Because dietary intake should be known and urinary N excretion is measured, the rate of whole-body protein breakdown can be determined: B = Q - I, as well as the rate of whole-body synthesis: S = Q - C. In these calculations, the standard value of 6.25 g protein = 1 g N is used to interconvert protein and urinary N. Attention to the units (g of protein vs. g of N) is important, as both units are often used concurrently in the same report.

Occasionally in the literature a term called "net protein balance" or "net protein gain" appears in papers. Net protein balance is defined as the difference between the measured protein synthesis and breakdown rates (S - B), which can be determined from whole-body protein breakdown and synthesis measured as shown above. However, as can seen by rearranging the balance equation for Q above: S - B = I - C, which is simply the difference between intake and excretion, i.e., nitrogen balance. The S - B term is a misnomer, in that it is based solely upon the N balance measurement, not upon the administration of the 15N tracer.

There is no question that the end-product method of Picou and Taylor-Roberts is a cornerstone method for protein metabolic research in humans and is especially well suited for studies of infants and children because it is noninvasive, requiring only oral administration of tracer and collection only of urine. However, the end-product method is not without its problems; the most serious of which are mentioned below.

When the [15N]glycine tracer is given orally at short intervals (e.g., every 3 h) the time required to reach a plateau in urinary urea 15N is about 60 h regardless of whether adults (23, 68), children, or infants (69, 70) are studied. The delay in attaining a plateau is due to the time required for the 15N tracer to equilibrate within the free glycine, serine, and urea pools (23, 67). An additional problem is plateau definition. Often the urinary urea 15N time course does not show by either visual inspection or curve-fitting regression the anticipated single exponential rise to plateau. To avoid this problem, Waterlow et al. ( 71) suggested measuring the 15N in ammonia after a single dose of [15N]glycine. The advantage is that the 15N tracer passes through the body ammonia pool within 24 hours. Tracer administration and urine collection are greatly simplified, and the modification does not depend on defining a plateau in urinary urea 15N. The caveat here is the dependence of the single-dose end-product method upon ammonia metabolism. Urinary ammonia 15N enrichment usually differs from urinary urea 15N enrichment (72) because the amino-15N precursor for ammonia synthesis is of renal origin, while the amino-15N precursor for urea synthesis is of hepatic origin. Which enrichment should be used? Probably the urea 15N, but it is difficult to prove either way (42).

The primary difficulty with the end-product method is highlighted from a report in which several different 15N-labeled amino acid tracers, including 15N-glycine and some 15N-labeled proteins, were compared as tracers for the end-product method. Widely divergent results were determined for protein turnover (from 2.6 to 17.8 g/kg/day), depending upon the 15N label administered (73). The differences reflect differences in the metabolism and distribution of the 15N label when placed into different amino acids and illustrate how dependent the end-product approach is on the metabolism of the amino acid tracer. Therefore, it is difficult to determine whether a change in end-product 15N enrichment may be attributable either to a change in protein turnover or to a change in the distribution of 15N due to changes in tracer metabolism that may be independent of changes in protein metabolism. To make these distinctions, the kinetics of the amino acid tracer in the body must be measured as well.

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