Validation of a soy food-frequency questionnaire and evaluation of correlates of plasma isoflavone concentrations in postmenopausal

Background: Soy foods may have various health benefits, but lit- tle is known about the patterns and correlates of soy consumption among postmenopausal women in the United States. Objective: We assessed the reliability and validity of a soy food- frequency questionnaire (FFQ) and examined demographic, lifestyle, and dietary correlates of plasma isoflavone concentra- tions in postmenopausal women. Design: In this cross-sectional study, soy isoflavone intake and plasma isoflavone concentration were analyzed in 96 post- menopausal women aged 50-79 y; the data were obtained at 2 vis- its that were 1 wk apart. Intake was determined with a 20-item soy FFQ and a comprehensive FFQ that included questions about tofu and soymilk. Fasting plasma daidzein and genistein concentrations were determined with liquid chromatography-mass spectrometry. Results: Intraclass correlations between week 1 and week 2 val- ues were > 0.98 for both the soy and comprehensive FFQs. Median reported isoflavone intake was < 2 mg/d. Pearson's product- moment correlation coefficients relating isoflavone intakes with plasma isoflavone concentrations ranged from 0.35 to 0.43. Plasma isoflavone concentrations were positively associated with age, fiber consumption, servings of fruit and vegetables, and dietary supplement use and were inversely associated with caf- feine consumption; no associations with body mass index, educa- tion, dietary beliefs, activity level, alcohol intake, or fat intake were observed. Conclusions: Within a population with low soy consumption, the soy FFQ and comprehensive FFQ showed good reliability and moderate validity. Associations of plasma isoflavone concentra- tions with other dietary behaviors suggest that these compounds may serve as biomarkers of health behaviors in populations with low soy consumption. Am J Clin Nutr 2003;77:674-80.