Studies of fuel utilization in subjects with CPT II and VLCAD deficiencies, with stable isotopes during exercise, have shown that in vivo oxidation of LCFA was severely impaired during prolonged, low-intensity exercise. These findings indicated that residual CPT II and VLCAD activities are sufficient to maintain normal oxidation of fat at rest, but that fat oxidation rate cannot increase above basal level during exercise (Ørngreen et al., 2004; 2005).