
Glycogen storage disease type V, also known as McArdle's disease is an autosomal recessive disorder caused by mutations in the gene that encodes myophosphorylase, an enzyme that is essential for glycogenolysis. Exercise intolerance usually develops during childhood in patients with McArdle's disease, along with pain, cramps, and fatigue in exercised muscle. These symptoms are more likely to be induced by brief, intense activities but can also occur after prolonged, low-intensity exercises. Severe muscle damage can lead to myoglobinuria and renal failure. Many patients note a "second-wind" phenomenon, whereby exercise becomes more easily tolerated after approximately 10 minutes of continued activity. This phenomenon is attributable to higher extramuscular fuel delivery to contracting muscle that partially rescues substrate-limited oxidative metabolism. In the present study Dr. Susanne Andersen and colleagues investigated whether ingesting a low dose of sucrose just before exercise would increase the availability of glucose to muscles, thus improving exercise tolerance. They aimed to achieve the benefit of sucrose administration while reducing caloric intake and to make treatment more convenient to patients by decreasing the time between sucrose ingestion and exercise. To this end, six patients were blinded to treatment and tested after ingestion of either 75 g of sucrose or a placebo 40 minutes before exercise or 37 g of sucrose or a placebo 5 minutes before exercise. Treatment effectiveness was analyzed by monitoring heart rate and perceived exertion during exercise. In all patients, sucrose ingestion 5 minutes before exercise alleviates the muscle symptoms and abolishes the second-wind phenomenon that occur during the early stages of exercise, when patients are prone to muscle injury. Compared to the 75 g dose of sucrose 40 minutes before exercise, the lower dose of sucrose was superior in providing an improved and prolonged effect on exercise tolerance in McArdle patients. Pre-exercise sucrose ingestion is not a perfect answer to McArdle disease because of the potential for causing weight gain and for potentiating the early onset of diabetes mellitus. However, until better treatments become available, the oral ingestion of sucrose before exertional activity is a reasonable approach to the management of this difficult myopathy.
Arch Neurol. 2008 Jun;65(6):786-9.