Invited commentary: a further look at dietary questionnaire validation.

Received for publication August 6, 2001, and accepted for publication September 7, 2001. Abbreviation: EATS, Eating at America’s Table Study. From the Departments of Nutrition and Epidemiology, Harvard School of Public Health, and from the Channing Laboratory, Departments of Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA (e-mail: dosulliv@hsph.harvard.edu). (Correspondence to Dr. Walter C. Willett, Department of Nutrition, Harvard School of Public Health, 665 Huntington Avenue, Boston, MA 02115). Knowledge about the long-term consequences of diet is centrally dependent on the methods used to measure the intake of foods by individual participants in epidemiologic studies. To avoid recall and selection biases, which occur in many case-control investigations of diet, large-scale prospective studies are highly desirable. In this context, selfadministered questionnaires are usually a practical necessity, and several different self-administered food frequency questionnaires have been developed. Because of the complex nature of individual diets and the substantial variability of food intake over time, measurement errors are inevitable. Thus, the evaluation and refinement of dietary questionnaires are appropriately an important component of nutritional epidemiology. The paper by Subar et al. (1) in this issue is a valuable contribution, as it provides further insights into progress in the development of dietary assessment methods as well as limitations in the methods to assess validity. In the analysis by Subar et al. (1), three food frequency questionnaires are each compared with a “gold standard” method—up to four 24-hour dietary recalls administered by telephone over a 1-year period. In principle, this design can be informative about the relative validity of different food frequency questionnaires, even when there is some uncertainty regarding the validity of the telephone-administered recalls. The core finding of the Subar analysis is that, after adjusting for total energy intake, the correlations between all three of the food frequency questionnaires and the recalls were remarkably similar (corrected for variation in the recalls, the correlations averaged across nutrients were all approximately 0.60–0.70). The similarity in results for energy-adjusted intakes is remarkable given the quite different nature of the dietary questionnaires: The Eating at America’s Table Study (EATS) questionnaire was a 34-page form based on very recent food pattern data, and the Willett form was an older four-page version that did not reflect the refinements we have made in recent years to account for changes in food availability. Clear reasons exist to focus on energy-adjusted nutrient intakes, which specifically measure the nutrient composition of diets, in evaluations of questionnaire validity. The most fundamental reason is that the central aim of observational epidemiology is to predict what would happen to disease risk were the exposure, nutrient intake in this case, to change. Changes in nutrient intake must be made primarily by altering the composition of diet with total energy intake held constant; unless physical activity is substantially modified, even small changes in total energy intake would result in major long-term changes in weight. Thus, nutritional epidemiology should fundamentally be the study of dietary composition, that is, energy-adjusted intakes, in relation to disease occurrence. In contrast, absolute nutrient intakes reflect both dietary composition and total energy intake, which makes them difficult to interpret with regard to disease causation or prevention. This is because many of the determinants of total energy intake, such as age, gender, height, lean mass, physical activity, and metabolic efficiency, are not directly modifiable or are variables that would typically be included as covariates in epidemiologic analyses. The fact that correlation coefficients in validation studies, including that by Subar et al., tend to increase when nutrients are adjusted for total energy intake (due to “cancellation” of correlated measurement errors in energy and specific nutrients) is fortunate but not the primary reason to emphasize energy adjustment (2). The greater biologic relevance of energy-adjusted nutrients is documented by generally stronger associations with biochemical measures of diet than is seen with absolute nutrient intakes (3–6). The report by Subar et al. (1) adds to a now voluminous literature indicating that carefully designed questionnaires can provide reasonably good, although not perfect, measures of dietary composition as assessed by comparison with more detailed assessments of diet. Many other studies have documented the validity of dietary questionnaires by comparisons with biochemical measures of intake (2), which adds important evidence because errors in the biochemical measurements should be independent of those in the dietary questionnaires. For example, in this issue of the Journal, we show that the percentage of energy from total fat intake assessed by our semiquantitative food frequency questionnaire is associated with fasting triglyceride levels, as predicted by metabolic studies (7). In addition, we and others have documented the ability of this questionnaire to predict reproducibly important diet and disease relations in prospective studies (8). More recently, we have shown that both baseline dietary patterns and change in these patterns predict