N end-product methods for the study of whole body protein turnover
Proceedings of the Nutrition Society
and Tropical Medicine This workshop is concerned with the use of stable isotopes in clinical nutrition, and my job is to discuss methods in which the tracer is given as an amino acid labelled with I5N and turnover rates are determined from measurements of 15N in urinary urea or ammonia. I will deal with the subject under three headings: practical advantages, theoretical problems and results. Practical advantages of end-product methods The main advantage is that, since measurements are made on
... ments are made on urine, the methods are not invasive. However, if we are using urea as end-product and want to make the measurements over a fairly short period of time, because of the slow turnover rate of the urea pool, it is necessary to take at least one blood sample to determine the isotope abundance in the urea which has not yet been excreted (see Fern, Garlick, McNurlan & Powell-Tuck, 1981) . With urea or ammonia as end-product there is no problem about having enough N for measurement, nor about measuring the lSN abundance with sufficient accuracy. With the extremely primitive single-collector instrument which we use, the SD is of the order of 0.2-10/0, depending on the degree of enrichment. This is adequate for most purposes. The main advantage of a more accurate instrument is that the dose of tracer can be reduced by an order of magnitude. This will save money. More important, it means that the amount of labelled amino acid given will be more truly a tracer dose. In our work we use doses of 100-200 mg [15Nlglycine. If an oral dose is assumed to be absorbed over a period of 10 min, during this time it will double or treble the glycine flux. However, the duration of this perturbation is short compared with the total time of the test (usually 9 h). With ammonia as end-product it is possible to repeat measurements after 2 d or so, because the residual labelling is very low (Waterlow, Golden et al. 1978). This is obviously a great advantage in clinical studies.