Blocked muscle fat oxidation during exercise in neutral lipid storage disease.

OBJECTIVE To determine whether impaired exercise capacity in neutral lipid storage disease with myopathy is solely caused by muscle weakness or whether a defect in energy metabolism (blocked fat oxidation) may also play a role. DESIGN We studied a 37-year-old woman with neutral lipid storage disease with myopathy, who cycled while lipid oxidation was assessed using U-(13)C palmitate tracer dilution technique. The effect of a glucose infusion during exercise was also studied. SETTING Neuromuscular research unit. RESULTS The exercise-induced increase in fat oxidation was virtually abolished in the patient. Treatment with intravenous glucose infusion improved maximal oxygen uptake from 23 to 27 mL × kg(-1) × min(-1), and maximal workload from 75 to 100 W. CONCLUSIONS These results demonstrate that in addition to fixed weakness, neutral lipid storage disease with myopathy is also characterized by a profound block in fat oxidation, which limits exercise tolerance.