The 'single-dose end-product' approach for the measurement of protein turnover is the only method suited for application to free-living individuals and for field studies. However, the method has never been widely accepted because the results obtained appear to vary from one situation to another. There is the need for a formal comparison of the different approaches that have been used and the assumptions on which they are based in order to be able to understand the extent to which the variation in the results obtained is real or might be attributed to methodological differences. The present study used the 'primelintermittent-dose end-product' approach over 18 h as a reference with which to compare the 'singledose end-product' approach, with urine being collected for periods of 9 to 48 h. N flux was derived in a total of thirteen normal men using oral ['%]glycine and measurement of l3N enrichment in urinary NH, or urea, with isotope being given either as prime/intermittent doses or a single dose in separate studies. The pattern of results was similar to those reported in the literature. In all studies the rates of N flux derived from urea were higher than those derived from NH,, under equivalent conditions, by about 2530%. The single-dose approach, with urine collection over 9 h, gave results which were consisteutly higher than the prime/intermittentdose approach. The extent of the difference was influenced by the duration of time over which the cumulative excretion of isotope in urine was determined, and with NH, as the end-product the most consistent estimates of N flux could be obtained with a urine collection of at least 12 h and no greater than 24 h. With urea as the end-product, correction for the label retained in the body pool at 9 h gave similar results for N flux to those derived from the total excretion of label in urea over 24 h. The derivation of values for protein synthesis and protein degradation from measures of flux requires care to ensure that the time periods over which N intake and excretion are measured accord with those for which the measurement of flux apply. It is concluded that measurements of protein turnover similar to those obtained with the prime/intermittent-dose approach can be obtained with the single-dose approach in the fed state during the daytime, either from the excretion of label in urinary NH, over a period of 12 h or with the excretion of label in urinary urea over a period of 24 h. The suitability of the method for use in the fasted state or at night time remains to be determined. Urea: Ammonia: Nitrogen fluxThe measurement of protein turnover has provided invaluable information on the dynamic changes in the relative rates of protein synthesis and degradation which take place in a range of physiological or pathological states (Waterlow et al. 1978a;Waterlow, 1984). For studies in adults there has been a tendency to use the relatively invasive precursor methods with the intravenous administration of isotope, either as a prime/continuous infusion or as a flooding dose of isot...
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