Understanding the complex relationship between amino acid absorption kinetics and postprandial muscle protein synthesis rates in healthy adults and critically ill patients

Abstract

Purpose of review: protein digestion and amino acid absorption kinetics are quantifiable metrics commonly utilized to determine the quality of a protein source. This review critically evaluates recent evidence (primarily from studies that provided commonly consumed protein-rich foods) regarding the relationship between in vivo protein digestion and amino acid absorption rates with the postprandial stimulation of muscle protein synthesis (MPS), with an emphasis on healthy adults and critically ill patients.

Recent findings: ingested protein sources that elicit moderate amino acid bioavailability, including leucine, stimulate MPS rates to a comparable extent as protein sources that elicit high amino acid bioavailability in healthy young adults. Amino acid absorption kinetics appear to be modulated in critically ill patients, leading to a marked reduction in postprandial MPS rates. Preliminary studies demonstrate that enteral feeding of high dose free amino acids increase amino acid bioavailability to a greater extent than intact protein, leading to a positive whole-body net protein balance in critically ill patients. However, in practice, the high osmolarity of free amino acids leads to a high prevalence of diarrhoea and thus limits the clinical application of this intervention.

Summary: the enteral provision of free amino acids represents a theoretical, but not practically-relevant, clinical nutrition strategy to mitigate the catabolic response to critical illness. Future studies are warranted to establish targeted protein/amino acid-based interventions to mitigate skeletal muscle atrophy during the metabolic care of critically ill patients.

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