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1 Departments of Surgery and Shriners Hospitals for Children-Galveston, University of Texas Medical Branch, Galveston, Texas, USA
2 AminoScience Laboratories, Ajinomoto Co, Inc, Kawasaki, Japan
3 Department of Internal Medicine, University of Texas Medical Branch, Galveston, Texas, USA
4 Department of Anesthesiology, University of Texas Medical Branch, Galveston, Texas, USA
* To whom correspondence should be addressed. E-mail: cskatsan{at}utmb.edu.
This study was designed to evaluate the effects of enriching an essential amino acid (EAA) mixture with leucine on muscle protein metabolism in elderly and young individuals. Four (two elderly and two young) groups were studied before and following ingestion of 6.7 g of EAA. EAA were based on the composition of whey protein (26% Leu) or were enriched in leucine (41% Leu). A primed, continuous infusion of L-[ring-2H5]phenylalanine was used together with vastus lateralis muscle biopsies and leg arteriovenous blood samples for the determinations of fractional synthetic rate (FSR) and balance of muscle protein. FSR increased following amino acid ingestion in both the 26% (basal: 0.048 ± 0.005%.h -1; post-EAA: 0.063 ± 0.007%.h -1) and the 41% (basal: 0.036 ± 0.004%.h -1; post-EAA: 0.051 ± 0.007%.h -1) Leu young groups (P < 0.05). In contrast, in elderly, FSR did not increase following ingestion of 26% Leu EAA (basal: 0.044 ± 0.003%.h -1; post-EAA: 0.049 ± 0.006%.h -1; P > 0.05), but increased following ingestion of 41% Leu EAA (basal: 0.038 ± 0.007%.h -1; post-EAA: 0.056 ± 0.008%.h -1; P < 0.05). Similar to the FSR responses, the mean response of muscle phenylalanine net balance, a reflection of muscle protein balance, was improved (P < 0.05) in all groups with the exception of the 26% Leu elderly group. We conclude that increasing the proportion of leucine in a mixture of EAA can reverse an attenuated response of muscle protein synthesis in elderly but does not result in further stimulation of muscle protein synthesis in young subjects.
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