The investigators propose to evaluate the effect of bezafibrate on metabolism during exercise in 22 adult patients affected with carnitine palmitoyltransferase II (CPTII) or very-long chain acyl-CoA-dehydrogenase (VLCAD) deficiencies. This study will be an 9-month, randomized, double-blind, placebo-controlled crossover trial. The trial will be conducted in two centers: Institut de Myologie, Pitié-Salpêtrière Hospital in France, and Rigshospitalet, University of Copenhagen, in Denmark. The main criteria for assessing the potential effect of this drug will be the fat oxidation rate studied during a moderate workload on cycle ergometer, after infusion of stable isotopes (palmitate and glucose tracers).