Frequency of soy food consumption and serum isoflavone concentrations among Chinese women in Shanghai

OBJECTIVE: The food-frequency questionnaire (FFQ) can be an efficient tool to evaluate dietary intake in large, population-based studies, especially for specific foods. The objective of this study was to validate the assessment of soy and isoflavone (daidzein and genistein) intakes, measured by an FFQ, by comparing intakes with serum isoflavone concentrations. DESIGN AND SETTING: Soy and isoflavone intakes and serum isoflavone concentrations were determined as part of a case-control study of dietary factors and risks of benign breast disease and breast cancer. The FFQ, administered during an in-person interview, included six soy-specific line items. Blood was drawn within one week of FFQ completion. SUBJECTS: In total, 1823 women living in Shanghai, People's Republic of China. RESULTS: In this population, soybean milk, fresh bean curd and other bean foods were eaten once per week, and fermented bean curd, fried bean curd puff and soybeans were eaten less than once per week. A significant linear trend (P<0.01) in serum isoflavone concentrations across increasing categories of soy and isoflavone intakes was observed, indicating that soy and isoflavone intakes, measured by the FFQ, well distinguished serum isoflavone concentrations. Linear trends were also observed in both case and control groups in stratified analyses, suggesting little differential bias by case-control status. CONCLUSIONS: The results suggest that the FFQ provides a useful marker of soy food consumption and isoflavone exposure in this population.