Validation of protein intake assessed from weighed dietary records against protein estimated from 24 h urine samples in children, adolescents and young adults participating in the Dortmund Nutritional and Longitudinally Designed (DONALD) Study

Objective. To date, only a few nutritional assessment methods have been validated against the biomarker of urinary-N excretion for use in children and adolescents. The aim of the present study was to validate protein intake from one day of a weighed dietary record against protein intake estimated from a simultaneously collected 24 h urine sample. Design: Cross-sectional analyses including 439 participants of the Dortmund Nutritional and Longitudinally Designed (DONALD) Study from four age groups (3-4, 7-8, 11-13 and 18-23 years). Mean differences, Pearson correlation coefficients (r), cross-classifications and Bland-Altman plots were used to assess agreement between methods. Results: Weighed dietary records significantly underestimated mean protein intake by -6.4 (95% CI -8.2, -4.7) g/d or -11%, with the difference increasing across the age groups from -0.6(95% CI -2.7, 1.5) g/d at age 3-4 years to -135(95% CI -18.7, -8.3)g/d at age 18-23 years. Correlation coefficients were r=0.7 for the total study sample and ranged from r=0.5 to 0.6 in the different age groups. Both methods classified 85% into the same/adjacent quartile for the whole study group (83-86% for the different age groups) and 2.5% into the opposite quartile (1.9-3.1% for the different age groups). Bland-Altman plots for the total sample indicated that differences in protein intake increased across the range of protein intake, while this bias was not obvious within the age groups. Conclusions: Protein intake in children and adolescents can be estimated with acceptable validity by weighed dietary records. In this age-heterogeneous sample, validity was lower among adolescents and young adults.