title: Relative validity of a food frequency questionnaire used in the Inter99 study.
abstract: OBJECTIVE
  To evaluate the validity of the Inter99 food frequency questionnaire (FFQ) compared with a 28-days' diet history and biomarkers.
  
  
  METHODS
  A random sample of 13 016 individuals were drawn from a general population and invited for a health screening programme. Participation rate was 52.5%. All high-risk individuals were re-invited for assessment after 1 and 3 years and completed a 198-item FFQ at all three occasions. Participants attending for 3 years follow-up were invited to participate in the validation study, including a 28-days' diet history, a 24-h urine collection and a fasting blood sample. Overall, 264 subjects participated.
  
  
  RESULTS
  Spearman's rank correlation coefficients between the two dietary methods ranged from 0.31(beta-carotene) to 0.64 (fruits) in men and from 0.31 (polyunsaturated fat and sodium) to 0.64 (fruits) for women. The proportion of individuals classified in the same or adjacent quintiles were, on average, 72% for men and 69% for women. Gross misclassification was found on average in 2%. The correlation coefficients of the residuals ranged from 0.27 (sodium) to 0.61 (fruits) for men and from 0.21 (sodium) to 0.62 (B12-vitamin) for women. Correlation coefficients between fruit and vegetable intake and carotenoids ranged from -0.08 (lycopene) to 0.44 (alpha-carotene). For the residuals the correlation coefficients ranged from -0.004 (lycopene) to 0.47 (alpha-carotene).
  
  
  CONCLUSIONS
  The Inter99 FFQ and the residuals of the intake provide acceptable classification of individuals according to their dietary intakes and the FFQ gives a good quantitative measurement of key dietary components.
authors.affiliation: Research Centre for Prevention and Health, Glostrup University Hospital, Glostrup, Denmark. ulto@glostruphosp.kbhamt.dk
authors.forename: U
authors.initials: U
authors.lastname: Toft
authors.forename: L
authors.initials: L
authors.lastname: Kristoffersen
authors.forename: S
authors.initials: S
authors.lastname: Ladelund
authors.forename: A
authors.initials: A
authors.lastname: Bysted
authors.forename: J
authors.initials: J
authors.lastname: Jakobsen
authors.forename: C
authors.initials: C
authors.lastname: Lau
authors.forename: T
authors.initials: T
authors.lastname: Jørgensen
authors.forename: K
authors.initials: K
authors.lastname: Borch-Johnsen
authors.forename: L
authors.initials: L
authors.lastname: Ovesen
mesh_terms: D010129:4-Aminobenzoic Acid / Q000652:urine; D000328:Adult; D015415:Biomarkers / Q000097:blood / Q000652:urine; D002338:Carotenoids / Q000097:blood; D003718:Denmark; D004032:Diet; D015930:Diet Records; D004034:Diet Surveys; D005260:Female; D005638:Fruit; D006801:Humans; D008297:Male; D008875:Middle Aged; D009584:Nitrogen / Q000652:urine; D015596:Nutrition Assessment; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D018709:Statistics, Nonparametric; D011795:Surveys and Questionnaires / Q000592:standards*; D014675:Vegetables
chemical_list: D015415:Biomarkers; D002338:Carotenoids; D009584:Nitrogen; D010129:4-Aminobenzoic Acid