The impact of dietary protein intake on urinary creatinine excretion in a healthy pediatric population

Objective: The aim of the study was to investigate the effect of daily protein intake on urinary creatinine (CR) excretion in healthy children. Methods: In a cross-sectional study, 24-hour urinary CR excretion rates were measured in 350 children aged 4 to 13 years. Body composition was assessed according to anthropometric measurements, and intake of protein was estimated from dietary records. Results: CR excretion adjusted for fat-free mass (FFM) showed a strong age dependency and correlated clearly with total protein intake and animal protein intake. Stepwise regression analyses revealed that age was the main predictor; however, animal protein intake improved the proportion of explained variance in FFM-related CR excretion. Comparisons of urinary CR output in subgroups of children with low (Ist tertile) or high (3rd tertile) animal protein intake showed that on average, a difference of about 9% in CR excretion corresponded to a difference of 1.1 g/kg of body weight per day of animal protein intake. Conclusions: Animal protein intake must be taken into account when urinary CR output is used for evaluating body composition in children. Additionally, the finding of a clearly stronger increase in urinary CR than in FFM with age offers an explanation for the commonly observed phenomenon of a decreasing ratio of numerous urinary analytes related to CR during childhood.