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

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 exogenous phenylalanine appearance rates increased after ingestion of PROT and AA (both, P