Defining meal requirements for protein to optimize metabolic roles of amino acids

Authors: LAYMAN, DONALD - University Of Illinois; ANTHONY, TRACY - Rutgers University; RASMUSSEN, BLAKE - University Of Texas Medical Branch; Adams, Sean; LYNCH, CHRISTOPHER - Pennsylvania State University; BRINKWORTH, GRANT - Commonwealth Scientific And Industrial Research Organisation (CSIRO); Davis, Teresa

Submitted to: The American Journal of Clinical Nutrition
Publication Type: Review Article
Publication Acceptance Date: 6/6/2014
Publication Date: 4/29/2015
Citation: Layman, D.K., Anthony, T.G., Rasmussen, B.B., Adams, S.H., Lynch, C.J., Brinkworth, G.D., Davis, T.A. 2015. Defining meal requirements for protein to optimize metabolic roles of amino acids. American Journal of Clinical Nutrition. pii: ajcn084053.

Interpretive Summary: Dietary protein provides essential amino acids (EAA) for the synthesis of new proteins plus an array of other metabolic functions. Recent research has focused on amino acids as metabolic signals that influence the rate of protein synthesis, inflammation responses, mitochondrial activity and satiety. These signals represent meal-based responses to dietary protein. The best characterized of these signals is the leucine–induced activation of mTORC1 which leads to the stimulation of skeletal muscle protein synthesis after ingestion of a meal containing protein. The response of this metabolic pathway to dietary protein (i.e., meal threshold) declines with advancing age or reduced physical activity. Current dietary recommendations for protein are focused on total daily intake of 0.8 g per kilogram of body weight or ~ 15% of total daily energy, but new research suggests daily needs for older adults at or > 1.0 g/kg and identifies anabolic and metabolic benefits to consuming 20 g to 30 g of protein at a given meal. The protein intake to meet this level falls well below current estimates for safe upper limits for protein or EAA. Resistance exercise appears to increase the efficiency of EAA use for muscle anabolism and lower the meal threshold for stimulation of protein synthesis. The meal threshold concept for protein emphasizes a need for temporal redistribution of dietary protein for optimum metabolic health, which should be considered when formulating dietary guidelines.

Technical Abstract: Dietary protein provides essential amino acids (EAA) for the synthesis of new proteins plus an array of other metabolic functions; many of these functions are sensitive to post-prandial plasma and intracellular amino acid concentrations. Recent research has focused on amino acids as metabolic signals that influence the rate of protein synthesis, inflammation responses, mitochondrial activity and satiety, exerting their influence through signaling systems including mammalian target of rapamycin complex 1 (mTORC1), general control nonrepressed 2 (GCN2), glucagon-like peptide-1 (GLP-1), peptide YY (PYY), serotonin and insulin. These signals represent meal-based responses to dietary protein. The best characterized of these signals is the leucine–induced activation of mTORC1 which leads to the stimulation of skeletal muscle protein synthesis after ingestion of a meal containing protein. The response of this metabolic pathway to dietary protein (i.e., meal threshold) declines with advancing age or reduced physical activity. Current dietary recommendations for protein are focused on total daily intake of 0.8 g per kilogram of body weight or ~ 15% of total daily energy, but new research suggests daily needs for older adults at or > 1.0 g/kg and identifies anabolic and metabolic benefits to consuming 20 g to 30 g of protein at a given meal. Applying this information to a typical 3 meal a day dietary plan results in protein intakes that are well below current estimates for safe upper limits for protein or EAA. Resistance exercise appears to increase the efficiency of EAA use for muscle anabolism and lower the meal threshold for stimulation of protein synthesis. The meal threshold concept for protein emphasizes a need for temporal redistribution of dietary protein for optimum metabolic health.