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Am J Clin Nutr. 2015 Apr 29. pii: ajcn084053. [Epub ahead of print]

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

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From the Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, Urbana, IL (DKL); the Department of Nutritional Sciences, Rutgers University, New Brunswick, NJ (TGA); the Department of Nutrition and Metabolism, Division of Rehabilitation Science, and Sealy Center on Aging, University of Texas Medical Branch, Galveston, TX (BBR); Arkansas Children's Nutrition Center and Department of Pediatrics, University of Arkansas for Medical Sciences, Little Rock, AR (SHA); the Department of Cellular and Molecular Physiology, the Pennsylvania State University College of Medicine, Hershey, PA (CJL); the Commonwealth Scientific and Industrial Research Organization-Food and Nutritional Sciences, Adelaide, Australia (GDB); and the USDA-Agricultural Research Service Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX (TAD).


Dietary protein provides essential amino acids (EAAs) for the synthesis of new proteins plus an array of other metabolic functions; many of these functions are sensitive to postprandial 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/mechanistic 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 that contains 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/kg body weight, but new research suggests daily needs for older adults of ≥1.0 g/kg and identifies anabolic and metabolic benefits to consuming at least 20-30 g protein at a given meal. Resistance exercise appears to increase the efficiency of EAA use for muscle anabolism and to lower the meal threshold for stimulation of protein synthesis. Applying this information to a typical 3-meal-a-day dietary plan results in protein intakes that are well within the guidelines of the Dietary Reference Intakes for acceptable macronutrient intakes. The meal threshold concept for dietary protein emphasizes a need for redistribution of dietary protein for optimum metabolic health.


leucine; mTOR; muscle protein synthesis; nitrogen balance; satiety

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