Background: For use in the German part of the European Prospective Investigation into Cancer and Nutrition (EPIC) a self-administered, optically readable food frequency questionnaire (FFQ) was developed to assess individuals' usual food and nutrient intake over the past year. The food list comprised 158 food items for which the typical portion size, the consumption frequency (1-6 times) and the time period (day, week, month, or year) were requested. This paper reports the results of the reproducibility and relative validity of nutrient intakes obtained by this instrument.
Methods: The study was carried out from October 1991 to October 1992. During this period, 104 men and women, aged 35-64 years, completed a 24-hour recall once a month, and the FFQ in the middle and at the end of this study. A short questionnaire on global consumption patterns was used to adjust the food consumption frequency given in the FFQ.
Results: Reproducibility correlations varied from 0.59 for saturated fat to 0.88 for alcohol, with most values falling between 0.60 and 0.70. Energy-adjustment decreased the observed correlations for all nutrients. Correlations between nutrient intake values from the 12-day average of dietary recalls and the FFQ corrected for food consumption frequency ranged from 0.42 for polyunsaturated fat to 0.88 for alcohol, followed by 0.65 for dietary fibres. After de-attenuation and energy-adjustment the range of correlations was somewhat wider (0.43-0.75, without alcohol). With regard to classification, on average, 36% of subjects fell into the same quintile, and 75% into within-one quintile when classified by the frequency-corrected FFQ compared to the quintiles based on recalled data.
Conclusions: In summary, the newly developed FFQ showed fairly good reproducibility and relative validity for most evaluated nutrients. The between-subject variation of nutrients in the German cohort (including the participants from East Germany) is likely to be greater than that among the subjects of the pilot phase. A revised version of the FFQ corrected for food consumption frequency is used in the EPIC study.