Validity and reproducibility of a self-administered questionnaire to determine dietary supplement users among Japanese

Objective: To evaluate reproducibility and validity of a self-administered food frequency questionnaire (FFQ) to determine the dietary supplement use. Design: Questionnaire data were compared with dietary: records (BR) in foil s 7 day periods for validity. FFQ1 and FFQ2 administered at an interval of approximately one year were compared for reproducibility. Setting and subjects: A total of 361 samples were selected fel a validation study of a questionnaire from the subgroup of the Japan Public Health Cent er-based prospective Study on cancer and cardiovascular disease (JPHC Study ) cohort II. Categories of dietary supplements in the FFQ were 'multivitami ns', 'beta -carotene'. 'vitamin C', 'vitamin E', and 'other supplements'. F or each category. inquiries were made as to the brand name, frequency anti duration of use. a dietary supplement user was defined as a subject who use d a dietary supplement one or more times a week for a year or longer. Results: Prevalence of overall supplement use was 10.9%, Percentage agreeme nt between FFQ1 and FFQ2 was 91.7%. Sensitivity and specificity of suppleme nt use by FFQ were 80.6% and 89.2%, respectively. Sensitivity was high for 'vitamin C'. When the self-reported categories were corrected, sensitivity for 'multivitamins' and 'vitamin E' improved. Conclusions: The FFQ on dietary supplements used for the JPHC 5 y follow-up survey was valid to determine overall use and well-defined dietary supplem ent use such as vitamin C, Categories must be corrected when determining mu ltivitamin and 'vitamin E'.