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 24weeks of high-intensity interval training (HIIT) at 100% maximal exercise capacity (Wattmax ). Twenty-one healthy male volunteers (age 23±2years; BMI 24.2±1.1kg/m2 ) performed 2×3minute bouts of cycling exercise at 100% Wattmax , separated by 5minutes of rest. Fourteen volunteers repeated this protocol following 24weeks of HIIT and twice-daily consumption of 80g carbohydrate (CON) or 3g 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 24weeks of HIIT did not alter this, nor did it enhance muscle metabolic adaptations or performance gains beyond those with HIIT alone.