USING FOOD FREQUENCY QUESTIONNAIRES TO ESTIMATE FRUIT AND VEGETABLE INTAKE - ASSOCIATION BETWEEN THE NUMBER OF QUESTIONS AND TOTAL INTAKES

The purpose of this study was to determine how estimates of fruit and vegetable intake, as determined from food frequency questionnaires, may be affected by the number of foods included. Three different national surveys of adults were used. Each of the surveys employed a food frequency questionnaire, but each included a different number of questions related to fruits and vegetables. Compared across surveys, values for median frequency of intake of individual fruits and vegetables were generally similar. However, values for median frequency of total fruit and vegetable intake, determined by summing across all fruits and vegetables included in the respective questionnaires, were very different and were associated with the number of questions asked: the 1987 National Health Interview Survey included 20 questions regarding fruits and vegetables, and the median frequency of intake was 23.8 times per week; the baseline survey for the national 5 A Day for Better Health Program included 33 questions, and the median intake was 34.6; and the 1982-1984 NHANES I Epidemiologic Follow-up Survey included 40 questions, and the median intake was 38.8. These findings suggest that summing across frequencies for individual foods may not be a valid way to assess intakes of total fruits and vegetables. Summary questions and/or adjustment factors may need to be employed to increase precision of the estimates.