Introduction The food frequency questionnaire (FFQ) has been one the most widely used method for assessment of food consumption, as well as being able to provide the usual intake, is considered easy to use and of low cost.
Objective To assess the validity and reproducibility of a quantitative FFQ with 66 food items (QFFQ) developed based on a representative sample of adolescents from São Paulo, Brazil.
Methods 250 adolescents filled two FFQ (1-year interval) and 5 24-h recalls (3-month interval) as part of a population-based survey conducted in São Paulo, Brazil, in 2007. To quantify the nutrient intake, the software Nutrition Data System for Research was used. The intake was adjusted for within-person variability by the method proposed by Iowa State University, using the software PC-SIDE, and energy-adjusted for the residual method. The validity for 23 nutrients was assessed by Spearman correlation coefficient and weighted κ. Intra-class correlation coefficient and weighted κ was used to reproducibility assessment.
Results In the validation, the Spearman correlation coefficients ranged from 0.25 (riboflavin) to 0.57 (phosphorus) of which eight nutrients showed acceptable correlation (>0.4). In the reproducibility analysis, the intra-class correlation coefficients ranged from 0.18 (vitamin C) to 0.57 (niacin), of which five nutrients showed acceptable correlation. The weighted κ ranged from 0.18 (sodium) to 0.67 (riboflavin) to reproducibility and from 0.19 (polyunsatured fat) to 0.56 (calcium) to validation.
Conclusion The results support the use of this instrument to assess food intake in epidemiological studies conducted in São Paulo, Brazil.
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