Ingestion of Free Amino Acids as Opposed to Intact Protein Increases Amino Acid Absorption but Does Not Further Augment Postprandial Muscle Protein Synthesis Rates

Abstract

Objectives: The rate of protein digestion and amino acid absorption determines the postprandial rise in circulating amino acids and, as such, modulates postprandial muscle protein synthesis rates. This study compares protein digestion and amino acid absorption kinetics and the subsequent muscle protein synthetic response following ingestion of intact protein versus an equivalent amount of free, crystalline amino acids. Methods: Twenty-four healthy, young subjects (age: 22 ± 3 y, BMI: 23 ± 2 kg·m −2 , sex: 12 M/12F) ingested 30 g intrinsically L-[1-13 C]-phenylalanine and L-[1-13 C]-leucine labeled milk protein (PROT; n = 12) or an equivalent amount of free amino acids (AA; n = 12). In addition, subjects received primed continuous L-[ring-2 H 5 ]-phenylalanine, L-[ring-3,5-2 H 2 ]-tyrosine, and L-[1-13 C]-leucine infusions. Blood samples and muscle biopsies were obtained frequently to assess protein digestion and amino acid absorption kinetics and subsequent muscle protein synthesis rates over a 6 h postprandial period. An unpaired t-test was used to compare overall exogenous phenylalanine release in plasma. For other parameters repeated measures ANOVA were applied to determine differences between groups over time (time as within, and group as between-subjects factor). Data are expressed as mean ± SD. Results: Postprandial plasma amino acid concentrations and exoge-nous phenylalanine appearance rates increased after ingestion of PROT and AA (both, P < 0.001), with a greater increase following ingestion of AA when compared to PROT (time * group interaction P < 0.001). Exogenous phenylalanine release in plasma assessed over the 6 h postprandial period, was greater in AA (76 ± 9%) compared with PROT (59 ± 10%; P < 0.001). Ingestion of AA and PROT strongly increased muscle protein synthesis rates based upon L-[ring-2 H 5 ]-phenylalanine (time effect P < 0.001), with no differences between groups (from 0.037 ± 0.015 to 0.053 ± 0.014%·h −1 and from 0.039 ± 0.016 to 0.051 ± 0.010%·h −1 , respectively; time * group interaction P = 0.629). Conclusions: Ingestion of free amino acids as opposed to intact milk protein is followed by more rapid amino acid absorption and greater postprandial plasma amino acid availability, but this does not further augment postprandial muscle protein synthesis rates.