Objective To develop a dietary assessment instrument to measure soy food consumption and isoflavone intake and test it for reliability.

Design A soy food frequency questionnaire, designed for use in this study, was administered twice to participants, separated by a 2-week interval. The questionnaire was tested for reproducibility of estimates of soy food consumption and isoflavone (genistein and daidzein) intake.

Subjects/setting A convenience sample of 51 faculty, staff, and students was recruited from a naturopathic university.

Statistical analyses performed Intraclass correlation coefficients were used to assess reproducibility of estimates of soy food intake and genistein and daidzein intake.

Results Correlation coefficients comparing mean soy food servings per month between the 2 administrations of the questionnaire ranged from 0.50 for soy yogurt to 0.89 for tempeh. Correlation coefficients for genistein and daidzein intake estimated by the 2 administrations of the questionnaire were the same: 0.89. Mean intake (±standard deviation) of genistein and daidzein was 7±10 and 4±6mg/day, respectively. Fifteen soy foods contributed 95% of the total genistein and daidzein intake: tofu, soy yogurt, tempeh, soy milk, low-fat tofu, soy flour, miso, soy protein isolate, low-fat soy milk, veggie soy burger, textured vegetable protein, miso soup, cooked soybeans, soy hot dogs, and natto (fermented soybeans).

Applications The soy food frequency questionnaire developed in this study provided highly reproducible estimates of soy food consumption and isoflavone intake. This instrument may be a useful tool in studies of the associations between isoflavone exposure and risk for chronic disease. J Am Diet Assoc. 1999;99:558–563.