Like cohort studies, these studies follow a group of people over time. 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. As a group, these types of studies have provided valuable information about the link between lifestyle factors and disease. Cohort studies gather the information all along and before anyone develops the disease being studied. 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.
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). Researchers regularly gather information from the people in the study on a wide variety of variables (like meat intake, physical activity level, and weight). These studies follow large groups of people over a long period of time. This opens such studies to potential inaccuracy and bias in the information they gather.
People with illnesses often recall past behaviors differently from those without illness. 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. 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). Animals or cells are no substitute for humans. However, laboratory studies are only a starting point. Laboratories provide strictly controlled conditions and are often the genesis of scientific ideas that go on to have a broad impact on human health. These are studies done in laboratories on cells, tissue, or animals. 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). There are many different types of research studies, and each has distinct strengths and weaknesses.
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