Objective: To assess the degree of agreement (comparability) between dietary fiber intakes reported on a food frequency questionnaire (FFQ) with 4-day food records (4DFR) and determine whether demographic, behavioral and biological factors influence comparability.
Methods: At baseline and year one, all participants in the Polyp Prevention Trial (PPT), a multi-center randomized, clinical trial of a low-fat, high fiber, high fruit/vegetable eating plan and recurrence of large bowel adenomatous polyps were instructed in dietary assessment and completed a 106-item FFQ and 4DFR that trained nutritionists reviewed. A random sub-cohort of participants (n = 399) was selected from the intervention and control arms of the PPT for analysis of both FFQ and 4DFR.
Results: Baseline crude and energy-adjusted fiber intakes were significantly higher in the 4DFR than the FFQ (P = 0.001). Using Bland-Altman statistics, the mean difference (FFQ-4DFR) was -0.11 g/MJ; while the limits of agreement were -1.45, 1.23 g/MJ. The mean fiber difference increased with increasing average intake (FFQ + 4DFR)/2, (P = 0.004) for men, but not women (P = 0.10), suggesting that fiber intake was under-estimated in the FFQ, relative to the 4-DFR, for men with low fiber intakes and over-estimated for men with high intakes. Smoking and gender significantly influenced the average intake at baseline, whereas other demographic and behavioral factors did not. Education was significantly associated with average difference in fiber intake at baseline, but not at year 1.
Conclusions: This study of clinical trial volunteers revealed differences in the ability to comparably report fiber intake across tools by gender, smoking, and education, however participants' repeated training in dietary assessment improved comparability in reporting over time.