Journal Title (Medline/Pubmed accepted abbreviation): Am J Clin Nutr
Page numbers: 322-331
doi (if applicable): 10.3945/ajcn.2010.29649
Summary of Background and Research Design
Background: Sarcopenia, or age-related loss of skeletal muscle mass, may result from impaired protein digestion and absorption or a blunted muscle protein synthesis response (ie, anabolic resistance). Little evidence exists in support of either mechanism.
Hypothesis/purpose of study: The study compared dietary protein digestion and absorption kinetics and subsequent muscle protein synthesis at rest and after exercise in young vs elderly men.Subjects:
The study included 24 nonobese elderly men age 74 ± 1 years and 24 body weight-matched young men age 21± 1 years who did not participate in regular exercise. None of the subjects had diabetes, glucose intolerance, or history of heart failure.
Experimental design: Single-institution, randomized study.
Treatments and protocol: Individuals within each group were randomized to rest or exercise. A standardized meal was ingested the evening before the testing. On the morning of the test, all participants received an amino acid tracer and rested for 90 minutes followed by an additional 30 minutes of rest or standardized exercise (5 min low-intensity cycling, 6 sets of 10 repetitions leg press and leg extension). Participants then received a 250 mL drink containing 20 grams of 13C-phenylalanine–labeled casein protein. Blood samples for amino acid levels were collected before tracer and drink consumption and approximately every 15 to 30 minutes for 6 hours after drink consumption (postprandial). Muscle biopsies were collected before the drink was consumed and at 6 hours after the drink.
Summary of research findings:
- In general, plasma amino acid concentrations increased from before drink ingestion and remained elevated throughout the 6 hours postprandial in all activity and age groups.
- Regardless of activity, phenylalanine levels were higher in the elderly than in the young (P < .05).
- After exercise, leucine levels were higher in the elderly compared with the young (P < .05).
- Regardless of activity, rates of tracer appearance and disappearance were lower in the elderly compared with the young (P < .05).
- Appearance and disappearance rates of ingested 13C-phenylalanine were similar between activity and age groups.
- Protein-bound enrichments for labeled phenylalanine and muscle protein synthesis rates were higher after exercise compared with rest, with similar magnitudes of elevation in each age group.
Interpretation of findings/Key practice applications:
There was no difference in dietary protein digestion and absorption between young and elderly men at rest or after exercise. However, muscle protein synthesis rates, although similar between age groups, were elevated to the same degree by exercise. It is possible that the increased levels of postprandial amino acids in the elderly may represent a compensatory mechanism to maintain muscle protein synthesis rates and an early stage of anabolic resistance. Physical activity appears to be an important tool to improve muscle protein synthesis and support aging muscles against sarcopenia.