BACKGROUND. Energy metabolism in muscles relies predominantly on the breakdown of glycogen early in exercise. In patients with McArdle's disease, blocked glycogenolysis in muscles results in low exercise tolerance and can lead to muscle injury, particularly in the first minutes of exercise. We hypothesized that ingesting sucrose before exercise would increase the availability of glucose and would therefore improve exercise tolerance in patients with McArdle's disease. METHODS. In a single-blind, randomized, placebo-controlled crossover study, 12 patients with McArdle's disease drank 660 ml of a beverage that had been sweetened with artificial sweeteners (placebo) or with 75 g of sucrose after an overnight fast. Thirty to 40 minutes later, the patients rode a stationary bicycle at a constant workload for 15 minutes while the heart rate, level of perceived exertion, and venous blood glucose levels were monitored. RESULTS. Supplemental sucrose increased the mean plasma glucose level by more than 36 mg per deciliter (2.0 mmol per liter) and resulted in a marked improvement in exercise tolerance in all patients. The mean (±SE) heart rate dropped by a maximum of 34±3 beats per minute (P<0.001), and the level of perceived exertion fell dramatically when the patients ingested glucose as compared with when they received the placebo. CONCLUSIONS. This study suggests that the ingestion of sucrose before exercise can markedly improve exercise tolerance in patients with McArdle's disease. The treatment takes effect during the time when muscle injury commonly develops in these patients. In addition to increasing the patients' exercise capacity and sense of well-being, the treatment may protect against exercise-induced rhabdomyolysis.
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