The Effect of an Increase of Protein Intake on Whole-Body Protein Turnover in Elderly Women Is Tracer Dependent

Abstract

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) consuming two levels of protein (10 and 20% of total energy, diets A and B, respectively). Protein turnover was measured during 12 h of overnight fasting with 15N-glycine with urea and ammonia as end products. During the last 4 h of the interval, protein turnover was also estimated by L-[1-13C]-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 not differ significantly between the diet periods, although all subjects were in negative nitrogen balance at the end of diet A. Protein breakdown, as measured with 15N-glycine, did not differ from results obtained using L-[1-13C]-leucine, whereas protein synthesis was found to be significantly lower using the former isotope. The 15N-glycine method indicated that protein turnover (both synthesis and breakdown) was higher in fasting elderly women when they consumed a 20% rather than a 10% protein diet, whereas the L-[1-13C]-leucine method did not show significant differences between the diet periods in the last 4 h of the overnight fasting period. However, the relative increase in net protein breakdown when comparing diet B with diet A, was comparable for both tracers. These data indicate that care is needed with the choice of the tracer used in measuring the components of protein turnover in elderly women with the aim of understanding the physiological basis behind the adequacy of the level of protein intake.