320 Introduction to Human Nutrition
dietary intake is culturally determined, such as in
Indonesia, where the rice consumed is white rather
than brown, and beriberi is common as a result of
vitamin B 1 defi ciency.
Nonexperimental (observational) designs can be
further classifi ed into four main subtypes:
● cross-sectional studies
● case–control studies
● cohort studies
● ecological studies.
Among observational studies, the main differences
between study designs relate to the time when expo-
sure and outcome are measured. The initiative
“STrengthening the Reporting of OBservational
studies in Epidemiology (STROBE), http://www.
strobe-statement.org” provides a check-list to assess
the methodological quality of the three major epide-
miological designs: cohort studies, case–control
studies, and cross-sectional studies (Von Elm and
Egger 2004).
Cross-sectional (prevalence) studies
Cross-sectional or prevalence studies measure both
exposure and outcome in the present and at the same
point in time. Cross-sectional surveys provide a snap-
shot of descriptive epidemiological data on nutrition,
identifying nutritional needs in the population and
forming a basis for health promotion and disease pre-
vention programs at a single point in time. Several
countries conduct regular cross-sectional surveys on
representative samples of their populations focusing
on dietary habits and frequencies of illness. Dietary
factors can then be correlated with prevalence of
illness, which may be helpful for national nutrition
policy.
Case–control studies
In case–control studies, outcome is measured in the
present, and past exposure is ascertained. Usually the
dietary and lifestyle patterns of patients with a disease
(cases) are compared with those of age- and gender-
matched people without disease (controls).
Subjects are identifi ed and recruited on the basis of
the presence or absence of the disease or the health
outcome variable of interest. Ideally, the controls are
randomly selected from the same study base as the
cases, and identical inclusion and exclusion criteria
are applied to each group. The presence of specifi c
dietary exposures or other factors of etiological
interest in subjects is generally established using
interviews, questionnaires, or medical record reviews.
Within the general framework for case–control
studies, there are several options for study design and
control selection.
For example, controls may be matched with cases
at an individual level on the basis of age, gender, or
other variables believed to affect disease risk. Match-
ing eliminates variability between cases and controls
with respect to the matching variables and thus leads
to a higher effi ciency in the analysis. Nevertheless,
matching does not control for the confounding effects
of these risk factors on the observed relationship.
Case–control studies are by far the most logistically
feasible of the analytical study designs in epidemiol-
ogy, but their application to questions of interest to
nutritionists is limited by the particular nature of
diet–disease relationships.
The insight to be gained from a comparison of
dietary exposures between cases and controls is
limited by the possibility that the dietary patterns of
subjects have changed since the time when diet was
most important to the disease initiation process. Ret-
rospective case–control studies attempt to overcome
this limitation by measuring past diet using food fre-
quency or diet history methods. One concern is that
recall of past diet by cases may be infl uenced by their
present disease status. For example, patients who have
had a heart attack may attach an unfair level of impor-
tance to their intake of specifi c foods, based on
misinformation.
A primary factor in choosing between a case–
control design and a cohort design is whether the
exposure or the outcome is rare. If the outcome is
rare, case–control studies are preferable, because a
cohort would need a very large sample to observe a
suffi cient number of events. If the exposure is rare,
cohort studies are preferable.
A nested case–control design consists of selecting
as cases only those members of a previously defi ned
cohort who develop the disease during their follow-
up period. A random sample or a matched sample of
non-cases is also selected from the cohort to make up
the control series as the comparison group.Cohort studies
In cohort studies exposure is evaluated in the present
and outcome ascertained in the future.