1. Total body protein turnover was studied in six elderly patients.

2. During the study they were fed by continuous infusion of a liquid formula through a nasogastric tube. l-[1-14C]Leucine and [15N]glycine were infused at a constant rate for 30 h. The labelled glycine was infused into the intragastric line; the labelled leucine was given either by this route or intravenously.

3. The specific radioactivity of free leucine in plasma and the rate of output of 14CO2 in expired air both reached a plateau at 10 h, and remained constant until the end of the infusion at 30 h.

4. The 15N abundance in urinary urea and total N was very similar. In neither was a plateau reached by 30 h but in four out of the six patients the abundance in urinary NH4+ had attained a plateau by the end of the infusion.

5. Flux rates and rates of protein synthesis were calculated in four ways: (A) from the specific radioactivity of plasma leucine at plateau; (B) from the proportion of dose excreted as 14CO2 at plateau; (C) from the final rates of 15N excretion in urea or total urinary N; (D) from the final or plateau rates of 15N excretion in urinary NH4+.

6. On average, the estimates of synthesis rate obtained by methods B and C agreed closely; those given by methods A and D were lower. Methods A, B and D ranked the individual patients in almost identical order.

7. The comparison of methods makes it possible to examine the validity of the assumptions on which the different methods depend. Method B is probably theoretically the most satisfactory and, of the methods used in this work, probably gives the best estimate of the absolute turnover rate. The other methods can be used for comparative purposes.

8. The results suggest that the rate of protein turnover is reduced in the elderly, compared with younger subjects.

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