Relative validity of a semi-quantitative, web-based FFQ used in the 'Snart Foraeldre' cohort - a Danish study of diet and fertility

Objective: To assess the relative validity of a semi-quantitative, web-based FFQ completed by female pregnancy planners in the Danish 'Snart Foraeldre' study. Design: We validated a web-based FFQ based on the FFQ used in the Danish National Birth Cohort against a 4 d food diary (FD) and assessed the relative validity of intakes of foods and nutrients. We compared means and medians of intakes, and calculated Pearson correlation coefficients and de-attenuated coefficients to assess agreement between the two methods. We also calculated the proportion correctly classified based on the same or adjacent quintile of intake and the proportion of grossly misclassified (extreme quintiles). Setting: Participants (n 128) in the 'Snart Foraeldre' study who had completed the web-based FFQ were invited to participate in the validation study. Subjects: Participants in the 'Snart Foraeldre' study, in total ninety-seven women aged 20-42 years. Results: Reported intakes of dairy products, vegetables and potatoes were higher in the FFQ compared with the FD, whereas reported intakes of fruit, meat, sugar and beverages were lower in the FFQ than in the FD. Overall the de-attenuated correlation coefficients were acceptable, ranging from 0.33 for energy to 0.93 for vitamin D. The majority of the women were classified in the same or adjacent quintile and few women were misclassified (extreme quintiles). Conclusion: The web-based FFQ performs well for ranking women of reproductive age according to high or low intake of foods and nutrients and, thus, provides a solid basis for investigating associations between diet and fertility.