Neutral Lipid Storage Disease With Myopath (NLSDM) is a disease caused by a defect in the PNPLA2 gene encoding ATGL. Patients with NLSDM accumulate triglycerides and exhibit muscle weakness, cardiac failure and hepatosteatosis. Most of these patients die at young age due to cardiac failure. Not much is known about the underlying mechanisms, though recently it was discovered that PPAR activation in ATGL-/- mice was impaired leading to decreased mitochondrial function, lipid accumulation and cardiac failure resulting in death at young age. Activation of PPARs, by treatment with fibrates rescued the phenotype and reduced mortality rates in these mice. These findings may have a major impact for patients with NLSDM if these results can be translated to humans. Therefore, the investigators would like to evaluate the beneficial effects of fibrate treatment on muscle mitochondrial and cardiac function in patients with NLSDM.