McArdle disease and carbohydrate ingestion before exercise: Timing on exercise tolerance, clinical relevance, and application to real world settings

McArdle disease (MD) is a rare (1:100 000) genetic myopathy whereby the enzyme glycogen phosphorylase is not expressed in skeletal muscle, resulting in the blockage of glycogenolysis and presenting as physical activity intolerance. To circumvent the blockage in glycogenolysis, current clinical practice guidelines advise individuals with MD (IWMD) to ingest 37 g of sucrose 5-10 minutes (min) before undertaking exercise. The current investigation undertook randomized repeated measure clinical exercise trials on five IWMD to compare exercise tolerance under different timing of sucrose ingestion before exercise. Results showed significantly higher plasma glucose availability at the beginning of exercise when sucrose was ingested 25-min before exercise ([7.4 ± 0.8] mmol⋅L -1 ) compared to 5-min before exercise ([5.6 ± 0.3] mmol⋅L -1 , p = 0.024) or a placebo ([5.3 ± 0.4] mmol⋅L -1 , p = 0.017). Peak heart rates we significantly lower when sucrose was ingested 25-min before exercise ([117 ± 14] beats.min -1 ) compared to 5-min before exercise ([129 ± 14] beats.min -1 , p = 0.022) or a placebo ([135 ± 20] beats.min -1 , p = 0.039) which contrasts with recommendations in current clinical practice guidelines. While the ingestion of sucrose (carbohydrate) before exercise for IWMD may provide statistically significant improvements in exercise tolerance in lab-base settings, its clinical relevance and application to real world settings (RWS) is likely negligible based on the extremely low power outputs IWMD achieve during lab studies. Clinical practice guidelines should be updated to better reflect the magnitude of benefit ingesting carbohydrate before exercise may provide to set realistic expectations for both medical professionals and IWMD. The ingestion of sucrose (carbohydrate) before exercise for IWMD may provide statistically significant improvements in exercise tolerance in lab-base settings, but its clinical relevance and application to real world settings is likely negligible based on the extremely low power outputs achieved by IWMD during lab studies.