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 (DR) in four 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 for a validation study of a questionnaire from the subgroup of the Japan Public Health Center-based prospective Study on cancer and cardiovascular disease (JPHC Study) cohort II. Categories of dietary supplements in the FFQ were 'multivitamins', 'beta-carotene', 'vitamin C', 'vitamin E', and 'other supplements'. For each category, inquiries were made as to the brand name, frequency and duration of use. A dietary supplement user was defined as a subject who used a dietary supplement one or more times a week for a year or longer.

RESULTS

Prevalence of overall supplement use was 10.9%. Percentage agreement between FFQ1 and FFQ2 was 91.7%. Sensitivity and specificity of supplement 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 supplement use such as vitamin C. Categories must be corrected when determining multivitamin and 'vitamin E'.