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/)