Urea production in normal breast‐fed infants measured with primed/intermittent oral doses of [15N,15N]urea

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Abstract
Urea kinetics were measured on 11 occasions in six normal, breast-fed infants aged 29-88 days. Prime and intermittent oral doses of [15N, 15N]urea with measurement of enrichment of urea in urine were used. The rate at which urea appeared in the urea pool was 265 mgN/kg per hour, 85% of which derived from endogenous production and 15% from the diet. Urinary excretion of urea was 87 mgN/kg per hour. Therefore, 60% of the urea entering the pool each day was hydrolysed by the metabolic activity of the colonic microflora and the nitrogen was made available for further metabolic interaction. The rate of urea appearance and the extent to which urea nitrogen was salvaged were greater in infants under 6 weeks than in those over 6 weeks, indicating that urea kinetics is a more active process at an early age, and slows with time. With respect to factors influencing urea kinetics, the apparently conflicting results which have appeared in the literature may be explained. The results may help explain the growth of breast-fed infants on low protein intakes.