L-carnitine and exercise tolerance in medium-chain acyl-coenzyme A dehydrogenase (MCAD) deficiency: a pilot study.

Skeletal muscle function may be impaired in patients with medium-chain acyl-CoA dehydrogenase (MCAD) deficiency, but the value of L-carnitine in their long-term management is not clear. This study was designed as a pilot to examine the effects of L-carnitine on exercise tolerance in patients with MCAD deficiency. Four clinically asymoptomatic MCAD-deficient patients, aged 8 to 20 years, were studied. Incremental ramp exercise tests were carried out before and after 4 weeks' treatment with oral L-carnitine (100 mg/kg per day). During exercise without L-carnitine supplementation, plasma carnitine concentrations fell, associated with an increased excretion of urinary acylcarnitines, notably acetylcarnitine, hexanoylcarnitine and octanoylcarnitine. L-carnitine treatment prevented this fall in plasma carnitine and resulted in greater increases in excretion of acylcarnitines. All four patients showed biologically significant improvement in peak oxygen uptake (peak VO2, 18-32% improvement), VO2 at a heart rate of 170 beats/min (15-23% improvement), VO2 at anaerobic threshold (27-42% improvement), and/or oxygen pulse (10-32% improvement). Exercise tolerance in MCAD-deficient patients may be improved by short-term L-carnitine supplementation. This may be the direct result of improved intramitochondrial homeostasis induced by L-carnitine in removing accumulating acyl moieties.