Nutrition Science and Everyday Application - beta v 0.1

TYPES OF RESEARCH STUDIES AND HOW TO INTERPRET THEM 89
Figure 2.3. The hierarchy of evidence shows types of research studies relative to their strength
of evidence and relevance to real-life nutrition decisions, with the strongest studies at the top and
the weakest at the bottom.
The pyramid also represents a few other general ideas. There tend to be more studies
published using the methods at the bottom of the pyramid, because they require less time,
money, and other resources. When researchers want to test a new hypothesis, they
often start with the study designs at the bottom of the pyramid, such as in vitro,
animal, or observational studies. Intervention studies are more expensive and resourceintensive,
so there are fewer of these types of studies conducted. But they also give us higher
quality evidence, so they’re an important next step if observational and non-human studies
have shown promising results. Meta-analyses and systematic reviews combine the results of
many studies already conducted, so they help researchers summarize scientific knowledge
on a topic.
NON-HUMAN STUDIES: IN VITRO & ANIMAL STUDIES
The simplest form of nutrition research is an in vitro study. In vitro means “within glass,”
(although plastic is used more commonly today) and these experiments are conducted
within flasks, dishes, plates, and test tubes. These studies are performed on isolated cells or
tissue samples, so they’re less expensive and time-intensive than animal or human studies.
In vitro studies are vital for zooming in on biological mechanisms, to see how things work at
the cellular or molecular level. However, these studies shouldn’t be used to draw conclusions
about how things work in humans (or even animals), because we can’t assume that the
results will apply to a whole, living organism.