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.

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.

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 –13·5 (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.

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.