Ingestion of Casein in a Milk Matrix Modulates Dietary Protein Digestion and Absorption Kinetics but Does Not Modulate Postprandial Muscle Protein Synthesis in Older Men

BACKGROUND: The slow digestion and amino acid absorption kinetics of isolated micellar casein have been held responsible for its relatively lower postprandial muscle protein synthetic response compared with rapidly digested proteins such as isolated whey. However, casein is normally consumed within a milk matrix. We hypothesized that protein digestion and absorption kinetics and the subsequent muscle protein synthetic response after micellar casein ingestion are modulated by the milk matrix. OBJECTIVE: The aim of this study was to determine the impact of the milk matrix on casein protein digestion and absorption kinetics and postprandial muscle protein synthesis in older men. METHODS: In a parallel-group design, 32 healthy older men (aged 71 +/- 1 y) received a primed continuous infusion of L-[ring-2H5]-phenylalanine, L-[ring-3,5-2H2]-tyrosine, and L-[1-13C]-leucine, and ingested 25 g intrinsically L-[1-13C]-phenylalanine and L-[1-13C]-leucine labeled casein dissolved in bovine milk serum (Cas+Serum) or water (Cas). Plasma samples and muscle biopsies were collected in the postabsorptive state and for 300 min in the postprandial period to examine whole-body and skeletal muscle protein metabolism. RESULTS: Casein ingestion increased plasma leucine and phenylalanine concentrations and L-[1-13C]-phenylalanine enrichments, with a more rapid rise after Cas vs. Cas+Serum. Nonetheless, dietary protein-derived phenylalanine availability did not differ between Cas+Serum (47 +/- 2% mean +/- SEM) and Cas (46 +/- 3%) when assessed over the 300-min postprandial period (P = 0.80). The milk matrix did not modulate postprandial myofibrillar protein synthesis rates from 0 to 120 min (0.038 +/- 0.005 vs. 0.031 +/- 0.007%/h) or from 120 to 300 min (0.052 +/- 0.004 vs. 0.067 +/- 0.005%/h) after Cas+Serum vs. Cas. Similarly, no treatment differences in muscle protein-bound L-[1-13C]-phenylalanine enrichments were observed at 120 min (0.003 +/- 0.001 vs. 0.002 +/- 0.001) or 300 min (0.015 +/- 0.002 vs. 0.016 +/- 0.002 mole percent excess) after Cas+Serum vs. Cas. CONCLUSIONS: Casein ingestion in a milk matrix delays protein digestion and absorption but does not modulate postprandial muscle protein synthesis when compared to the ingestion of micellar casein only in healthy older men. This trial was registered at Nederlands Trial Register as NTR4429