Self-administered food frequency questionnaire: the effect of different designs on food and nutrient intake estimates

Our objective was to determine the possible influence that different designs of a food frequency questionnaire might have on food, energy and nutrient intake estimates.

A population-based survey included 6783 women, 40-70 years old, living in central Sweden. Using a factorial study design, we compared eight different types of questionnaire covering combinations of three factors: increasing/decreasing frequency categories; addition of portion sizes; and addition of non-dietary questions. All questionnaires included the same list of 60 food items. One of the eight questionnaires was mailed to each subject according to a random assignment. The overall response rate was 77%.

Compared with increasing frequencies, decreasing order of frequency categories entailed 3-11% higher estimates of mean intake for 7 of 14 food groups, 4% higher estimates for energy and 3-6% higher estimates for 13 of 18 nutrients. Addition of portion sizes had heterogeneous effects, both on dietary items (e.g. from -30% decrease for eggs to +76% increase for coffee) and on calculated nutrients (from -7% for beta-carotene to +19% for vitamin C). The inclusion of some additional non-dietary questions did not influence the estimated mean intake of any food or nutrient.

The results of this study have implications for the design of questionnaires and for pooled analyses in nutritional epidemiology, when different food questionnaires are used.