Measuring Food Intake 273availability and intake of foods in addition to infor-
mation on nutrient intake. Similarly, nutritional epi-
demiologists are also interested in the relationship
of different foods and dietary patterns with specifi c
health outcomes. The use of dietary data in the context
of epidemiological studies is covered in the textbook
Public Health Nutrition (Gibney et al. 2004).
The analysis and presentation of food intake data
depends on the objectives of the study. When the
purpose is to examine intakes of specifi c foods, intakes
of foods may be expressed as means, medians, or
frequency distributions of intakes, as the number or
percentage of respondents consuming specifi c foods,
or as the percentage contribution of food items to the
total food intake, energy intake, or intake of nutrients
of interest. Since not all members of a sample consume
a given food, it is always important to indicate whether
the total sample size or only the number of respon-
dents consuming the food has been used in statistical
calculations.
Although intakes of individual food items may be
reported, food intake data are usually reduced to
more manageable proportions by grouping foods into
appropriate categories. While this can be done in
different ways, for example in terms of composition,
biological origin, or cultural use, the process is
relatively straightforward within a given culture or
country. It is more diffi cult, however, to develop a
classifi cation that can be used consistently across dif-
ferent countries or food cultures. National food clas-
sifi cation systems tend to differ not only because the
type and range of foods differs but also because the
same foods are used in different ways. For the purpose
of comparing food intake patterns between countries
or regions, it is, therefore, necessary to develop a food
classifi cation or coding system that allows food data
from individual regions or countries to be assigned in
a consistent way.
The United Nations University Food and Nutrition
Program for an International Network of Food Data
Systems (INFOODS) was developed for the purpose
of supporting work on the classifi cation and naming
conventions for individual foods and food groups
(see Chapter 2).
Indirect information on food consumption, such
as that provided by FAO food balance sheets and by
data from household budget and similar surveys, is
usually presented in terms of foods or food groups,
but may also be converted to nutrients to provide
information on the nutrient contribution of individ-
ual foods or groups of foods.
Tracking changes in the food sources of nutrients
and nonnutrients is particularly important in the
context of technological developments in food pro-
duction and manufacture that result in the addition
of nutrients to foods, in the development of foods for
specifi c functional purposes, and in the genetic modi-
fi cation of foods. A specifi c example of the need for
individual food, rather than nutrient, intake data is
provided by exposure assessments to dietary non-
nutrients such as food additives, pesticide residues,
and other possible food contaminants.10.10 Food safety assessments
Safety assessments for food additives are expressed in
terms of the acceptable daily intake (ADI) estimated
on the basis of lifetime exposure. While it is clearly
not possible to collect food consumption data over
the lifetimes of individuals, it is important that the
dietary data used for the purpose of estimating accept-
able levels of intake over a lifetime refl ect, as far as is
possible, the habitual level of intake of the foods being
assessed.
For the purpose of food safety assessments only the
intake of “consumers” is of interest. It follows, there-
fore, that the dietary data need to be adequate to
obtain both an accurate estimate of the proportion of
the population who are consumers and of the average
habitual intake of consumers. Because the frequency
of consumption varies between foods (some foods are
eaten by most people on most days, but many other
foods are eaten less frequently), the duration of the
dietary recording period infl uences both the esti-
mate of the proportion of consumers and the average
intake of consumers. Intake data for 1 day will inevi-
tably underestimate the true number of consumers
for most foods and overestimate the average habitual
intake of those consumers because not all foods are
eaten every day. However, it appears that 75% or more
of household menu items are normally consumed
within a 14 day period and that a 14 day diary pro-
vides a good estimate of the habitual intake of most
foods by consumers.
Most studies of the food intake of individuals,
however, do not last for 14 days because of the