Co-ingesting milk fat with micellar casein does not affect postprandial protein handling in healthy older men

Background & aim: Dietary protein digestion and absorption plays an important role in modulating postprandial muscle protein synthesis. The impact of co-ingesting other macronutrients with dietary protein on protein digestion and absorption and the subsequent muscle protein synthetic response remains largely unexplored. This study investigated the impact of co-ingesting milk fat with micellar casein on dietary protein-derived amino acid appearance in the circulation and the subsequent postprandial muscle protein synthetic response in healthy older men. Methods: Twenty-four healthy, older males ( age: 65 ± 1 y, BMI: 25.7 ± 0.5 kg/m2 ) received a primed continuous infusion of L-[ring-2H5]-phenylalanine and L-[1–13C]-leucine and ingested 20 g intrinsically L-[1–13C]-phenylalanine and L-[1–13C]-leucine-labeled casein with ( PRO + FAT; n = 12 ) or without ( PRO; n = 12 ) 26.7 g milk fat. Plasma samples and muscle biopsies were collected in both the postabsorptive and postprandial state. Results: Release of dietary protein-derived phenylalanine into the circulation increased following protein ingestion ( P < 0.001 ) and tended to be higher in PRO compared with PRO + FAT ( Time × Treatment P = 0.076 ). No differences were observed in dietary protein-derived plasma phenylalanine availability ( 52 ± 2 vs 52 ± 3% in PRO vs PRO + FAT, respectively; P = 0.868 ). Myofibrillar protein synthesis rates did not differ between treatments, calculated using either the L-[ring-2H5]-phenylalanine ( 0.036 ± 0.003 vs 0.036 ± 0.004 %/h after PRO vs PRO + FAT, respectively; P = 0.933 ) or L-[1–13C]-leucine ( 0.051 ± 0.004 vs 0.046 ± 0.004 %/h, respectively; P = 0.480 ) tracer. In accordance, no differences were observed in myofibrillar protein-bound L-[1–13C]-phenylalanine enrichments between treatments ( 0.018 ± 0.002 vs 0.014 ± 0.001 MPE, respectively; P = 0.173 ). Conclusion: Co-ingesting milk fat with micellar casein does not impair protein-derived phenylalanine appearance in the circulation and does not modulate postprandial myofibrillar protein synthesis rates. Clinical Trial Registration Number: NCT01680146 ( http://www.clinicaltrials.gov/ )