There are many different types of research studies, and each has distinct strengths and weaknesses. In general, randomized trials and cohort studies provide the best information when looking at the link between a certain factor (like diet) and a health outcome (like heart disease).
These are studies done in laboratories on cells, tissue, or animals. Laboratories provide strictly controlled conditions and are often the genesis of scientific ideas that go on to have a broad impact on human health. However, laboratory studies are only a starting point. Animals or cells are no substitute for humans.
These studies look at the characteristics of one group of people who already have a certain health outcome (the cases) and compare them to a similar group of people who do not have the outcome (the controls). While case-control studies can be done quickly and relatively cheaply, they aren’t ideal for studying diet because they gather information from the past. People with illnesses often recall past behaviors differently from those without illness. This opens such studies to potential inaccuracy and bias in the information they gather.
These studies follow large groups of people over a long period of time. Researchers regularly gather information from the people in the study on a wide variety of variables (like meat intake, physical activity level, and weight). Once a specified amount of time has elapsed, the characteristics of people in the group are compared to test specific hypotheses (like the link between carotenoids and glaucoma, or meat intake and prostate cancer).
Though time-consuming and expensive, cohort studies generally provide more reliable information than case-control studies because they don’t rely on information from the past. Cohort studies gather the information all along and before anyone develops the disease being studied. As a group, these types of studies have provided valuable information about the link between lifestyle factors and disease. Two of the largest and longest-running cohort studies of diet are the Harvard-based Nurses’ Health Study and the Health Professionals Follow-up Study.
Like cohort studies, these studies follow a group of people over time. However, with randomized trials, the researchers actually intervene to see how a specific behavior change or treatment, for example, affects a health outcome. They are called “randomized trials” because people in the study are randomly assigned either to receive or not receive the intervention. This randomization helps researchers hone in on the true effect the intervention has on the health outcome.
However, randomized trials also have drawbacks, especially when it comes to diet. While they are good at looking at topics like vitamin supplements and cancer, when the change in diet is more involved than say taking a vitamin pill, participants begin to have trouble keeping to their prescribed diets. Such involved interventions can also become very expensive.
Read an example of how different types of studies shaped health recommendations about fiber and colon cancer.
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