Carnitine palmitoyltransferase II (CPTII) and very-long chain acyl-CoA-dehydrogenase (VLCAD) deficiencies are the two most common inherited disorders of mitochondrial fatty acid oxidation (FAO) in adults, both inherited in an autosomal recessive manner. Mitochondrial FAO plays a pivotal role for maintaining energy homeostasis in situations such as fasting, fever or prolonged exercise that require both glucose sparing and major energy supply. In these situations, a number of tissues such as skeletal muscle, heart, and liver, favour fatty acids as the main source of energy. Long-chain fatty acids (LCFA), that represent the major part of endogenous free fatty acids cannot enter the mitochondrial matrix compartment by simple diffusion, and the transfer of LCFA across the mitochondrial membranes is governed by a multienzymatic system named carnitine palmitoyltransferase (CPT) consisting of two enzymes: CPT I and II. CPT I, a key regulatory step of LCFA oxidation, is located within the outer mitochondrial membrane, while CPT II is appended to the inner face of the inner mitochondrial membrane. CPTs I and II catalyze a single reaction (carnitine + acyl-CoA ⇔ acylcarnitine + CoA~SH) in the forward and reverse directions, respectively. VLCAD is bound to the inner mitochondrial membrane, and catalyses the first step of the long-chain fatty acid β-oxidation spiral (Izai et al., 1992).