Food frequency dietary assessment: how bad is good enough?

This issue of the Journal features a diverse set of papers on dietary epidemiology. Although these papers might not help us decide which party buffet items to select in the coming days of the holiday food season, they do represent a cross section of the current status of the field of dietary epidemiology. This field includes a rapidly growing number of studies using variations of the food frequency method of dietary assessment and, at the same time, studies that continue to critically examine the validity of that very same method. The food frequency validation studies in this issue (1–3) and their accompanying commentaries (4, 5) are noteworthy contributions to the process of building a consensus about the proper use of the food frequency method of dietary estimation. There is perhaps no other epidemiologic discipline that has attracted as much public attention and, at the same time, as much scientific criticism as has dietary epidemiology. That is because the exposure is both of immediate interest to the public and notoriously difficult to measure. A small preliminary study suggesting an effect of the frequency of broccoli consumption on breast cancer survival might make headlines when even more definitive findings from studies on other important topics would not. Publicity surrounding variations in findings between dietary epidemiologic studies has led to both scientific and public cynicism about the field. Some excess publicity can be blamed on epidemiologists, but most has been a consequence of the high public appetite for information about diet and health. As we have learned the sometimes difficult lessons that popular hypotheses about diet and disease may well be null, the validity of dietary measurements has been openly questioned. Experimental nutritional scientists, who have been trained to consider nutrients as precisely quantitated factors, often scoff at dietary epidemiologic methods. When nutritional scientists ponder epidemiologic findings relating long-term diet to disease based on dietary estimates from such questions as " How often do you eat peas? " their skepticism is understandable. The disagreement between experimental nutritional scientists and nutritional epidemiologists appears to result from not only their different standards of precision for exposures but also their use of different end-points. Although experimental nutrition can precisely measure nutrient exposures, their outcomes, such as disease in laboratory animals or short-term biomarkers in human volunteers , are often only tangentially related to the human diseases of interest. In contrast, epidemiologic dietary studies relate the relevant …