The precision of measuring the rate of whole-body nitrogen flux and protein synthesis in man with a single dose of [15N]-glycine.

The rates of nitrogen flux and protein synthesis in the whole body were measured in two fed volunteers on at least five occasions over a period of 3-4 years. Each time the experimental protocol and the amount of energy and protein consumed by the subjects were directly comparable. Paired measurements, separated by a period of 6 months or more, were also made in five other volunteers. Rates of flux and synthesis were estimated independently from 15N excretion in urinary ammonia and total urea (excreted plus retained within the body) during a 9-h period after administration of [15N]-glycine. The results obtained for the first two subjects indicate that the overall precision of measuring nitrogen flux by this method is between 5 and 11 per cent (coefficient of variation) when based on ammonia or urea alone and between 3 and 6 per cent when based on the arithmetic or harmonic average of the rates given by these two end products. For synthesis the variation was slightly larger - up to 15 per cent for ammonia or urea and between 5 and 7 per cent for the two end-product averages.