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

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.

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.

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).

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.

NCT01680146 (http://www.clinicaltrials.gov/).