Human Nutrition

HOW IS INFORMATION ABOUT PEOPLE’S DIETS COLLECTED? ❚
reasonable degree of reliability. A second more
difficult question to answer relates to the validity
of the information, i.e. how well it measures
the subject’s intake. Since food and, therefore,
nutrient intakes are not constant from day to
day for the majority of people, an average intake
over a period of time would reflect ‘habitual’
intakes. The duration over which food intake has
to be recorded to obtain a valid measure of
habitual intake has been studied and shown to
vary for different nutrients. Ultimately, the decision
on how long a recording period should be
depends on the nutrients to be analysed. For
example, for energy and macronutrient intake,
Black (2001) reports that a 7-day intake record
will provide a result within 15–20 per cent of the
true level of intake. For micronutrients, the variability
is up to 30–40 per cent for those that are
widely distributed in foods, and very large for
vitamins, such as A and C, which occur in large
amounts in fewer foods. Some of the methods
used are discussed below.
The weighed inventory
This is considered to be the ‘gold standard’ of
dietary intake studies. In this method, all the food
eaten by the subject during a period, usually
1 week, is weighed and recorded, together with
any plate waste. Actual nutrient intakes are then
calculated, using data from food composition
tables applicable to the particular country (in the
UK McCance and Widdowson’s tables, published
by the Royal Society of Chemistry, are the most
widely used – see Food Standards Agency,
2002b). The major drawback of the method is
that it requires a considerable degree of motivation
and cooperation on the part of the subject. It
is quite an intrusive method, which takes time at
meals and may thus deter a busy person.
Most subjects tend to underrecord their
habitual food intake, possibly because they
actually eat less during the study period, or
forget/omit to record some of the foods eaten.
This seems to be a particular problem in those
who are trying to restrain their food intake in
some way. Snack foods are often omitted, perhaps
because of inconvenience or forgetfulness.
Recent work has shown that the fat and carbohydrate
intakes are underreported to a greater
extent than protein intakes. Food choice may
also be altered to facilitate weighing.
Food diaries
In this technique, the food eaten is simply
recorded in a notebook, without being weighed.
Comprehensive instructions are provided to the
subject to explain the procedure. Cooking methods,
brand names and recipes are requested.
The respondent is asked to provide an estimate of
the portion size using household measures, e.g.
spoons, cups, units, slices or recording packet
weights. The researcher then has the task of
quantifying portions eaten. Food models or pictures
may help in the quantification of portion
sizes. A number of photographic atlases of food
portions have been developed and validated in
recent years. Other visual images, including those
generated by computer may be used in the future,
but would also need to be validated. Tables of
average portion sizes are available in the UK,
based on measurements of typical portions (Food
Standards Agency (FSA), 2002a). Database information
is available for a large number of typical
servings of foods, although they remain only an
estimate for each particular subject. A large database,
known as DINER (Data Into Nutrients for
Epidemiological Research) has been developed as
part of the EPIC study, which contains information
on over 7000 foods and portion sizes.
The food diary method requires that the subject is
literate and physically able to write. Alternative
ways of recording the size of portion eaten include
photographing the meal, and the use of computerized
scales with an associated tape-recorder, for
example, the PETRA (Portable Electronic Tape
Recording Automated) Scales system, which can
both weigh and store a description of the meal. In
both cases, however, the data still require interpretation
and collation by the researcher.
The diary method remains subject to possible
changes in the diet by the respondent and
failure to record all foods eaten. However, if
respondents are adequately instructed, reasonably
comprehensive records can be obtained.
Generally, women produce more reliable records
by this method than men.
A record of foods eaten with no attempt to
assess the quantity (a menu record) is a further
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