Protein quality and urea kinetics in prepubertal Chilean schoolboys

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Abstract
Urea kinetics were measured non-invasively in 12 Chilean schoolboys aged 8–10 years who were receiving one of two diets, either predominantly animal protein or predominantly vegetable protein. Both the diets provided an equivalent level of gross protein, 1.2 g/kg/day. The study diets were given for 10 days to enable adaptation to take place. On the eighth day a single oral dose of 15N15N-urea, 100mg, was given and the amount of label excreted as 15N15-urea in urine over the subsequent 48 hours was measured. There was little difference in any aspect of urea kinetics between the two diets with urea production (animal, 173 ± 50 mgN/kg/day; vegetable 179 ± 53 mgN/kg/day), urea excretion (animal, 86 ± 19 mgN/kg/day; vegetable, 105 ± 13 mgN/kg/day), urea nitrogen hydrolysis (animal, 87 ± 49 mgN/kg/day; vegetable, 74 ± 42 mgN/kg/day), and the salvaged urea-nitrogen derived from hydrolysis which returned to urea formation (animal, 12 ± 5 mgN/kg/day; vegetable, 17 ± 9 mgN/kg/day) all being similar. A very high proportion of the salvaged nitrogen derived from urea hydrolysis was maintained within the metabolic pool, about 80%, which was equivalent to 0.4 g protein/kg/day. This is the first time urea kinetics have been measured in children of this age and shows that 57% of the urea produced is excreted in urine on average with about 43% of the urea-nitrogen being salvaged for further metabolic interaction. It is concluded that the vegetable based protein diet taken habitually by Chilean children is metabolically equivalent in terms of urea kinetics to a diet based upon animal protein at this level of intake, but that high rates of salvage of urea nitrogen are found on both diets.