title: Validity and reproducibility of a self-administered questionnaire to determine dietary supplement users among Japanese.
abstract: OBJECTIVE
  To evaluate reproducibility and validity of a self-administered food frequency questionnaire (FFQ) to determine the dietary supplement use.
  
  
  METHODS
  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.
  
  
  METHODS
  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'.
authors.affiliation: National Cancer Center Research Institute, Tokyo and Kashiwa, Japan.
authors.forename: J
authors.initials: J
authors.lastname: Ishihara
authors.forename: T
authors.initials: T
authors.lastname: Sobue
authors.forename: S
authors.initials: S
authors.lastname: Yamamoto
authors.forename: S
authors.initials: S
authors.lastname: Sasaki
authors.forename: M
authors.initials: M
authors.lastname: Akabane
authors.forename: S
authors.initials: S
authors.lastname: Tsugane
mesh_terms: D001205:Ascorbic Acid / Q000008:administration & dosage; D015331:Cohort Studies; D015930:Diet Records; D019587:Dietary Supplements / Q000706:statistics & numerical data*; D005260:Female; D006801:Humans; D007564:Japan; D008297:Male; D008875:Middle Aged; D015995:Prevalence; D011446:Prospective Studies; D015203:Reproducibility of Results; D012650:Self Disclosure; D012680:Sensitivity and Specificity; D011795:Surveys and Questionnaires / Q000592:standards*; D014810:Vitamin E / Q000008:administration & dosage
chemical_list: D014810:Vitamin E; D001205:Ascorbic Acid