[Validity assessment of a food frequency questionnaire by comparison with a 4-day diet record]

When dietary methods are used to evaluate nutritional intake, it is important to know which methods are most appropriate to ensure validity in nutrient intake studies. Our main objective was to assess the validity of a semi-quantitative food-frequency questionnaire (FFQ) developed for Portuguese adults by comparing nutrient intake (energy and 44 nutrients) assessed by FFQ with the nutritional data from a four-day diet record (DR). The FFQ was developed by the Epidemiology Department, Faculty of Medicine, University of Porto, and allowed the subjects to indicate their daily, weekly or monthly intake of 89 foods. Subjects were 246 university students (159 females and 87 males, aged 18 to 29 years). Mean intakes from the FFQ were generally higher than those from the DR. Pearson correlation coefficients between FFQ and DR ranged from 0.21 for vitamin D, to 0.73, for iodine. When energy-adjusted, this coefficients ranged from 0.20, for selenium, to 0.75 for iodine. Misclassification in opposite extreme quintiles varied from 0% to 16%, and the proportion of subjects correctly classified within the lowest two quintiles ranged between 52% and 84%. The distribution of those correctly classified within the highest two quintiles ranged between 51% and 88%. Exact classification in the same quintile of ingestion varied from 26% and 41%. These data suggest that this FFQ provides valid estimates of intakes for major nutrients, comparable to those reported from other studies in the literature.