THE EFFECT OF AN INCREASE OF PROTEIN-INTAKE ON WHOLE-BODY PROTEIN-TURNOVER IN ELDERLY WOMEN IS TRACER DEPENDENT

To compare the response of whole-body protein turnover with variations in dietary protein level, whole-body protein turnover was measured by different stable isotope methods in six elderly women (69 +/- 5 y) co nsuming two levels of protein (10 and 20% of total energy, diets A and B, respectively). Protein turnover was measured during 12 h of overni ght fasting with N-15-glycine with urea and ammonia as end products. D uring the last 4 h of the interval, protein turnover was also estimate d by L-[1-C-13]-leucine infusion. Nitrogen balance [diet A, -0.040 +/- 0.015 g/(kg.d); diet B, 0.002 +/- 0.053 g/(kg.d); mean +/- SD] did no t differ significantly between the diet periods, although all subjects were in negative nitrogen balance at the end of diet A, Protein break down, as measured with N-15-glycine, did not differ from results obtai ned using L-[1-C-13]-leucine, whereas protein synthesis was found to b e significantly lower using the former isotope. The N-15-glycine metho d indicated that protein turnover (both synthesis and breakdown) was h igher in fasting elderly women when they consumed a 20% rather than a 10% protein diet, whereas the L-[1-C-13]-leucine method did not show s ignificant differences between the diet periods in the last 4 h of the overnight fasting period. However, the relative increase in net prote in breakdown when comparing diet B with diet A, was comparable for bot h tracers, These data indicate that care is needed with the choice of the tracer used in measuring the components of protein turnover in eld erly women with the aim of understanding the physiological basis behin d the adequacy of the level of protein intake.