Abstract
BackgroundAssessment of habitual diet is important in investigations of diet-disease relationships. Many epidemiological studies use the food frequency questionnaire (FFQ) to evaluate dietary intakes but few studies validate the instrument against biological markers. The aim of this study was to assess the validity and reproducibility of a previously validated 70-item food frequency questionnaire (FFQ) that was expanded to 120-items to assess diet - cancer relations.MethodsRelative validity of the FFQ was assessed against twelve 24-hour recalls administered over 12 months in 70 subjects. The FFQ was repeated after one year (FFQ2) to assess reproducibility. The validity of the FFQ was evaluated by comparing nutrient and food group intakes from 24-hour recalls with the first and second FFQ. In addition, FFQ validity for cholesterol and folate were determined through correlation with biomarkers (serum cholesterol, serum folate and whole blood folate) in 159 control subjects participating in a case-control prostate cancer study.ResultsCompared to recalls the FFQ tended to overestimate energy and carbohydrate intakes but gave no differences in intake for protein and fat. Quartile agreement for energy-adjusted nutrient intakes between FFQ2 and recalls ranged from 31.8% - 77.3% for the lowest quartile and 20.8% - 81.0% in the highest quartile. Gross misclassification of nutrients was low with the exceptions of protein, vitamin E and retinol and weighted kappa values ranged from 0.33 to 0.64 for other nutrients. Validity correlations for energy-adjusted nutrients (excluding retinol) were moderate to high (0.38- 0.86). Correlation coefficients between multiple recalls and FFQ1 ranged from 0.27 (fruits) to 0.55 (red meat); the second FFQ gave somewhat higher coefficients (0.30 to 0.61). Reproducibility correlations for the nutrients ranged from 0.50 to 0.84.Calibration of the FFQ with biochemical markers showed modest correlations with serum cholesterol (0.24), serum folate (0.25) and whole blood folate (0.33) adjusted for age, energy, body mass index and smoking.ConclusionsThe expanded FFQ had good relative validity for estimating food group and nutrient intakes (except retinol and vitamin E) and was a reliable measure of habitual intake. Associations with biomarkers were comparable to other studies.
Highlights
Assessment of habitual diet is important in investigations of diet-disease relationships
Biochemical markers have been used as surrogate markers of dietary intakes as the potential sources of random errors are different from errors in any dietary assessments [6] and are largely independent of measurement errors associated with memory [7]
Validity was assessed by comparing estimates obtained from the food frequency questionnaire (FFQ) with those from multiple 24-h recalls in both genders in the general population and calibrated in men enrolled in a study of prostate cancer, using biological markers
Summary
Assessment of habitual diet is important in investigations of diet-disease relationships. Many epidemiological studies use the food frequency questionnaire (FFQ) to evaluate dietary intakes but few studies validate the instrument against biological markers. Accurate estimates of habitual dietary intake remain a challenge in the study of diet-disease relationships. Food frequency questionnaires (FFQs) have been used to assess long-term dietary intakes, an important exposure factor for conditions such as cardiovascular diseases and Validation and calibration studies of FFQs are essential in nutritional epidemiology for the interpretation of findings and comparability between studies. Random errors between the two methods cannot be assumed and could lead to higher estimates of correlation between the FFQ and the reference method [4].
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