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

• Published in: The Journal of nutrition 2022-01, Vol.152 (1), p.59-67
• Main Authors: Weijzen, Michelle E G, van Gassel, Rob J J, Kouw, Imre W K, Trommelen, Jorn, Gorissen, Stefan H M, van Kranenburg, Janneau, Goessens, Joy P B, van de Poll, Marcel C G, Verdijk, Lex B, van Loon, Luc J C
• Format: Article
• Language: English
• Subjects: Absorption; Adult; Amino acids; Amino Acids - metabolism; Availability; Biopsy; Dietary Proteins; Digestion; Double-blind studies; Eating; Editor's Choice; Equivalence; Female; Humans; Ingestion; Kinetics; Male; Milk; Muscle Proteins - metabolism; Muscle, Skeletal - metabolism; Muscles; Muscular system; Nuclear Techniques in Nutrition Research; Nutrition; Phenylalanine; Postprandial Period; Protein biosynthesis; Protein synthesis; Proteins; skeletal muscle; splanchnic extraction; stable isotopes; Tyrosine; Young Adult; Young adults
• ISSN: 0022-3166; 1541-6100
• DOI: 10.1093/jn/nxab305

The rate of protein digestion and amino acid absorption determines the postprandial rise in circulating amino acids and modulates postprandial muscle protein synthesis rates. We sought to compare protein digestion, amino acid absorption kinetics, and the postprandial muscle protein synthetic response following ingestion of intact milk protein or an equivalent amount of free amino acids. Twenty-four healthy, young participants (mean ± SD age: 22 ± 3 y and BMI 23 ± 2 kg/m2; sex: 12 male and 12 female participants) received a primed continuous infusion of l-[ring-2H5]-phenylalanine and l-[ring-3,5–2H2]-tyrosine, after which they ingested either 30 g intrinsically l-[1–13C]-phenylalanine–labeled milk protein or an equivalent amount of free amino acids labeled with l-[1–13C]-phenylalanine. Blood samples and muscle biopsies were obtained to assess protein digestion and amino acid absorption kinetics (secondary outcome), whole-body protein net balance (secondary outcome), and mixed muscle protein synthesis rates (primary outcome) throughout the 6-h postprandial period. Postprandial plasma amino acid concentrations increased after ingestion of intact milk protein and free amino acids (both P