Evaluation of the relative validity of a Web-based food frequency questionnaire used to assess Soy Isoflavones and nutrient intake in adolescents

Abstract

Background: Dietary assessment during adolescence is crucial in determining how adolescents' consumption habits potentially affect current and long-term health. However, assessment methods for adolescents need to be relevant to their emerging technological culture. We developed a web-based food frequency questionnaire (webFFQ) to assess the habitual soy isoflavones and nutrient intake of adolescents. Our purpose was to validate this webFFQ against multiple 1-day photograph-assisted food records (FR). Methods: Adolescents aged 12-18 years (n = 70) attending middle and high schools completed the webFFQ and provided six 1-day FR. Fifteen participants were excluded due to improbable energy intake. Statistical agreements were determined using Wilcoxon signed-ranks test, Pearson's bivariate correlations of normalized values with energy-adjustment and correction for attenuation, cross-classification, and Bland-Altman plots. Results: Based on n = 55, the webFFQ had higher intake estimates for all isoflavones and most nutrients compared to the FR. Energy-adjusted and deattenuated correlations were moderately strong for total isoflavones (r = 0.67), daidzein (r = 0.63), genistein (r = 0.64), and glycitein (r = 0.54). They ranged from a high of 0.82 (animal proteins) to a low of 0.11 (vitamin B12) for nutrients. Cross-classification agreement up to within 1 quartile ranged from 99.0 % (vegetable protein) to 61.8 % (vitamin B12) with low gross misclassifications (0.0-12.7 %). Bland-Altman plots for the isoflavones showed consistent overestimation and wide variation but nevertheless good agreement between the two methods. Conclusions: The webFFQ is relatively valid in ranking adolescents according to their isoflavones and nutrient intake. However, while it significantly overestimates the absolute intake of most nutrients, results are comparable to other food frequency questionnaires developed for adolescents.