title: Co-ingesting milk fat with micellar casein does not affect postprandial protein handling in healthy older men.
abstract: 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/).
authors.affiliation: NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht University Medical Centre+, Maastricht, The Netherlands; Top Institute Food & Nutrition (TIFN), Wageningen, The Netherlands.
authors.forename: Stefan H M
authors.initials: SHM
authors.lastname: Gorissen
authors.affiliation: NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht University Medical Centre+, Maastricht, The Netherlands; Top Institute Food & Nutrition (TIFN), Wageningen, The Netherlands.
authors.forename: Nicholas A
authors.initials: NA
authors.lastname: Burd
authors.affiliation: NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht University Medical Centre+, Maastricht, The Netherlands; Top Institute Food & Nutrition (TIFN), Wageningen, The Netherlands.
authors.forename: Irene Fleur
authors.initials: IF
authors.lastname: Kramer
authors.affiliation: NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht University Medical Centre+, Maastricht, The Netherlands; Top Institute Food & Nutrition (TIFN), Wageningen, The Netherlands.
authors.forename: Janneau
authors.initials: J
authors.lastname: van Kranenburg
authors.affiliation: NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht University Medical Centre+, Maastricht, The Netherlands.
authors.forename: Annemie P
authors.initials: AP
authors.lastname: Gijsen
authors.affiliation: Department of Anesthesia and Intensive Care, Karolinska University Hospital, Huddinge, Sweden.
authors.forename: Olav
authors.initials: O
authors.lastname: Rooyackers
authors.affiliation: NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht University Medical Centre+, Maastricht, The Netherlands; Top Institute Food & Nutrition (TIFN), Wageningen, The Netherlands. Electronic address: L.vanLoon@maastrichtuniversity.nl.
authors.forename: Luc J C
authors.initials: LJC
authors.lastname: van Loon
mesh_terms: D000368:Aged; D000818:Animals; D001786:Blood Glucose / Q000378:metabolism; D015992:Body Mass Index; D002364:Caseins / Q000008:administration & dosage* / Q000493:pharmacokinetics; D004032:Diet; D004044:Dietary Proteins / Q000008:administration & dosage / Q000493:pharmacokinetics; D015444:Exercise; D006017:Glycolipids / Q000008:administration & dosage* / Q000493:pharmacokinetics; D006023:Glycoproteins / Q000008:administration & dosage* / Q000493:pharmacokinetics; D006801:Humans; D007930:Leucine / Q000097:blood; D066292:Lipid Droplets; D008297:Male; D008823:Micelles; D008875:Middle Aged; D008892:Milk / Q000737:chemistry; D009124:Muscle Proteins / Q000096:biosynthesis; D018482:Muscle, Skeletal / Q000378:metabolism; D010649:Phenylalanine / Q000097:blood; D019518:Postprandial Period; D014176:Protein Biosynthesis
chemical_list: D001786:Blood Glucose; D002364:Caseins; D004044:Dietary Proteins; D006017:Glycolipids; D006023:Glycoproteins; D008823:Micelles; D009124:Muscle Proteins; C110367:milk fat globule; D010649:Phenylalanine; D007930:Leucine
keywords: Casein; Fat; Leucine; Muscle protein synthesis; Sarcopenia