Nutrition Science and Everyday Application - beta v 0.1

TYPES OF RESEARCH STUDIES AND HOW TO INTERPRET THEM 91
ask a group of people to describe their diet, and then researchers would track
them over time to see if those eating a Mediterranean diet had a lower incidence
of cardiovascular disease.
• Case-control studies compare a group of cases and controls, looking for differences
between the two groups that might explain their different health outcomes. For
example, researchers might compare a group of people with cardiovascular
disease with a group of healthy controls to see whether there were more controls
or cases that followed a Mediterranean diet.
• Cross-sectional studies collect information about a population of people at one point
in time. For example, a cross-sectional study might compare the dietary patterns of
people from different countries to see if diet correlates with the prevalence of
cardiovascular disease in the different countries.
Prospective cohort studies, which enroll a cohort and follow them into the future, are usually
considered the strongest type of observational study design. Retrospective studies look at
what happened in the past, and they’re considered weaker because they rely on people’s
memory of what they ate or how they felt in the past. There are several well-known examples
of prospective cohort studies that have described important correlations between diet and
disease:
• Framingham Heart Study: Beginning in 1948, this study has followed the
residents of Framingham, Massachusetts to identify risk factors for heart disease.
• Health Professionals Follow-Up Study: This study started in 1986 and enrolled
51,529 male health professionals (dentists, pharmacists, optometrists, osteopathic
physicians, podiatrists, and veterinarians), who complete diet questionnaires every
2 years.
• Nurses Health Studies: Beginning in 1976, these studies have enrolled three large
cohorts of nurses with a total of 280,000 participants. Participants have completed
detailed questionnaires about diet, other lifestyle factors (smoking and exercise,
for example), and health outcomes.
Observational studies have the advantage of allowing researchers to study large groups
of people in the real world, looking at the frequency and pattern of health outcomes and
identifying factors that correlate with them. But even very large observational studies may
not apply to the population as a whole. For example, the Health Professionals Follow-
Up Study and the Nurses Health Studies include people with above-average knowledge of
health. In many ways, this makes them ideal study subjects, because they may be more
motivated to be part of the study and to fill out detailed questionnaires for years. However,
the findings of these studies may not apply to people with less baseline knowledge of health.
We’ve already mentioned another important limitation of observational studies—that
they can only determine correlation, not causation. A prospective cohort study that finds
that people eating a Mediterranean diet have a lower incidence of heart disease can only
show that the Mediterranean diet is correlated with lowered risk of heart disease. It can’t
show that the Mediterranean diet directly prevents heart disease. Why? There are a huge
number of factors that determine health outcomes such as heart disease, and other factors
might explain a correlation found in an observational study. For example, people who eat