Introduction to Human Nutrition

246 Introduction to Human Nutrition

or estimated in terms of standard household mea-
sures. In recalls and questionnaires quantities always
have to be estimated. Dimensions, photographs of
foods, food models, and, sometimes, actual foods may
be used to assist in this process. Each of these
approaches has specifi c advantages and disadvantages
and no single method of measuring food intake can
be regarded as the ideal method for all situations.
Until recently, weighed intake recorded over a 7 day
period was taken as the reference method against
which less detailed methods were compared. It has,
however, been realized that this method has its limita-
tions and that it is not only desirable but also neces-
sary to use physiological and biochemical measures
to determine whether any method of measuring
food intake is actually measuring what it sets out to
measure. This will be discussed in more detail in
Section 10.6.

Basic concepts

Before describing the most commonly used direct
dietary assessment methods it is appropriate to
introduce four fundamental concepts relevant to the
process of dietary assessment and evaluation. A brief
defi nition of terms related to these concepts is given
in Box 10.2. Terms are listed in the box in alphabetical
order for ease of reference.

Habitual intake
The objective of virtually all dietary assessments is to
obtain an estimate of the habitual or average long-
term intake for the group or the individual of interest.
Habitual intake represents what is “usual” in the long
term and not simply at a specifi c moment in time. It
is this level of intake that is relevant for maintenance
of energy balance and nutrient status, and for the
assessment of relationships between nutrient intake
and health in the long term. Habitual intake, however,
is diffi cult to measure because food intake varies
widely from day to day and, to a lesser extent, from
week to week and month to month. Figure 10.2 illus-
trates the energy intake of one individual who main-
tained a weighed record every sixth day for 1 year. The
horizontal line indicates the overall average intake
over the year in MJ per day. The open circles show the
intake on individual days and the solid circles the
average intake over 7 day periods. It is obvious that
intake on a single day does not provide a reliable
estimate of habitual intake and that even average

Box 10.2

Accuracy: the extent to which an estimate approximates the true

value.

Bias: the extent by which an estimate differs from the true value.

Coeffi cient of variation: the standard deviation of a set of obser-

vations expressed as a percentage of their mean.

Habitual intake: an estimate of the long-term average intake of

an individual.

Precision: the extent to which a method is repeatable. Usually

expressed in terms of the coeffi cient of variation, i.e., the standard

deviation of the results of repeated analyses of the same para-

meter expressed as a percentage of their mean.

Random errors: errors that are randomly distributed about the

true value. Random errors increase the variability of a set of obser-

vations but do not affect their mean.

Repeatability (reproducibility): a method is repeatable or repro-

ducible when it gives the same result on repeated measurement.

Systematic errors: errors that are not randomly distributed about

the true value. Systematic errors can increase or decrease the vari-

ability of a set of observations and also affect the estimate of their

mean. The effect on the mean is referred to as bias.

Validity: a method is valid if it measures what it is intended to

measure, i.e., the true value:

• Absolute validity or accuracy: terms used to describe the
  extent to which a method measures the true value. It is not pos-
  sible to determine the absolute accuracy of a dietary method by
  comparison with another dietary method.


• Face or content validity: a method which gives results that are
  consistent with other data related to the parameter they are
  intended to measure.


• Relative (comparative) validity: the extent to which a test
  method performs in relation to a criterion or reference method,
  i.e., the dietary method judged to provide the “best” available
  measure of the true value.


• Validity at group level: a method is valid at the group level if
  it provides an unbiased estimate of mean intake for the group.


• Validity at individual level: a method is valid at the individual
  level if it is able to rank respondents correctly (usually evaluated
  in terms of the same proportion in the same quintile or tertile
  of the distribution compared with the “true” or more usually a
  “reference” value).
  Variance: statistical term to describe the variation that occurs in a
  set of observations. It is equal to the square of the standard devia-
  tion of the individual observations:


• Between-person variance: the variance arising from differ-
  ences between individuals.


• Within-person variance: the variance arising from differences
  within individuals.

intake over 7 days can differ by as much as 20% from

the overall mean.

The nature of error

There is not, and probably never will be, a method

that can estimate dietary intake without error. This