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Relative validity of fruit and vegetable intake estimated by the food frequency questionnaire used in the Danish National Birth Cohort*

Maternal Nutrition Group, Danish Epidemiology Science Centre, Statens Serum Institut, Copenhagen S, Denmark, tbs{at}ssi.dk

Sjurdur F. Olsen

Maternal Nutrition Group, Danish Epidemiology Science Centre, Statens Serum Institut, Copenhagen S, Denmark

Salka E. Rasmussen

Department of Toxicology and Risk Assessment, Danish Institute for Food and Veterinary Research, Denmark

Merete Osler

Department of Social Medicine, Institute of Public Health, University of Copenhagen, Denmark

Objective: To validate the fruit and vegetable intake estimated from the Food Frequency Questionnaire (FFQ) used in the Danish National Birth Cohort (DNBC). Subjects and setting: The DNBC is a cohort of 101,042 pregnant women in Denmark, who received a FFQ by mail in gestation week 25. A validation study with 88 participants was made. A seven-day weighed food diary (FD) and three different biomarkers were employed as comparison methods. Results: Significant correlations between FFQ and FD-based estimates were found for fruit (r=0.66); vegetables (r=0.32); juice (r=0.52); fruit and vegetables (F&V) (r=0.57); and fruit, vegetables, and juice (F&V&J) (r=0.62). Sensitivities of correct classification by FFQ into the two lowest and the two highest quintiles of F&V&J intake were 58—67% and 50— 74%, respectively, and specificities were 71—79% and 65—83%, respectively. F&V&J intake estimated from the FFQ correlated significantly with urinary flavonoids (r=0.39, p<0.005), plasma β-carotene (r=0.21, p<0.05) and erythrocyte folate (r=0.33, p<0.005); all average biomarker levels increased across increasing quintiles of F&V&J intake. Conclusion: Data on F&V&J recorded by FFQ in DNBC can be used in etiological analyses of associations between fruit and vegetable intake and health outcomes, provided that the potential misclassification of dietary exposure is kept in mind.

Key Words: Biomarkers • Danish National Birth Cohort • erythrocyte folate • food diaries • food frequency questionnaire • plasma beta-carotene • urinary flavonoids • validation

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