intake data, and in the use of energy

expenditure data to define energy requirements. In

particular, the data presented here illustrate the

important contribution of doubly labelled water

measurements of total free-living energy expenditure.

These have provided points of reference that can be

used to assess and define both energy intakes and

energy requirements.

Energy Intake

Gross, digestible, and metabolizable energy

obtained from food

0002The energy in food is provided by the four energy-

yielding macronutrients, protein, fat carbohydrate

and alcohol. The energy obtained from macronutri-

ent oxidation is utilized in a number of ways. It can be

used to perform mechanical, electrical, and chemical

work; be deposited as new tissue; or exported from

the body (e.g., as milk). The gross energy of a food is

the energy obtained when it is totally combusted in a

bomb calorimeter, and is the maximum amount of

energy that can be extracted. However, not all the

energy in food is available for use by the body. Not

all the macronutrients are digested completely, not all

ENERGY/Intake and Energy Requirements 2091

the products of digestion are absorbed completely,

and energy is lost in urine as nitrogenous compounds

(including urea, uric acid and creatinine), because the

protein that is digested and absorbed is not oxidized

completely.

Gross energy

food

 gross energy

feces

digestible energy

Digestible energy  protein energy

urine and feces

metabolizable energy:

Estimates of Energy Content Using Food Tables

0003 Tables of food composition usually list the energy

content of foods not as gross, but as available or

metabolizable energy, i.e., allowances have already

been made for digestibility, and for the incomplete

oxidation of protein by using food specific or mixed

diet factors. Differences in the methods used to deter-

mine the carbohydrate content of individual foods

and inclusion of the nondigestible fractions may

lead to an overestimation of the energy content, par-

ticularly in fruits and vegetables. However, when cal-

culating the total energy content of a mixed diet, these

differences are of little practical significance. The

inherent variability of foodstuffs and how they are

prepared and cooked are greater sources of error. For

example: food tables may not represent the value of

foods and drinks as consumed, particularly when

dietary intake is recorded in terms of composite dishes

and not as separate ingredients; the energy content of

foods varies with the proportions of water and fat

that they contain; and recipes used by subjects for

dishes such as stews, casseroles, cakes, and puddings

may differ considerably from the recipe used to

calculate the food table value.

Measurements of Energy Intake

0004 -Rationale Dietary assessments form a central com-

ponent of many areas of clinical and research studies.

Various national and international committees make

recommendations with respect to the proportions of

the dietary energy intake from different food sources

that are considered to be best for optimal health. Data

from large epidemiological surveys provide vital

information for expert committees to consider when

formulating allowances and deciding issues of food

policy. Energy and nutrient intakes and requirements

in different groups of people need to be quantified at

an individual level to determine the relationships

between nutrition and health. Intakes also need to

be quantified at the population level to detect and

establish the prevalence of people who are not achiev-

ing adequate levels of energy and nutrient intake.

Some reported energy intake data from national

surveys conducted in the UK and published in the

last 10 years are shown in Table 1.

0005-Methods used to measure energy intake Measure-

ments of energy intake in free-living individuals are

obtained by dietary surveys. A record or estimate of

all food and drink consumed is collected, and the

energy content is then most usually estimated using

data from food tables. In some experimental settings,

where very accurate measurements are required, du-

plicate diets comprising foods actually consumed are

collected by researchers and subjected to bomb calor-

imetry. However, under free-living conditions, the

effort required of a subject to collect duplicate diets

leads to alteration of food patterns and to an under-

estimate of food consumption.

0006There are a number of different survey techniques

for estimating energy and nutrient intakes. The

methods used to collect data about food consumption

include weighed intake records, food diaries, collec-

tion of duplicate diets, diet history, 24-h recalls, and

food-frequency questionnaries. Recalls and question-

naires may also include methods of estimating

quantity by reference to models or photographs. The

chosen method will depend on a number of factors

such as resources available, number of subjects, foods

and nutrients of interest, other parameters to be meas-

ured, the purpose of study, and subject characteristics

such as age, behavioral, and health indices.

tbl0001Table 1 Mean reported energy intakes of free-living children

and adults in the UK

Age range

(years)

Males Females

MJ per day kcal per day MJ per day kcal per day

1.5–2.5 4.39 1045 4.39 1045

2.5–3.5 4.88 1160 4.88 1160

3.5–4.5 5.36 1273 4.98 1183

4–6 6.39 1520 5.87 1397

7–10 7.47 1779 6.72 1600

11–14 8.28 1971 7.03 1672

15–18 9.60 2285 6.82 1624

16–24 10.33 2460 7.14 1700

25–34 10.25 2440 7.02 1670

35–49 10.50 2500 7.27 1730

50–64 9.96 2380 6.76 1610

65–74 8.21 1954 6.07 1445

75–84 7.75 1845 5.88 1400

85þ 7.20 1713 5.77 1374

Sources: Gregory J, Foster K, Tyler H, and Wiseman M (1990) The Dietary

and Nutritional Survey of British Adults. London: The Stationery Office,

Gregory JR, Collins DL, Davies PSW, Hughes JM, and Clarke PC (1995)

National Diet and Nutrition Survey: Children Aged 1 to 4 Years.Volume 1: Report

of the Diet and Nutrition Survey. London: The Stationery Office, Gregory J,

Lowe S, Bates CJ et al. (2000) National Diet and Nutrition Survey: Young

People Aged 4 to 18 Years.Volume 1: Report of the Diet and Nutrition Survey.

London: The Stationery Office, Finch S, Doyle W, Lowe C et al. (1998)

National Diet and Nutrition Survey: People Aged 65 Years and Over.Volume 1:

Report of the Diet and Nutrition Survey. London: The Stationery Office, with

permission.

2092 ENERGY/Intake and Energy Requirements

0007 -Validity of energy-intake measurements It is often

assumed that energy intake is the easiest and most

straightforward component of the energy-balance

equation to measure accurately. Whilst this may be

so under strictly controlled experimental conditions,

it is far more problematical in free-living subjects.

Estimates of energy intake are subject to many errors,

some of which have been well recognized and under-

stood for many years. For example, food table values

do not necessarily reflect the actual food as eaten (see

above), inadequate descriptions of foods lead to

errors in choosing appropriate foods from the food

tables, and there may be errors in estimating the

quantities of food and drink consumed. However,

the magnitude and direction of other errors, and

their implications, only began to be appreciated in

the last 10 years. Up until then, the mean results for

energy intake in studies of diet and health were rarely

questioned. It was recognized that during a study,

some individuals’ reported energy intake would be

lower than their habitual intake, but it was assumed

that this was balanced by overestimates of intake at

the other end of the distribution, thus yielding valid

data for a group. Studies were based on the premise

that the methods used were accurate and precise and

that they provided valid estimates of habitual intake.

0008 -Evaluation of energy-intake measurements The de-

velopment in the early 1980s of the doubly labeled

water (DLW) technique for measuring energy expend-

iture in free-living people also provided an external

validation of measurements of energy intake. On a

group basis, energy intake may be presumed to be

equal to energy expenditure. Data from DLW meas-

urements have, therefore, not only provided informa-

tion of energy expenditure and energy requirements

(see below) under many physiological, behavioral and

experimentally imposed conditions, but also given

important insights into the validity and interpretation

of energy intake measurements. Of particular concern

is that the majority of dietary surveys are significantly

biased towards the underestimation of energy intake.

0009 Invalid dietary data have serious implications for

the interpretation of results. Apparent underestimates

of intake will falsely inflate the prevalence of inad-

equate nutrient intake, and between-subject differ-

ences in qualitative or quantitative reporting creates

bias that distorts the real associations between diet

and health.

0010 It is not feasible or necessary to carry out concur-

rent DLW measurements of energy expenditure when

the prime interest of a study is the measurement of

energy intake. However, the large body of DLW data

yielded from many studies conducted over the last 20

years, together with statistical considerations, that

take into account the number of subjects, duration

of measurements, and variability in energy intake and

expenditure, can be used as independent points of

reference for evaluating measurements of energy

intake.

0011-Developments in methodology of energy-intake

measurements Over the past 10 years, a great deal

of research has been undertaken by workers in many

countries. The work includes the exploration of the

statistical nature of food intakes in order to develop

better study designs, and strategies for obtaining reli-

able and habitual-intake data. Energy intake data are

now more closely scrutinized, and critically evaluated

and interpreted. The subgroups most likely to provide

records of poor validity are also being characterized.

This is turn means that psychological assessments

(dietary restraint, attitudes to food, health and other

topics) as well as the more traditional elements

(height, weight, assessment of occupation and leisure

activities need to be incorporated into study protocols

as a matter of routine. The origins and causes of mis-

reporting due to food- and meal-specific bias, and the

socioeconomic, demographic, behavioral, and psy-

chological characteristics of subjects are all the focus

of much research.

Energy Requirements

Definition of Energy Requirements

0012In 1985, the report of the Joint Food and Agriculture

Organization, World Health Organization and

United Nations University Expert Consultation stated

that:

The energy requirement of an individual is the level of

energy intake from food that will balance energy ex-

penditure when the individual has a body size and com-

position, and level of physical activity, consistent with

long-term good health; and that will allow for the main-

tenance of economically necessary and socially desirable

physical activity. In children and pregnant or lactating

women, the energy requirement includes the energy

needs associated with the deposition of tissues or the

secretion of milk at rates consistent with good health.

Rationale for Defining Energy Requirements

0013Estimates of energy requirements are required as the

basis of nutrition policy and dietary advice, to assess

the adequacy of diets and evaluate nutritional status,

to plan the food supplies of populations, and to plan

diets for institutionalized individuals. In many coun-

tries, expert committees are charged with estimating

the energy requirements of their own populations,

usually divided into different age and sex groups.

ENERGY/Intake and Energy Requirements 2093

The average energy requirements currently used in

the UK are shown in Table 2.

Determinants of Energy Requirements

0014 All body movements require energy in addition to

that required for maintenance. Strenuous activities

require more energy than sedentary activities, and

the cost of weight-bearing activities increases with

body weight. Whilst the gross energy costs of certain

activities may be very high during that activity, e.g.,

during laboring tasks or very athletic recreations,

these activities are rarely maintained for long periods

of time. The energy costs are diluted by the time spent

undertaking sedentary activities, e.g., sitting or stand-

ing around, and the average overall daily energy re-

quirement is further reduced by the time spent asleep.

The amount of physical activity due to differences in

occupation, leisure pursuits, and domestic circum-

stances determines the overall energy expenditure,

and therefore the energy requirements, of otherwise

similar individuals.

0015 In addition to the energy required for maintenance

and for physical activity, the biochemical synthesis of

new tissues for growth, development, repair, and milk

production incurs extra energy costs, and therefore the

energy requirements, for infants, children, pregnant

women and lactating women, and in some clinical

conditions. The energy requirements under these cir-

cumstances are difficult to quantify, and estimates are

often derived from stoichiometric calculations based

on rates and composition of weight gain, milk produc-

tion and tissue repair, and the effects of infection,

disease, trauma, and drug therapy on appetite and

energy expenditure.

Estimation of Energy Requirements

0016The 1985 FAO/WHO/UNU Expert Consultation

heralded a significant change in the way that recom-

mendations for dietary energy were determined and

defined. It was considered that observed energy

intakes, even if they were accurate, did not necessarily

represent desirable levels for body weight, physical

activity, and optimal health. It was concluded that

estimates of requirements should ‘as far as possible

be based on estimates of actual or desirable energy

expenditure.’ The 1985 Committee also stated that

‘Although, in principle, it would be desirable to de-

termine the requirements of children in the same way

as for adults, from measurements of energy expend-

iture, this approach involves many difficulties in prac-

tice.’ It was concluded that the necessary data were

not yet available to base recommendations for energy

requirements of infants and children on estimates of

energy expenditure, and so the use of information

about normally growing infants and children has

continued.

0017-Physical activity levels The 1985 FAO/WHO/UNU

report also established the principle of calculating

energy requirements for adults (although not preg-

nant and lactating women) as multiples of measured

or predicted basal metabolic rate (BMR). Multiples of

BMR are now widely referred to as physical activity

levels (PALs). These are a succinct way of expressing

the energy requirements of diverse groups of people,

and the values provide a means of comparing activity

levels between and within individuals. Because total

energy expenditure is expressed as a multiple of BMR,

most of the interindividual variability due to weight,

height, age, and sex is removed.

0018-Estimates of physical activity levels using factorial

calculations The PALs derived by the 1985 Consult-

ation were based on literature values of the energy

costs of various occupational and discretionary activ-

ities, and assumptions about their duration and inten-

sity. A factorial calculation was then used to determine

the theoretical overall 24-h energy requirement.

For example, it was estimated that the PAL for a

typical rural woman in a developing country was

1.76 BMR. This was based on the assumptions

that within each 24-h period, 8 h were spent asleep

(0.33 1.0 BMR); 3 h were spent on housework

and preparing food (0.13 2.7 BMR); 4 h were

spent working in fields (0.17 2.8 BMR); 2 h were

spent on discretionary activities (0.08 2.5 BMR);

and the remainder of the time (7 h), was spent on

sedentary activities (0.29 1.4 BMR). A similar

calculation indicated that the typical PAL of a man

tbl0002 Table 2 Estimated average requirements for energy in the UK

Age range Males Females

MJ per day kcal per day MJ per day kcal per day

0–3 months 2.28 545 2.16 515

4–6 months 2.89 690 2.69 645

7–9 months 3.44 825 3.20 765

10–12 months 3.85 920 3.61 865

1–3 years 5.15 1230 4.86 1165

4–6 years 7.16 1715 6.46 1545

7–10 years 8.24 1970 7.28 1740

11–14 years 9.27 2220 7.92 1845

15–18 years 11.51 2755 8.83 2110

19–50 years 10.60 2550 8.10 1940

51–59 years 10.60 2550 8.00 1900

60–64 years 9.93 2380 7.99 1900

65–74 years 9.71 2330 7.96 1900

75þ years 8.77 2100 7.61 1810

Source: Department of Health (1991) Dietary Reference Values for Food

Energy and Nutrients for the United Kingdom. London: The Stationery Office.

2094 ENERGY/Intake and Energy Requirements

employed as an office clerk was 1.54 BMR. The

survival requirement, with no allowance for energy

needed to earn a living or prepare food, was con-

sidered to be 1.27 BMR; a level of expenditure

expected to be found only in totally inactive depend-

ent persons and not compatible with long-term health

(see also Table 8). The estimated average daily energy

requirements of adults whose work is classified as

light, moderate, or heavy, are given in Table 3.

0019 The UK Committee on Dietary Reference Values

followed the approach of the FAO/WHO/UNU panel

and arrived at PALs considered to be more appropri-

ate for UK populations. In countries like the UK, it

is likely that the overall energy expenditure of an

individual may be influenced more by recreational

activities than by work. PAL values taking both occu-

pational and nonoccupational activity levels into ac-

count are shown in Table 4.

0020 -Measurements of physical activity levels from DLW

measurements The development in the early 1980s

of the DLW technique for measuring total energy

expenditure (TEE) in free-living people revolution-

ized knowledge and assumptions about energy ex-

penditure. The measurement of TEE over relatively

long periods is likely, in the majority of people, to be

representative of their habitual energy expenditure

and, by inference, their energy requirements. TEE

measured by DLW, used together with measured or

predicted BMR, has allowed PALs to be measured, as

well as estimated factorially. Studies conducted over

the last 20 years mean that there is now a large data-

set of DLW measurements of energy expenditure and

PALs measured under many physiological, behav-

ioral, and experimentally imposed conditions. Some

of these data are summarized in Tables 5, 6 and 7.

These tables illustrate the range of PALs depending on

age, and on activity levels that are typical of popula-

tions in developed countries.

0021There are also data that have been obtained from

individuals at the extremes of physical activity that

can be used to define the upper and lower limits of

human energy expenditure and, by inference, the

limits of energy requirements and energy intake.

Some of these data are summarized in Tables 8

and 9. It should be noted that at the upper level of

energy expenditure, it is important to draw a distinc-

tion between the maximum achievable level over a

limited period of time (e.g., intensive periods of

sports) and the maximum sustainable as a long-term

way of life given physical fitness and adequate food.

tbl0004 Table 4 Estimated average daily energy requirements of

adults at three levels of occupational and nonoccupational

activity, expressed as a multiple of BMR

Nonoccupational

activity

Occupational activity

Light Moderate Moderate/heavy

Male Female Male Female Male Female

Nonactive 1.4 1.4 1.6 1.5 1.7 1.5

Moderately active 1.5 1.5 1.7 1.6 1.8 1.6

Very active 1.6 1.6 1.8 1.7 1.9 1.7

Source: Department of Health (1991) Dietary Reference Values for Food

Energy and Nutrients for the United Kingdom. London: The Stationery Office.

tbl0005Table 5 Average total energy expenditure and PALs measured

by doubly labelled water in different age and sex groups

Age range

(years)

Males Females

per day

kcal

per day

PAL M J

per day

kcal

per day

PAL

1–6 6.1 1452 1.64 5.5 1310 1.57

7–12 9.8 2333 1.74 8.0 1905 1.68

13–17 14.1 3357 1.75 11.4 2714 1.73

18–29 13.8 3286 1.85 10.4 2476 1.70

30–39 14.3 3405 1.77 10.0 2381 1.68

40–64 11.5 2738 1.64 9.8 2333 1.69

65–74 11.0 2619 1.61 8.6 2048 1.62

75 9.2 2190 1.54 6.1 1452 1.48

Source: Black AE, Coward WA, Cole TJ and Prentice AM (1996) Human

energy expenditure in affluent societies: an analysis of 574 doubly-

labelled water measurements. European Journal of Clinical Nutrition 50:

72–92.

tbl0003 Table 3 Estimated average daily energy requirements of

adults whose work is classified as light, moderate, or heavy,

expressed as a multiple of BMR

Light Moderate Heavy

Men 1.55 1.78 2.10

Women 1.56 1.64 1.82

Source: FAO/WHO/UNU (1985) Report of a Joint Expert Consultation: Energy

and Protein Requirements. Technical Report Series 724. Geneva: WHO.

tbl0006Table 6 Doubly labelled water measurements of total energy

expenditure in 2- to 7-year-old children

Age range

(years)

Males Females

MJper day kcalper day MJper day kcalper day

2–3 4.50 1071 4.45 1059

3–4 5.22 1243 4.73 1126

4–5 5.50 1309 4.87 1159

5–6 6.18 1471 5.71 1359

6–7 6.74 1605 6.46 1538

Source: Black AE, Coward WA, Cole TJ and Prentice AM (1996) Human

energy expenditure in affluent societies: an analysis of 574 doubly-

labelled water measurements. European Journal of Clinical Nutrition 50:

72–92.

ENERGY/Intake and Energy Requirements 2095

DLW data from men and women people living in

rural areas in developing countries have shown

that PALs for a very physically active lifestyle are

about 2.5.

Low Levels of Energy Expenditure and Energy

Requirements in Developed Countries

0022 Many studies conducted in developed counties over

the last 20–30 years show secular trends with respect

to decreases in the levels of both energy intake and

energy expenditure. The latter is due to decreases in

the energy expended on physical activity because of

labor-saving devices, increased use of mechanized

transport for even short journeys, and changes in

leisure-time activities. In many developed countries,

therefore, energy requirements are falling. Low levels

of physical activity are the reason why some expert

committees set average energy requirements at a rela-

tively low level. For example, in the UK, because of

the inactive lifestyle of much of the population due

to their light occupations and nonactive, nonoccu-

pational activity, in the absence of other information,

a PAL of only 1.4 is used to estimate energy re-

quirements.

Implications of Low Energy Intakes and Energy

Requirements

0023Current healthy eating guidelines in the UK, USA, and

many other countries recommend that people eat a

wide variety of foods. As energy requirements fall, so

it becomes more difficult for a low energy diet to

provide adequate intakes of macronutrients and

micronutrients from all the major food groups. Low

levels of energy expenditure and physical activity, or

more correctly inactivity, have been identified as

major risk factors for becoming overweight and

obese. This applies to both developed, and increas-

ingly to developing, countries. Low levels of physical

activity are also associated with increased morbidity

and mortality, and increased risk of diabetes, cardio-

vascular disease, and osteoporosis. Hence, there are

now many health-promotion initiatives that integrate

energy and nutrient intake messages with advice

about physical activity.

Developments in Recommendations for Energy

Requirements

0024Sections of the 1985 FAO/WHO/UNU report are

currently undergoing expert revision by the Inter-

national Dietary Energy Consultative Group. Data

have been reexamined and updated. With respect to

energy requirements, the topics under consideration

by expert working groups include the equations used

to predict BMR, physical activity levels in adults, and

the energy requirements of infants, children and ado-

lescents aged 1–18 years, pregnant and lactating

women, and older individuals.

tbl0007 Table 7 Doubly labelled water measurements of total energy expenditure at very low levels of physical activity

Subjects Totalenergy expenditure PAL

MJperday kcalperday

Demented elderly females 5.2 1238 1.27

Handicapped male and female adolescents 6.1 1452 1.22

Obese males in calorimeter – no exercise 10.6 2524 1.17

Female athletes in calorimeter – no exercise 7.0 1667 1.21

Source: Black AE, Coward WA, Cole TJ and Prentice AM (1996) Human energy expenditure in affluent societies: an analysis of 574 doubly-labelled water

measurements. European Journal of Clinical Nutrition 50: 72–92.

tbl0008 Table 8 Doubly labelled water measurements of total energy

expenditure at very high levels of physical activity

Subjects Total energy

expenditure

PAL

per day

kcal

perday

Soldiers on field training on hilly

terrain

14.4 3429 1.92

Soldiers on active service in cold

conditions

17.8 4238 2.43

Soldiers training in the Arctic 17.8 4238 2.42

Soldiers training in the jungle 19.9 4738 2.63

Soldiers training in the snow 20.6 4905 2.79

Female swimmers tapering training 10.9 2595 1.75

Male swimmers tapering training 16.7 3976 2.08

Female runners in training 11.8 2809 2.03

Female mountaineers (Mt Everest) 12.0 2857 2.00

Male mountaineers (Mt Everest) 14.7 3500 2.44

Female endurance runners in training 12.3 2929 2.25

Female athletes in rigorous training 14.6 3476 2.79

Female Nordic skiers 18.3 4357 2.81

Male Nordic skiers 30.3 7214 3.47

Male Tour de France cyclists 33.7 8024 4.69

Male Arctic explorers 33.1 7881 4.47

Source: Black AE, Coward WA, Cole TJ and Prentice AM (1996) Human

energy expenditure in affluent societies: an analysis of 574 doubly-

labelled water measurements. European Journal of Clinical Nutrition 50:

72–92.

2096 ENERGY/Intake and Energy Requirements