VALIDATION OF A DIETARY QUESTIONNAIRE USED IN A LARGE-SCALE PROSPECTIVE COHORT STUDY ON DIET AND CANCER

Objective: The study was conducted to assess the validity of a self-ad ministered 150-item food frequency questionnaire (FFQ), used in a coho rt study on diet and cancer (120852 men and women, aged 55-69). Design & subjects: The study was carried out in a subgroup of the cohort (59 men and 50 women) 2 years after the baseline FFQ was completed. A die tary record, kept over three 3-day periods, 4-5 months apart, served a s reference method. To evaluate the representativeness of the study po pulation for the entire cohort, a comparison was made with the baselin e questionnaire of a random sample of the cohort. Results: Pearson cor relation coefficients between nutrient intakes assessed by the record and the FFQ that was completed afterwards ranged from 0.40 (95% CI: 0. 22-0.54) for vitamin B1 to 0.86 (95% CI: 0.80-0.90) for alcohol intake , with correlations for most nutrients between 0.6 and 0.8. Adjustment for energy intake and sex did not materially affect these correlation s, except the correlation for fat intake, which changed from 0.72 to 0 .52. Correlation coefficients were only slightly modified when the res ults were extrapolated to the cohort at large. Correction of correlati on coefficients for attenuation by day-to-day variance in the record d ata improved them by 0.07 on average. Conclusions: It is concluded tha t the FFQ is able to rank subjects according to intake of food groups and nutrients. Despite a better performance of validation study partic ipants, this conclusion also applies to the cohort at large.