Common names Cori disease / Forbes disease
Deficient enzyme Amylo-1,6-glucosidase
This enzyme is sometimes referred to as the glycogen  debranching enzyme.


With your body having low levels of glucose, you need to produce energy in other ways. One way to do this involves your body using its fat stores to produce ketones. Your body can also start to use protein to produce energy.


Ketones are an alternative source of energy. In healthy individuals, ketone levels are typically very low. Extended fasting can lead to higher levels of ketones. Ketones are produced from the body’s fat stores. The brain needs a lot of energy to work properly but it cannot use fat to produce energy – it prefers glucose. However, the brain can get energy from ketones. If you have GSD type III, turning fat into ketones helps your brain get energy when glucose levels are low. Ketones also help keep your muscles supplied with energy when your glucose levels are low. Although ketones can be a very useful source of energy, it is important that your ketone levels do not go too high. High ketone levels indicate that GSD control could be better.


Good dietary management aims to reduce the risk of your blood glucose levels going too low. This means that your body will not need to make ketones or break down muscles to produce energy.


GSD III is split into two main types. Both GSD type IIIa and IIIb affect the liver but GSD IIIa also affects the muscles.

Muscle weakness

If you have GSD type IIIa, your body cannot use the muscle’s glycogen stores. This can lead to muscle weakness  - you may tire more easily than others. You may also have leg pain. Activity is important for everybody so regular exercise should be encouraged but you may need extra resting time and extra snacks. The Specialist Dietitian can advise on this.


The heart is arguably the most important muscle in the body. If you have GSD type IIIa, the heart can become enlarged. This is referred to as cardiomyopathy. Management of GSD type IIIa aims to reduce any complications. Regular heart scans are used to monitor the heart.