A NONINVASIVE METHOD FOR MEASURING UREA KINETICS WITH A SINGLE-DOSE OF [N-15N-15]UREA IN FREE-LIVING HUMANS

A noninvasive method is described in which the endogenous rate of urea production can be determined in normal, free-living adults. A single dose of [N-15N-15]urea was given orally, and the amount of label excreted as [N-15N-15]urea and [N-15N-14]urea in urine over the subsequent 48 h was measured. From the rates of excretion of labeled and unlabeled urea the rate of urea production was derived. Using this single-dose protocol the rate of urea production was 207 +/- 56 (mean +/- SD) mg N/(kg.d) in six normal adult men consuming 74 g protein/d. These results were not different when compared with rates of urea production obtained with a prime/intermittent protocol in an earlier study in the same individuals 1199 +/- 20 mg N/(kg.d)]. We conclude that urea kinetics can be measured noninvasively with a single dose of [N-15N-15]urea and that this method may be suitable for use in free-living individuals to determine urea production rates for habitual dietary intakes.