1 Fox Chase Cancer Center, Philadelphia, PA 19111
2 Roswell Park Cancer Institute, Buffalo, NY 14263
In their article comparing a dietary pattern based on the Dietary Approaches to Stop Hypertension (DASH) Study and patterns identified using exploratory factor analysis, Schulze et al. (1) address a crucial issue in dietary pattern research: What method of quantifying dietary patterns is most useful in examining diet-disease associations? They conclude that the "hypothesis-oriented" DASH dietary pattern is the more useful for predicting hypertension risk. The actual answer, however, depends on the goal of the research investigation; while a "hypothesis-oriented" pattern may be useful in some instances, factor analysis-based patterns may be more informative in others.
The goal of quantifying a "hypothesis-oriented" pattern is to provide a basis for dietary recommendations or interventions encouraging people to alter their eating behaviors. The "hypothesis-oriented" pattern represents a combination of individual foods or nutrients thought to be related to a specific outcome based on prevailing interpretations of epidemiologic, clinical, or other scientific evidence. In contrast, "exploratory" patterns represent combinations of foods that tend to occur together in a given population as the result of different cultural, social, and economic conditions. Although they are identified "independently of their relevance to any disease" (1, p. 366), they represent real-world dietary behaviors that are of interest by themselves. The goal of quantifying "exploratory" patterns is to understand how socioeconomically and culturally influenced food intake patterns are associated with disease occurrencefor example, does adoption of a Western diet increase risk of chronic disease among Mexican Americans (2)? Is a fast-food-based diet of meat and French fries less healthy than a more traditional meat-and-potatoes diet (3)? An important caveat is that patterns identified using exploratory factor analysis are not entirely exploratory; rather, they must be meaningful and interpretable, representing recognizable combinations of foods that have a firm social/historical basis and that might even have been expected a priori (4).
Three other aspects of Schulze et al.s work deserve comment. First, contrary to the authors conclusions, the "hypothesis-oriented" pattern did not perform substantially better than factor analysis-based patterns in predicting hypertension risk. Estimates did not show a clear trend of a protective effect, and the DASH dietary pattern was significantly protective only in the third quartile. In fact, estimates for the DASH dietary pattern were largely similar to estimates for the "traditional cooking" pattern, for which we might have expected a positive association with risk.
Second, other factors might have weakened associations for the factor analysis-based patterns. Besides the relative homogeneity in intake among study participants noted by Schulze et al., the manner in which they grouped the foods may have attenuated estimates, as it did in the only study to have examined the effect of grouping foods (5). Grouping potentially dissimilar foodsfor example, including ketchup with sauces and salad dressingmay weaken the factor structure of the food items and produce less internally consistent patterns.
Finally, comparison of estimates with those based on individual foods or nutrients, such as milk, might have been useful. Stronger associations for the DASH dietary pattern than for any individual foods or nutrients would demonstrate the utility of the hypothesis-oriented pattern over the usual, single-factor approach in predicting hypertension risk. It would also have been interesting to see how well the DASH dietary pattern fitted these data using a confirmatory analysis, such as that used to confirm the exploratory factor analysis-derived pattern structures.
In summary, Schulze et al. (1) raise a crucial issue that is seldom addressed in dietary pattern research. However, their work should not lead to the conclusion that dietary patterns identified using exploratory factor analysis are less valuable than hypothesis-oriented patterns in studies of diet and disease.
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