Free amino acids are more readily available than intact protein but do not induce a differential effect on muscle protein synthesis rates

Muscle protein synthesis is dependent upon an adequate supply of all proteinogenic amino acids, which are normally supplied by food proteins.  Some supplements and fortified foods contain crystalline free amino acids, yet the relative efficiency of those free amino acids, compared to intact proteins, to support protein synthesis is not known.  Weijzen and colleagues addressed this void in our understanding and published their observations in the January 2022 issue of The Journal of Nutrition.

Protein digestion, amino acid absorption kinetics, and postprandial muscle protein synthesis were determined in 24 young (average 22 years with BMI of 23 kg/m²) participants (12 male and 12 female).  The subjects received a primed continuous infusion of L-[ring-²H₅]-phenylalanine and L-[ring-3,5-²H₂]-tyrosine before they consumed 30 g of intrinsically L-[1-¹³C]-phenylalanine-labeled milk protein or an equivalent amount of L-[1-¹³C]-phenylalanine-labeled free amino acids.  During the 6-hour postprandial period, blood samples and muscle biopsies were collected.

Plasma amino acid concentrations increased after consumption of the intact milk protein or free amino acids, but the increase was greater with ingestion of free amino acids.  Appearance of ingested phenylalanine into plasma was also greater with free amino acids.  Muscle protein synthesis rates were increased with both free amino acids and with milk protein, but there was no difference between the two sources.  The authors concluded that free amino acid consumption leads to more rapid absorption and greater plasma amino acid availability, and thus may be beneficial in conditions where protein digestion or amino acid absorption may be compromised.

In an editorial, Tome noted the large (30 g) bolus of protein or amino acids may have maximized muscle protein synthesis rates, and may have obscured any differences in muscle protein synthesis that would have occurred if smaller amounts were consumed.  Tome indicated the results of Weijzen supports the need to include the rate of protein digestion and amino acid absorption along with amino acid digestibility and the profile of indispensable amino acids when assessing dietary protein.  Tome concluded that there may be situations when supplementation with crystalline free amino acids may be of benefit and the utility of this approach requires further research.

References

Michelle E G Weijzen, Rob J J van Gassel, Imre W K Kouw, Jorn Trommelen, Stefan H M Gorissen, Janneau van Kranenburg, Joy P B Goessens, Marcel C G van de Poll, Lex B Verdijk, Luc J C van Loon, Ingestion of Free Amino Acids Compared with an Equivalent Amount of Intact Protein Results in More Rapid Amino Acid Absorption and Greater Postprandial Plasma Amino Acid Availability Without Affecting Muscle Protein Synthesis Rates in Young Adults in a Double-Blind Randomized Trial, The Journal of Nutrition, Volume 152, Issue 1, January 2022, Pages 59–67, https://doi.org/10.1093/jn/nxab305.

Daniel Tome, Efficiency of Free Amino Acids in Supporting Muscle Protein Synthesis, The Journal of Nutrition, Volume 152, Issue 1, January 2022, Pages 3–4, https://doi.org/10.1093/jn/nxab370.

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