Study design and hypothesis testing: issues in the evaluation of evidence from research in nutritional epidemiology

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In addressing questions about the relations of dietary factors to disease in human populations, epidemiologic studies must account for the complexity of dietary habits, the intercorrelations among dietary habits, and the correlations of those habits with other behaviors. Furthermore, for studies of chronic disease, relevant dietary exposures may occur over decades. The classic epidemiologic study designs have been used to examine the associations between diet and disease; the strengths and weaknesses of those designs must be considered. Concerns have been raised regarding the validity of the measures of diet, the differential recall of diet by diseased individuals in case-control studies, and confounding by other related factors in both case-control and cohort studies. In clinical trials there may be difficulties in effecting the necessary dietary changes, especially for macronutrients, and there are also concerns about those circumstances in which participants cannot be blinded to their treatment. For case-control and cohort studies and for some clinical trials, intercorrelations among nutrients are a concern in the identification of factors that are important in the etiology of disease. It is important to understand these considerations when interpreting nutritional epidemiologic studies for the purpose of setting public policy. No one study can be considered definitive in the understanding of a diet-disease relation. However, epidemiologic findings from multiple studies taken together can contribute significantly to our understanding of diet in relation to disease in humans.