Patients suffering from the metabolic myopathy Glycogen Storage Disease type IIIa (GSDIIIa) have a problem releasing sugar stored in cells that is needed for energy production. This causes several systemic impairments, but only recently have the exercise-related symptoms in the muscles been examined. A previous study showed signs that intravenous infusion of glucose relieves some of these symptoms. The purpose of this study is to investigate in a randomized and placebo-controlled fashion whether oral ingestion of sugar can alleviate muscular symptoms in patients with GSDIIIa.

It has recently been documented how patients with GSDIIIa have a moderate to severely reduced exercise capacity, and that exercise induces muscle pain and cramps. These symptoms are caused by the inability to mobilize skeletal muscle glycogen and are most likely the consequence of a severe energy deficiency within muscles. The study changed the phenotype of GSDIIIa, to include exercise-induced symptoms, which is a typical presentation in other metabolic myopathies. It also documented that exercise capacity was significantly improved while exercise-induced muscular symptoms were relieved by an intravenous glucose infusion. Based on these findings, this study wishes to investigate if oral ingestion of sucrose has the same effects on work capacity on a larger number of patients, in a randomized, placebo-controlled, cross-over setup. Ingestion of sucrose has the potential to be an effective, cheap and easily accessible dietary treatment of muscular symptoms in GSDIIIa.

Inclusion Criteria:

• Genetically and/or biochemically verified GSDIIIa.
• 18 years or older.

Exclusion Criteria:

• Clinically significant cardiac or pulmonary disease.
• Pregnancy or lactation.
• Severe mental disorders or participants that are in other ways unable to understand the purpose of the trials.
• Subjects where the investigator assess that it is not possible or very difficult to place an intravenous catheters.
• Other conditions of the joints or skeletal muscle such as arthritis or sprains. If the condition is expected to resolve before the study inclusion period is stopped, the subject may be included at a later time.
• Moderate to severe muscle weakness, where the participants are not expected to complete 10 minutes of cycle-ergometry exercise at 70 % of VO2peak.
• Verified diabetes.
• Participation in other clinical trials that may interfere with the results.
• Medications that may interfere with the results or increase the risk of bleeding.
• Blood-clotting or bleeding disorders.
• Blood donation one month or less prior to inclusion.