title: Protein ingestion acutely inhibits insulin-stimulated muscle carnitine uptake in healthy young men.
abstract: BACKGROUND
  Increasing skeletal muscle carnitine content represents an appealing intervention in conditions of perturbed lipid metabolism such as obesity and type 2 diabetes but requires chronic L-carnitine feeding on a daily basis in a high-carbohydrate beverage.
  
  
  OBJECTIVE
  We investigated whether whey protein ingestion could reduce the carbohydrate load required to stimulate insulin-mediated muscle carnitine accretion.
  
  
  METHODS
  Seven healthy men [mean ± SD age: 24 ± 5 y; body mass index (in kg/m(2)): 23 ± 3] ingested 80 g carbohydrate, 40 g carbohydrate + 40 g protein, or control (flavored water) beverages 60 min after the ingestion of 4.5 g L-carnitine tartrate (3 g L-carnitine; 0.1% (2)[H]3-L-carnitine). Serum insulin concentration, net forearm carnitine balance (NCB; arterialized-venous and venous plasma carnitine difference × brachial artery flow), and carnitine disappearance (Rd) and appearance (Ra) rates were determined at 20-min intervals for 180 min.
  
  
  RESULTS
  Serum insulin and plasma flow areas under the curve (AUCs) were similarly elevated by carbohydrate [4.5 ± 0.8 U/L · min (P < 0.01) and 0.5 ± 0.6 L (P < 0.05), respectively] and carbohydrate+protein [3.8 ± 0.6 U/L · min (P < 0.01) and 0.4 ± 0.6 L (P = 0.05), respectively] consumption, respectively, compared with the control visit (0.04 ± 0.1 U/L · min and -0.5 ± 0.2 L). Plasma carnitine AUC was greater after carbohydrate+protein consumption (3.5 ± 0.5 mmol/L · min) than after control and carbohydrate visits [2.1 ± 0.2 mmol/L · min (P < 0.05) and 1.9 ± 0.3 mmol/L · min (P < 0.01), respectively]. NCB AUC with carbohydrate (4.1 ± 3.1 μmol) was greater than during control and carbohydrate-protein visits (-8.6 ± 3.0 and -14.6 ± 6.4 μmol, respectively; P < 0.05), as was Rd AUC after carbohydrate (35.7 ± 25.2 μmol) compared with control and carbohydrate consumption [19.7 ± 15.5 μmol (P = 0.07) and 14.8 ± 9.6 μmol (P < 0.05), respectively].
  
  
  CONCLUSIONS
  The insulin-mediated increase in forearm carnitine balance with carbohydrate consumption was acutely blunted by a carbohydrate+protein beverage, which suggests that carbohydrate+protein could inhibit chronic muscle carnitine accumulation.
authors.affiliation: Medical Research Council/Arthritis Research UK Centre for Musculoskeletal Ageing Research, School of Life Sciences, University of Nottingham, United Kingdom.
authors.forename: Chris E
authors.initials: CE
authors.lastname: Shannon
authors.affiliation: Medical Research Council/Arthritis Research UK Centre for Musculoskeletal Ageing Research, School of Life Sciences, University of Nottingham, United Kingdom.
authors.forename: Aline V
authors.initials: AV
authors.lastname: Nixon
authors.affiliation: Medical Research Council/Arthritis Research UK Centre for Musculoskeletal Ageing Research, School of Life Sciences, University of Nottingham, United Kingdom.
authors.forename: Paul L
authors.initials: PL
authors.lastname: Greenhaff
authors.affiliation: Medical Research Council/Arthritis Research UK Centre for Musculoskeletal Ageing Research, School of Life Sciences, University of Nottingham, United Kingdom francis.stephens@nottingham.ac.uk.
authors.forename: Francis B
authors.initials: FB
authors.lastname: Stephens
mesh_terms: D000328:Adult; D019540:Area Under Curve; D001132:Arm; D001628:Beverages; D002331:Carnitine / Q000378:metabolism*; D004032:Diet; D004040:Dietary Carbohydrates / Q000008:administration & dosage; D004435:Eating; D006801:Humans; D007328:Insulin / Q000097:blood* / Q000378:metabolism; D008297:Male; D018482:Muscle, Skeletal / Q000187:drug effects* / Q000378:metabolism; D012016:Reference Values; D000067816:Whey Proteins / Q000494:pharmacology*; D055815:Young Adult
chemical_list: D004040:Dietary Carbohydrates; D007328:Insulin; D000067816:Whey Proteins; D002331:Carnitine
keywords: doubly labeled water; energy requirement; resting energy expenditure; short bowel syndrome; total energy expenditure