McArdle disease is a rare muscle disorder. In this disease, the muscle cells can’t break down a complex sugar called glycogen. It is part of a group of diseases called glycogen storage diseases. Another name for McArdle disease is glycogen storage disease GSD 5 or GSD V.
Your cells use a simple sugar, called glucose, for energy. When you eat, your digestive system sends a large amount of glucose into your blood. This raises your blood glucose levels. Your body removes this extra glucose from the blood to stabilize the blood glucose level. Your body then converts the extra glucose into glycogen. It stores it in the liver, muscles, and other places in the body. Glycogen is a form of energy storage.
When you haven’t eaten in a while, the glucose level in your blood starts to drop. This tells your body to start using some of the glycogen it saved up earlier. The glycogen gets broken down into the glucose so that your body has a steady supply.
Your muscles need a constant supply of glucose to keep working well. In McArdle disease, your muscles can’t break down the saved up glycogen. That's because an important substance needed for that process is missing from your muscle cells. This means your muscles can’t use the stored glycogen to get the glucose they need. The key missing substance in your muscles is an enzyme called myophosphorylase. An enzyme is a substance that helps speed up chemical reactions in the body.
This may make it hard for you to exercise without becoming tired and having muscle pain. Most of the time, symptoms of this health problem appear by the time a person is age 15.