1 Applied Research Program, Division of Cancer Control and Population Sciences, National Cancer Institute, Bethesda, MD.
2 Biometry Research Group, Division of Cancer Prevention, National Cancer Institute, Bethesda, MD.
We thank Drs. Willett (1) and Block (2
) for their thoughtful comments. Our goal in creating the Diet History Questionnaire was to improve the accuracy of food frequency questionnaires. Table 7 of our paper (3
) indicates that, in comparing food frequency questionnaires with recalls, we were successful but that the improvement was small. Although we should continue to improve food frequency questionnaires, we agree with Dr. Willett that perhaps there is a ceiling on their ability to accurately measure diet.
For surveillance and public health, knowledge of absolute intakes is essential, as indicated by Dr. Block. Such information allows evaluation of progress toward meeting national dietary recommendations. Dr. Willett asserts that, for nutritional epidemiology, energy adjustment is essential because of the need to reflect dietary composition. Are we sure, however, that energy-adjusted nutrients are the only appropriate biologic exposures for nutritional epidemiology and that every nutrient exposure necessarily relates to energy intake? Using energy-adjusted nutrients allows one to assess the effect of substituting one nutrient in relation to another. We agree that this substitution effect is important. However, so too is the addition effect, which represents changes in absolute intakes. Knowledge of both the substitution and addition effects contributes to a more complete understanding of diet-disease associations (4). Further, although energy adjustment improves correlations between food frequency questionnaires and recalls/records, there is a wealth of literature showing that energy is poorly measured in all self-reported dietary instruments, including recalls/records (5
). Therefore, how can we be sure that energy adjustment really controls for true energy intake and that estimated associations between disease incidence and energy-adjusted nutrients do not suffer from residual confounding? Studies like the National Cancer Institute's Observing Protein and Energy Nutrition Study, a dietary biomarker-based validation study with doubly labeled water and urinary protein, sodium, and potassium, will provide data to address this question.
Dr. Willett (1) states that current food frequency questionnaires provide reasonably good measures of energy-adjusted intakes allowing detection of important diet-disease associations. However, the estimated attenuation coefficients for energy-adjusted nutrients in table 6 (3
) indicate that true relative risks would be substantially underestimated using any food frequency questionnaire. For example, attenuation coefficients for energy-adjusted fat intake are in the range of 0.40.5, meaning that a moderate true relative risk of 1.6 would be estimated as 1.201.26. Therefore, the necessary sample size to retain the nominal power to detect diet-disease associations would have to be 46 times greater. Furthermore, these results are based on using the 24-hour recall as a reference instrument and may substantially underestimate the true attenuation as is indicated by biomarker studies (6
).
We respectfully disagree with Dr. Willett about potential reasons for substantially lower correlations for absolute nutrient intakes seen with his food frequency questionnaire. First, he explains that the Willett food frequency questionnaire nutrient analysis program does not include age- and gender-specific portion sizes since age and gender are routinely used as covariates in epidemiologic studies. Covariate adjustment assumes the same linear relation across all items asked, whereas using individual portion sizes allows more complex relations that can differ across items. Second, Dr. Willett assumes that the validity of the Block food frequency questionnaire and Diet History Questionnaire may be overstated since their errors and those in the recalls would be more strongly correlated than errors with the Willett food frequency questionnaire. He suggests that the weighed food records would not have this problem and would, therefore, produce more realistic results. However, Kipnis et al. (6) have shown highly correlated errors between 4-day weighed food records and a modified version of the Willett food frequency questionnaire in a British validation study using urinary nitrogen as a biomarker for protein intake. The corresponding correlation is higher than that observed between errors in the recalls and Diet History Questionnaire in the Observing Protein and Energy Nutrition biomarker study we are currently analyzing (Kipnis et al., unpublished data). Clearly, the issue of correlated errors requires further research.
We would like to address a few specific concerns of Dr. Block's. First, as stated by Dr. Block, folate fortification was in transition during the study period. In an earlier draft, Dr. Block pointed out that the folate data among instruments were not consistent with respect to fortification. Thus, we removed folate data from the paper. At this time, however, all three food frequency questionnaire nutrient databases have been updated to reflect the new folate values. Second, as stated in the paper, other than for three standard software options ("Recalc," "Fruit-Adjust," and "Veg-Adjust"), we did not modify the Block food frequency questionnaire analysis options, nutrient database, or portion sizes. Although we appreciate Dr. Block's concern about the options we did change, we did so for specific reasons. The "Recalc" option identifies persons with extreme energy intakes and modifies their portion size data, thereby modifying nutrient intakes. When we were considering how to handle extreme nutrient intake values, we needed a consistent approach for all three food frequency questionnaires. We decided to use energy cutpoints. Applying energy exclusions to a distribution first modified by "Recalc" would have led to a differential probability of inclusion by instrument. As to "Fruit-Adjust" and "Veg-Adjust," both the Diet History Questionnaire and the Block food frequency questionnaire have this option, but the Willett food frequency questionnaire does not. Again, we wanted to treat all instruments consistently so we chose not to use these options. Third, the recall data did not include any supplements. Fourth, we have performed preliminary analyses on the effect of assigning a sex-specific medium portion size for each line item on the Diet History Questionnaire; it appears to decrease correlations with absolute intakes but not to affect energy-adjusted correlations.
We thank the editors of the American Journal of Epidemiology for allowing this discussion of dietary assessment issues to take place.
NOTES
Reprint requests to Dr. Amy F. Subar, 6130 Executive Boulevard, MSC 7344, EPN 4005, Bethesda, MD 20892-7344 (e-mail: amy_subar{at}nih.gov).
REFERENCES
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