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Optimizing Adult Protein Intake During Catabolic Health Conditions

I read this study this morning and found it very informative. Thanks to Dr. Gabrielle Lyon for sharing.

Things that stood out for me:

"The RDA for protein has been previously reviewed and found to be inadequate for older persons (2). A presumed reason for the inadequacy of the protein RDA for older persons is a phenomenon known as the “anabolic resistance” of skeletal muscle. Anabolic resistance is the phenomenon within skeletal muscles of older persons when there is an attenuated response of muscle protein synthesis (MPS) to resistance exercise (3) and ingestion of protein (4, 5). Protein synthesis is a critical component of the natural turnover of proteins required for continuous repair and remodeling of skeletal muscle to maintain strength and functional mobility.

The age-related reduction in the sensitivity of MPS to exercise or protein can be overcome with greater volumes of resistance exercise (6) or greater doses of protein (4); however, greater exercise volumes may be impractical for many older persons. There is also no doubt that a reduction in either habitual physical activity or muscle disuse also bring about a state of anabolic resistance (7, 8). Thus, inactivity or disuse as part of aging per se may be a predominant reason for older persons requiring more protein."

"In skeletal muscle, the essential amino acid leucine provides a unique translation signal. Indeed, the leucine content of a meal is an important determinant of the potential of a meal to support the complex process of protein synthesis. Specifically, leucine stimulates the mTORC1 (mechanistic target of rapamycin) signal cascade resulting in assembly of the eIF4F (initiation factor 4F) initiation complex and activation of the ribosomal protein S6. The eIF4F complex accelerates the assembly of ribosomes on available mRNAs and the S6 protein allows ribosomes to target mRNAs that enhance the overall capacity for protein synthesis. These signals are downregulated during short-term catabolic conditions such as an overnight fast, acute bed rest, or exhaustive exercise. When these signals are downregulated, the composition of the next meal is critical to optimize the anabolic recovery. This stimulation and regulation of muscle protein anabolism becomes increasingly important with advancing age as muscle becomes less sensitive to routine anabolic signals from hormones (i.e., insulin and IGF-1: insulin-like growth factor-1), protein ingestion, and physical activity (4, 17)."

"Thus, it may be, based on an increased dietary need for leucine to overcome the anabolic resistance of aging (3437), that the leucine content of proteins becomes an increasingly important component of protein quality in older persons."

"In terms of defining what constitutes “healthy” or “unhealthy,” we posit that any individual experiencing anabolic resistance (via any mechanism) or increased catabolic burden, warrants a dietary intervention that includes an appropriately greater absolute and relative amount of high-quality dietary protein (98)."

"Dietary protein and amino acid metabolism may be leveraged to optimize glycemic regulations.

TABLE 1 Metabolic regulation with diets high in glucose versus amino acids

High-carbohydrate, low-protein diet (>50% of energy from carbohydrates; 0.8 g/kg protein) 

  • Produces rapid postmeal increases in blood glucose

  • Postmeal hyperglycemia must be eliminated within 2 h

  • Rapid increase in insulin; biphasic response from the pancreas

  • Increased recycling of glucose via lactic acid to gluconeogenesis (Cori cycle)

  • Inhibition of fatty acid oxidation in skeletal muscle

Moderate carbohydrate, higher protein (25 to 40% of energy from carbohydrates; 1.6 g/kg protein) 

  • Free amino acids have slow postmeal metabolism; >5 h

  • Amino acids slowly produce glucose via gluconeogenesis

  • Amino acids stimulate Phase I insulin only

  • Amino acids recycle glucose via alanine and not lactic acid

  • Stimulation of fatty acid oxidation in skeletal muscle

  • Stimulation of muscle protein synthesis

"In young, physically active, normal weight adults with normal insulin sensitivity, glucose homeostasis can be achieved across a wide range of dietary carbohydrate and protein intakes. However, in aging adults, as muscle mass, physical activity, and insulin sensitivity decrease, and glucose tolerance declines, the potential use of diets with higher protein and reduced carbohydrates needs to be more fully studied.

Three aspects of metabolic regulation serve to highlight critical differences arising from shifting the balance between dietary carbohydrates and proteins. The first is postmeal utilization of the metabolic substrates (i.e., glucose versus amino acids), the second is the insulinogenic response of the pancreas, and the third is the regulatory response in skeletal muscle."

"Emerging evidence reveals the optimal protein intake is more than simply a percentage of daily energy but a meal-to-meal decision about protein quantity and quality. Factors including increasing age and declining physical activity reduce the efficiency of protein turnover especially in skeletal muscle resulting in reduced mass, strength, and metabolic regulation. The reduced efficiency, characterized as anabolic resistance, can be overcome, at least in part, by increasing protein quantity and quality at individual meals."

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