Introduction to Human Nutrition

Measuring Food Intake 265

doing so would be to simplify the process of record-
ing. Other reasons may include a desire to eat less in
order to lose weight or to be seen to conform with
dietary recommendations. If this is what happens in
practice, then what is measured in short-term dietary
records may be actual intake or desired intake, but not
usual intake.
Many studies have now demonstrated that there is
a tendency, in most population subgroups, for short-
term dietary records to provide estimates of energy
intake that are on average around 16% lower than
would be expected on the basis of measured and/or
estimated levels of energy expenditure.
These studies will be discussed further in the
section on precision and validity. The fact that for
some groups measurements of energy intake and
energy expenditure agree quite closely indicates that
it is possible to achieve recording without a concomi-
tant change in diet when there is full cooperation
from respondents, and highlights the importance of
efforts to achieve such cooperation.

10.5 Choosing a dietary
assessment method

It is not possible to decide which dietary method to
use until the purpose of the study has been clearly

defi ned, since this will determine the kind of informa-

tion and the length of time for which it needs to be

collected from each individual. Often, the purpose of

the study also determines the level of precision that is

required to meet the study objectives and therefore

the sample size. These two considerations are the

most important ones in determining the method to

be used, because both the method and the size of the

sample have implications for the human and fi nancial

resources needed for the study.

Purpose of the study

When dietary data are collected to describe the diet

of a group for comparison with that of another group

or groups, it is possible to use either a short-term

method such as a 24 hour recall or record, or a longer-

term method such as food records obtained over

several days, a diet history, or a questionnaire about

habitual intake. The fi nal choice will depend on

factors such as the importance of a representative

study sample, the resources available, and the level

of precision required. Usually, the most effi cient

approach is to measure the diet of as many individu-

als as possible for 1 day.

However, if the purpose of the dietary study is to

determine the proportion of individuals in the group

who are at risk of dietary inadequacy or excess, rela-

tive to some standard of reference, then a single day

of information on each individual is no longer ade-

quate because it is necessary to have a reliable esti-

mate of the distribution of habitual intake in the

group. As Figure 10.2 shows, a single day of intake is

generally not a reliable measure of an individual’s

habitual intake.

To determine the distribution of habitual food

intake in a group, at least 2 days (preferably not

consecutive) of information from each individual or

a representative subsample of individuals from the

group of interest are needed. If several days of intake

are available they can be used to derive a mean intake

for each individual and from this the distribution of

average intakes for the group. Alternatively, statistical

techniques can be used to adjust 1 day intake data, for

the day-to-day variation that occurs in individuals, to

provide a better estimate of the underlying distribu-

tion of habitual intake for the group than is given by

the 1 day data (Dodd et al., 2006). While the use of

appropriate statistical techniques can improve esti-

mates of the proportion of individuals at risk of

Table 10.4 Number of days of records required to enable 80% of men
to be assigned into their correct third of the intake distribution

Nutrient

British civil

servants

Random sample

of British men

Random sample

of Swedish men

Energy 7 5 7
Protein 6 5 7
Fat 9 9 7
Carbohydrate 4 3 3
Sugar 2 2 –
Dietary fi ber 6 10 –
P:S ratio 11 – –
Cholesterol 18 – –
Alcohol 4 – 14
Vitamin C – 6 14
Thiamin – 6 15
Ribofl avin – 10 –
Calcium – 4 5
Iron – 12 9

P:S ratio, ratio of polyunsaturated to saturated fatty acids in the
diet. (Reproduced from Margetts BM, Nelson M. Design Concepts in
Nutritional Epidemiology. Oxford: Oxford University Press, 1991, with
permission from Oxford University Press.)