How did the OPEN Study assess the FFQ and 24-hour recall?

Over the course of the study, OPEN participants completed 2 Food Frequency Questionnaires (FFQ), 2 24-recall interviews, and filled out several other health-related questionnaires. They were dosed with doubly labeled water (a biomarker used for measuring energy expenditure), provided several spot urine samples to complete the doubly labeled water assessment, completed two 24-hour urine collections and had their height and weight measured. Investigators analyzed the 24-hour urines for several nutrients: nitrogen, sodium and potassium, biomarkers that measure protein, sodium, and potassium intakes, respectively. The questionnaires and samples allowed study investigators to compare what participants said they ate and drank against the objective evidence provided by the biomarkers and thus, to get a better sense of the extent and nature of error in the FFQs and 24-hour recalls.