15N tracer investigations of the physiological availability of urea nitrogen in mother's milk.

The physiological availability of urea in mother's milk was investigated in tracer studies using [15N]2 urea and involving 22 infants. The incorporation of 15N into the body protein was established in 16 subjects by emission spectrophotometrical determination of the 15N excess in the serum protein. Between 2% and 3.6% of the serum protein is synthesized from the urea nitrogen in mother's milk. In further studies on the 15N balance in 6 infants, renal excretion of 15N after oral multiple and single impulse labelling with [15N]2 urea constituted 60% of the dose administered. Three-tenths-2.5% was excreted in the feces. The retention of 15N in the protein pool varied between 16.7 and 61.4%.