Introduction to Human Nutrition

316 Introduction to Human Nutrition

factors adequately is impossible. However, if they are
recorded, then, when the data are being evaluated on
a statistical basis, they can be included to ascertain
their effect on the measured outcome, bone mineral
density. To accomplish this aim, multivariate methods
such as multiple regression or logistic regression
should be used (see Section 13.2).

13.6 Epidemiological designs

Epidemiology is a health-related science dealing with
the distribution and determinants of health and
illness in populations. Nutritional epidemiology inte-
grates the knowledge derived from nutrition research,
to examine diet–disease relationships at the level of
free-living populations. Nutritional epidemiology
provides scientifi c evidence to understand the role of
nutrition in the cause and prevention of disease.
The comparison and choice of different epidemio-
logical study designs depends on exposure measures,
outcome measures, costs, and expected length of
follow-up. The selection of a study method is often
infl uenced by pragmatic issues such as feasibility,
as well as by ethical questions.
Epidemiological studies can be divided into two
broad categories (Figure 13.2): experimental and
nonexperimental (observational) studies. Observa-
tional studies can be further divided into descriptive
and analytical studies. In a wide sense, an experiment
is a set of observations, conducted under controlled

circumstances, in which the scientist manipulates the

conditions to ascertain the effect of such manipula-

tion on the observations.

Experimental studies in

nutritional epidemiology

It is necessary to consider that in biological experi-

mentation, it is not possible for the scientist to control

completely all of the relevant circumstances, and the

manipulation will consist of increasing at most the

degree of variation in the factor that the scientist is

investigating. The ideal will be to obtain two almost

identical sets of circumstances where all factors are

the same. If a strong variation is then introduced in

only one of these factors, all of the observed differ-

ences between the two sets that occurred thereafter

would be causally attributed to the single factor that

the investigator had manipulated.

Experimental epidemiological designs are those in

which the investigator assigns the exposure to each

subject. In these studies, the treatment (or exposure)

is assigned with the aim of attaining maximum com-

parability between treated and untreated groups

regarding all other characteristics of the subjects apart

from the treatment or exposure of interest. In epide-

miological research, the best way to achieve identical

sets of circumstances is to assign subjects randomly

to exposure (treatment) or control groups. This

process is called randomization. All randomized

studies are experimental designs.

Exposure, from an epidemiological point of view,

describes lifestyle or environmental factors that may

be relevant to health. Outcome is another generic

term used to describe the health-related events or

variables that are being studied in relation to the

effect of an exposure. In nutritional epidemiology, the

primary exposure of interest is dietary intake, whereas

outcome measures usually involve disease occurrence

or nutritional status indicators (anthropometry,

clinical signs of disease/health status, biological or

physiological measures or dietary habits).

It is also possible to design experimental studies

assigning whole population groups to different expo-

sures. These studies are called community trials.

For example, if a whole town is assigned to receive

an educational program about healthy eating and

another neighboring town is assigned to control (no

educational program), this would be a community

trial; when randomization is used, it is termed “cluster-

Clinical trial

Field trial

Community trial

Intervention trial

(quasi-experiment)

Cohort

Analytical

Descriptive

No

Yes

Yes

Randomization

Nonexperimental

(observational)

Experimental

Case–control

Cross-sectional

(prevalence)

Ecological

(correlation)

Assignment of

exposure by

investigator

No

Figure 13.2 Classifi cation of epidemiological designs.