Our skeletal muscles normally depend on energy from carbohydrates and fats. These fuels can be stored in the muscle (glycogen) or imported directly from the bloodstream (glucose and fatty acids). When a genetic defect ()interferes with the processing of specific fuels, energy shortages can occur and toxic byproducts may build up. Some people may be able to bypass their specific defects by adjusting diet or exercise habits to draw energy more efficiently from unaffected pathways.