Abstract
OBJECTIVE: To develop a method to validate an FFQ for reported intake of episodically consumed foods when the reference instrument measures short-term intake, and to apply the method in a large prospective cohort. DESIGN: The FFQ was evaluated in a sub-study of cohort participants who, in addition to the questionnaire, were asked to complete two non-consecutive 24 h dietary recalls (24HR). FFQ-reported intakes of twenty-nine food groups were analysed using a two-part measurement error model that allows for non-consumption on a given day, using 24HR as a reference instrument under the assumption that 24HR is unbiased for true intake at the individual level. SETTING: The National Institutes of Health-AARP Diet and Health Study, a cohort of 567 169 participants living in the USA and aged 50-71 years at baseline in 1995. SUBJECTS: A sub-study of the cohort consisting of 2055 participants. RESULTS: Estimated correlations of true and FFQ-reported energy-adjusted intakes were 0·5 or greater for most of the twenty-nine food groups evaluated, and estimated attenuation factors (a measure of bias in estimated diet-disease associations) were 0·4 or greater for most food groups. CONCLUSIONS: The proposed methodology extends the class of foods and nutrients for which an FFQ can be evaluated in studies with short-term reference instruments. Although violations of the assumption that the 24HR is unbiased could be inflating some of the observed correlations and attenuation factors, results suggest that the FFQ is suitable for testing many, but not all, diet-disease hypotheses in a cohort of this size.