Quality of meal protein determines anabolic response in older adults.

BACKGROUND & AIMS: It has been demonstrated that the relative content and profile of essential amino acids (EAA) play a determining role for stimulation of muscle protein synthesis (MPS) following intake of pure EAA or protein alone.

METHODS: To test if this also holds in the context of mixed meals at both whole body and muscle levels, twelve older subjects (57-74 yrs) received primed continuous infusion of L-[ring-2 H5 ]phenylalanine and L-[ring-2 H2 ]tyrosine over a 9-h experimental period to determine whole body protein kinetics and MPS in the fasted state and following consumption of egg-based (EGG) or cereal-based (CEREAL) isocaloric and isonitrogenous breakfast. A standardized lunch, primarily consisting of beef protein was also consumed by each group. Whole body protein kinetics [protein synthesis (PS), breakdown (PB), and net balance (NB)] were expressed as changes from basal fasted period.

RESULTS: We found that EGG breakfast resulted in a greater NB through a greater suppression of PB compared with the CEREAL breakfast. The greater NB during the post-breakfast period with the EGG was normalized following the standard lunch despite the sustained elevations in plasma EAA concentrations. However, the EGG breakfast stimulated both PS and PB compared with the CEREAL breakfast during the post-lunch period. MPS was not different between meals despite larges differences in the plasma EAA responses.

CONCLUSIONS: We conclude that in the context of mixed meals, quality of protein affects NB through changes in protein breakdown and affects protein turnover following subsequent meal intake.