The non-invasive measurement of urea kinetics in normal man by a constant infusion of 15N15N-urea.

A two-pool model is described for the non-invasive measurement of urea kinetics in man. The isotope, 15N15N-urea, was given until an isotopic steady state was reached in urine and the time taken to achieve this is defined. During an isotopic steady state, a comparison was made of the effect of giving the isotope orally, intravenously and intragastrically; no differences were found between the different routes. Measurements of enrichment were made on excretion products in urine. In six normal adults with a protein intake of 200 mg N/kg/d, the urea production rate was 139 +/- 15 mg N/kg/d, 70 per cent of which was excreted in urine. Of the 34 mg N/kg/d produced by hydrolysis of urea in the gastrointestinal tract, 41 per cent was resynthesized to urea, and about 48 per cent was available for other synthetic processes.