322 Introduction to Human Nutrition
Incidence
The incidence of an outcome is the proportion of new
cases that occur in a population during a period of
observation. The numerator of incidence is the
number of new cases developing during the follow-up
period, while the denominator is the total population
at risk at the beginning of the follow-up time:
Incidence
New cases
Population inititally at risk=When calculated in this fashion, incidence is a pro-
portion. However, incidence can also be expressed as
a rate (velocity or density), when the time during
which each person is observed (i.e., person-time of
observation) is included in the denominator. Then it
is called incidence rate or incidence density and it is
expressed as the number of new cases per person-
time of observation.
Other epidemiological methods
Epidemiological studies have also been conducted to
assess: consumer attitudes to and beliefs about food,
nutrition, physical activity patterns, and health to
provide policy-makers, researchers and the food
industry with data to promote health messages con-
cerning the relation between food or nutrient intake
and chronic diseases. These surveys seek information
about infl uences on food choice, health determinants,
criteria about perceptions of healthy eating, regular
sources of nutritional information, expected benefi ts
and barriers to healthy diet implementation, in order
to identify consumers’ knowledge, attitudes, and beliefs
concerning food and health interactions and to
promote more focused nutrition education messages.
Meta-analysis and pooled analysis
The role of meta-analysis for systematically combin-
ing the results of published randomized trials has
become routine, but its place in observational epide-
miology has been controversial despite widespread
use in social sciences. Some have argued that the com-
bining of data from randomized trials is appropriate
because statistical power is increased without concern
for validity since the comparison groups have been
randomized, but that in observational epidemiology
the issue of validity is determined largely by con-
founding and bias rather than limitations of statistical
power. Thus, the greater statistical precision obtained
by the combination of data may be misleading because
the fi ndings may still be invalid. An alternative to
combining published epidemiological data is to pool
and analyze the primary data from all available studies
on a topic that meets specifi ed criteria. Ideally, this
should involve the active collaboration of the original
investigators, who are fully familiar with the data and
its limitations. This kind of study conducted with a
combination of the original data from several studies,
is the basis of pooled analysis or pooling projects. In
a pooled analysis, the range of dietary factors that can
be addressed may be considerably greater than in the
separate analyses because any one study will have few
subjects in the extremes of intake and, sometimes, the
studies will vary in distribution of dietary factors. The
advantages of pooled analyses in nutritional epidemi-
ology are so substantial that they are becoming
common practice for important issues, such as alco-
holic intake and breast cancer, body size and breast
cancer, or alcohol beverages and coronary heart
disease.
Analysis of epidemiological data requires careful
consideration of the criteria for acceptable data
quality, but also of the presentation of categorized or
continuous independent variables and the applica-
tion of empirical scores. The study of subgroup analy-
sis and interactions and error correction are other
issues of interest. Other limitations are the require-
ment of sample to be considered as representative,
compliance, inaccuracies of information in retrospec-
tive studies, and confounding effects by factors that
are simultaneously associated with both the exposure
and the outcome.13.7 Perspectives on the future
Future nutrition research will develop new methods
for studying those processes whereby cells, tissues and
the whole body obtain and utilize substances con-
tained in the diet to maintain their structure and
function in a healthy manner. Particular emphasis
will be paid to molecular and cellular based strategies
devised to understand better the genetic basis of
nutritional outcomes.
It can also be anticipated that many ongoing large
cohort studies with tens of thousands of participants
will provide valuable information on the role of
nutrition in disease prevention, and also on nutri-
tional management of a large number of diseases by