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.