Validity of protein, retinol, folic acid and n–3 fatty acid intakes estimated from the food-frequency questionnaire used in the Danish National Birth Cohort

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Abstract
Objective To validate intakes of protein, folic acid, retinol and n–3 fatty acids estimated from a food-frequency questionnaire in week 25 of pregnancy (FFQ-25). Design Validation was done against a 7-day weighed food diary (FD) and biomarkers of the nutrients in gestation week 32–38. Subjects and setting The FFQ-25 to be validated was used in the Danish National Birth Cohort comprising 101 042 pregnant Danish women, of whom 88 participated in the present validation study. Results Estimated intakes of protein, retinol and folic acid did not differ significantly between the two dietary methods, but intake of n–3 fatty acids was one third larger when estimated from the FFQ-25. The intakes estimated from the two dietary methods were all significantly correlated, ranging from 0.20 for retinol intake to 0.57 for folic acid intake. Sensitivities of being correctly classified into low and high quintiles were between 0.22 and 0.77, and specificities were between 0.62 and 0.89. Urinary protein content did not correlate significantly with protein estimated from the FFQ (r = 0.17, P > 0.05), but did with intake estimated from the FD (r = 0.56, P < 0.0001). Erythrocyte folate correlated significantly with the estimated total intake from the FFQ (r = 0.55, P < 0.0001) and the FD (r = 0.52, P < 0.0001). No correlations with plasma retinol were found. Erythrocyte eicosapentaenoic acid (C20:5n-3) correlated significantly with n–3 fatty acids intake estimated from both the FFQ-25 (r = 0.37, P < 0.001) and the FD (r = 0.62, P < 0.0001). Conclusion The FFQ-25 gives reasonable valid estimates of protein, retinol and folic acid intakes, but seems to overestimate intake of n–3 fatty acids.