Introduction to Human Nutrition

Nutrition Research Methodology 321

Cohort studies are most commonly longitudinal or
prospective, with subjects being followed forward in
time over some predefi ned period to assess disease
onset. They may also be retrospective (historical
cohorts), with groups identifi ed on the basis of expo-
sure sometime in the past and then followed from that
time to the present to establish presence or absence of
the outcome. The feasibility of retrospective cohorts
depends on the availability of good-quality data from
pre-existing fi les. The research costs associated with
cohort study designs mean that such studies are less
common than other approaches. Nevertheless, a sub-
stantial effort to develop large cohort studies in nutri-
tional epidemiology has been made since the early
1980s. Cohort studies can assess multiple outcomes,
whereas case–control studies are restricted to assess-
ing one outcome, but may be able to assess many dif-
ferent exposures. If an absolute measure of the effect
of the exposure on the outcome is required, the only
design that is appropriate is a cohort study, as case–
control studies cannot be used to estimate incidence.
For example, to ascertain the relationship between
olive oil consumption and coronary heart disease, a
case–control study would compare the previous con-
sumption of olive oil between cases of myocardial
infarction and healthy controls. A cohort study would
start with a roster of healthy individuals whose base-
line diet would be recorded. They would then be fol-
lowed up over several years to compare the occurrence
of new cases of myocardial infarction between those
consuming different levels of olive oil as recorded
when they were healthy at baseline.

Ecological studies

Epidemiological studies can be classifi ed according to
whether measurements of exposure and outcome are
made on populations or individuals. Observational
investigations in which the unit of observation and
analysis is not the individual but a whole community
or population are called ecological studies. In ecologi-
cal studies, measures of exposure routinely collected
and aggregated at the household, local, district,
regional, national, or international level are compared
with outcome measures aggregated at the same level.
An example of an ecological study would be plotting
the mortality rates for colon cancer in several coun-
tries against the average intakes of saturated fat in
these same countries and calculating the correlation
between the two variables.

Studies considering the individual (instead of the

population) as the unit of observation are always

preferable because in an individually based study it is

possible to relate exposure and outcome measures

more directly, preventing many fl aws that are likely to

invalidate the fi ndings of ecological studies. One of

these fl aws is known as the “ecological fallacy” and it

is the bias resulting because an association observed

between variables on an aggregated level does not

necessarily represent the association that exists at an

individual level. A major advantage of individually

based studies over aggregated studies is that they

allow the direct estimation of the risk of disease in

relation to exposure.

Ecological studies measure diet less accurately

because they use the average population intake as the

exposure value for all individuals in the groups

that are compared, leading to a high potential for

biased ascertainment of diet–disease associations.

Ecological studies, also termed correlation studies,

may compare indicators of diet and health or disease

within a single population over time to look for

secular trends, or to compare the disease incidence

rates and dietary intake patterns of migrant groups

with those of comparable populations in the original

and new country. Ecological comparisons have been

important in hypothesizing diet and disease associa-

tions. Nevertheless, they are not able to establish

causal relationships.

Defi nition of outcomes and end-points

Epidemiological outcomes must be clearly defi ned at

the outset of a study. For example, a study of diet and

CVD may specify that the outcome (CVD) is verifi ed

by specifi c clinical tests such as cardiac enzyme level

or electrocardiographic changes. Taking the word of

the patient or the doctor is not suffi cient. Two main

measures of the frequency for an outcome are used in

epidemiology: prevalence and incidence.

Prevalence

The prevalence of an outcome is the proportion of

subjects in a population who have that outcome at a

given point in time. The numerator of prevalence is

the number of existing cases and the denominator is

the whole population:

Prevalence

Existing cases

Total population

=