title: Increasing skeletal muscle carnitine availability does not alter the adaptations to high-intensity interval training.
abstract: Increasing skeletal muscle carnitine availability alters muscle metabolism during steady-state exercise in healthy humans. We investigated whether elevating muscle carnitine, and thereby the acetyl-group buffering capacity, altered the metabolic and physiological adaptations to 24 weeks of high-intensity interval training (HIIT) at 100% maximal exercise capacity (Wattmax). Twenty-one healthy male volunteers (age 23±2 years; BMI 24.2±1.1 kg/m2) performed 2 × 3 minute bouts of cycling exercise at 100% Wattmax, separated by 5 minutes of rest. Fourteen volunteers repeated this protocol following 24 weeks of HIIT and twice-daily consumption of 80 g carbohydrate (CON) or 3 g l-carnitine+carbohydrate (CARN). Before HIIT, muscle phosphocreatine (PCr) degradation (P<.0001), glycogenolysis (P<.0005), PDC activation (P<.05), and acetylcarnitine (P<.005) were 2.3-, 2.1-, 1.5-, and 1.5-fold greater, respectively, in exercise bout two compared to bout 1, while lactate accumulation tended (P<.07) to be 1.5-fold greater. Following HIIT, muscle free carnitine was 30% greater in CARN vs CON at rest and remained 40% elevated prior to the start of bout 2 (P<.05). Following bout 2, free carnitine content, PCr degradation, glycogenolysis, lactate accumulation, and PDC activation were all similar between CON and CARN, albeit markedly lower than before HIIT. VO2max, Wattmax, and work output were similarly increased in CON and CARN, by 9, 15, and 23% (P<.001). In summary, increased reliance on non-mitochondrial ATP resynthesis during a second bout of intense exercise is accompanied by increased carnitine acetylation. Augmenting muscle carnitine during 24 weeks of HIIT did not alter this, nor did it enhance muscle metabolic adaptations or performance gains beyond those with HIIT alone.
authors.affiliation: MRC/ARUK Centre for Musculoskeletal Ageing Research, School of Life Sciences, University of Nottingham, Nottingham, UK.|Diabetes Division, University of Texas Health Science Centre, San Antonio, TX, USA.
authors.forename: Christopher E
authors.initials: CE
authors.lastname: Shannon
authors.affiliation: MRC/ARUK Centre for Musculoskeletal Ageing Research, School of Life Sciences, University of Nottingham, Nottingham, UK.
authors.forename: Reza
authors.initials: R
authors.lastname: Ghasemi
authors.affiliation: MRC/ARUK Centre for Musculoskeletal Ageing Research, School of Life Sciences, University of Nottingham, Nottingham, UK.
authors.forename: Paul L
authors.initials: PL
authors.lastname: Greenhaff
authors.affiliation: MRC/ARUK Centre for Musculoskeletal Ageing Research, School of Life Sciences, University of Nottingham, Nottingham, UK.|Sport and Health Sciences, University of Exeter, Exeter, UK.
authors.forename: Francis B
authors.identifier: http://orcid.org/0000-0001-6312-5351
authors.initials: FB
authors.lastname: Stephens
mesh_terms: D000107:Acetylation; D000222:Adaptation, Physiological; D000255:Adenosine Triphosphate / Q000378:metabolism; D000328:Adult; D002331:Carnitine / Q000008:administration & dosage* / Q000378:metabolism; D004040:Dietary Carbohydrates / Q000008:administration & dosage; D019587:Dietary Supplements; D004311:Double-Blind Method; D000072696:High-Intensity Interval Training; D006801:Humans; D019344:Lactic Acid; D008297:Male; D018482:Muscle, Skeletal / Q000378:metabolism*; D055815:Young Adult
chemical_list: D004040:Dietary Carbohydrates; D019344:Lactic Acid; D000255:Adenosine Triphosphate; D002331:Carnitine
keywords: carnitine supplementation; pyruvate dehydrogenase complex; repeated-bout exercise; training adaptation