Nutrition Research Methodology 315The more foods and dishes that can be prepared in
this way, the more successful the experiment will be.
The fi nal dilemma is where the test foods will be
consumed. A volunteer may share the test foods,
which are almost always supplied free of charge, with
friends or family. To be sure of consumption, volun-
teers may be asked to consume the test meal in some
supervised space, usually a metabolic suite. This,
however, is a very costly option. Nutritional interven-
tion studies with different macronutrient distribution
of food content within energy-restricted diets are
typical in nutrition research (Abete et al. 2006).
Study designs in human nutrition
The randomized clinical trial is the most powerful
design to demonstrate cause–effect relationships. It is
unique in representing a completely experimental
approach in humans. The major strength of random-
ized trials is that they are able to control most biases
and confounding even when confounding factors
cannot be measured. The CONSORT statement has
established the CONsolidated Standards Of Reporting
Trials (http://www.consort-statement.org/). The
CONSORT guidelines comprise a checklist and a fl ow
diagram offering a standard way for reporting the
research and assessing its quality. The major method-
ological issues to be considered and reported in a
randomized trial include the following aspects: enrol-
ment, allocation, follow-up, and inclusion in analysis
of participants, sample size, proceedings for the
randomization, blinding of the allocation, blinded
assessment of the outcome, comparability of groups
regarding major prognostic variables, ascertainment
and measurement of end-points, statistical analyses,
subgroup analyses, results description, ancillary anal-
yses, adverse events, interpretations, generalizability,
and overall quality of the reported evidence.
As the researcher designs the options for altering
the intake of nutrient under investigation, so too the
design of the study requires careful thought. The
metabolic effect of the nutrient in question may be
infl uenced by age, gender, and other variables, such as
high levels of alcohol intake or physical activity,
smoking, health status, prescribed drug use, and
family history. On an experiment-by-experiment
basis, the researcher must decide which attributes will
exclude a volunteer (exclusion criteria).
The volunteers recruited can now be assigned to the
various treatments. When the numbers are small, ran-
domly assigning subjects to the treatments may lead to
imbalances that could confound conclusions. For
example, if one has 45 volunteers for three treatments,
it could be that the 15 assigned to treatment A include
the fi ve heaviest subjects and the fi ve lightest subjects.
Another treatment may be predominantly one gender.
In such instances, a minimization scheme can be used.
Minimization is a technique in which individuals are
allocated to treatment groups, ensuring a balance by
minimizing the differences between groups in the dis-
tribution of important characteristics (age, weight,
physical activity). To apply minimization, during the
recruitment process the investigators must keep an
ongoing analysis of differences between groups in the
major variables that may affect the result and allocate
new individuals to the group that leads to a more bal-
anced distribution of these characteristics. Another
option is stratifi ed randomization in which strata are
identifi ed and subjects are randomly allocated within
each stratum. While stratifi cation and minimization
are potentially very useful, it is impractical to stratify
individuals for many variables at the same time or to
try to minimize every conceivable variable that may
affect the result. To a considerable extent, the need to
balance groups becomes less important when all sub-
jects are rotated through all treatments (crossover
designs). For this to happen, the number of experi-
mental periods must equal the number of treatments.
For any given period, all treatments must be repre-
sented. An important factor to consider in this type of
design is whether or not a washout period is needed
between treatments, and its duration.
Consider the situation above if the study was to
examine the effect of fi sh oil (treatment A) versus
olive oil (treatment B) on lymphocyte function. If it
is deemed necessary that 20 days are needed to alter
the membrane phospholipids of lymphocytes, then
it is likely that 30 days will be needed to return to
baseline. If it is necessary that each treatment should
commence at baseline, then a washout period, where
volunteers resume their normal routine, is needed.
A fi nal consideration is the occasion when it is not
possible to balance all confounding factors. Take as an
example a study to examine the effect of supplemental
calcium on bone mineral density in premenopausal
women. The treatment group will receive a supple-
ment of 1000 mg of calcium as a tablet and the control
will receive a placebo tablet. What factors might one
wish to balance in such a study? Among the possibili-
ties are age, parity, use of oral contraceptives, intake of
coffee, smoking, and physical activity. To balance these