The rate of nitrogen metabolism in the whole body of man measured with [15N]-glycine and uniformly labelled [15N]-wheat.

Seven fed volunteers were given either [15N]-glycine or uniformly labelled [15N]-wheat. The rate of nitrogen flux (turnover) in the whole body was estimated independently from the excretion of label in ammonia and urea over the following 12 h. The estimates obtained from urea took account of isotope retained within the body pool of urea at the end of the period of measurement. When [15N]-glycine was administered the rate of nitrogen flux based on the excretion of label in ammonia was, with one exception, less than that based on urea. In contrast, when 15N-labelled wheat was given the calculated rate of flux from ammonia was always higher than that from urea. Correlation of these differences was achieved by assuming a two-pool model for metabolic nitrogen in the body. The results suggest that, in the absorptive state at least, glycine is a satisfactory tracer for measuring rates of nitrogen metabolism.