Evaluating epidemiologic evidence of the effects of food and nutrient exposures.

The objective of this paper is to discuss some of the issues to be considered when evaluating and interpreting epidemiologic evidence from observational studies that collect data on dietary intake. The assessment of such evidence should include consideration of the study design, sample selection, and the measurements of exposure and disease. The degree and type of error in nutrient data can lead to analytic problems and potentially be a source of bias either toward or away from the null value. Because methods of statistical correction and adjustment for error, such as energy adjustment, cannot necessarily completely compensate for sources of bias in dietary data, additional research should be conducted on sources of error in dietary data. Published research using reported dietary data should include a discussion of potential sources of error and their effect on the results. The most useful studies are likely to be those designed to address a clearly defined prior hypothesis about a specific diet-disease relation. Because of the potential for bias and confounding, observational epidemiologic studies of diet and outcome cannot generally provide decisive evidence by themselves either for or against specific hypotheses. Although randomized clinical trials of the effects of specific nutrients or dietary modifications are not always feasible, they provide more definitive results and should generally be considered more valid than observational studies using self-reported dietary intake. Well-designed observational epidemiologic studies using self-reported dietary intake can provide valuable data to support or challenge hypotheses derived from clinical or laboratory data and to suggest further directions for investigation.