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    43 Glycogen.
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    43 Glycogen.

    Glycogen: glucose storage in liver and muscle, hormonal control, diabetes, hypoglycaemia and glycogen storage diseases.

    Glycogen is the body's storage form of glucose: a large, highly branched polymer of glucose molecules. It is stored mainly in the liver and skeletal muscle. Liver glycogen keeps blood glucose stable between meals and overnight: it is broken down and released as glucose into the blood. Muscle glycogen is used only by the muscle itself as fuel during exercise, because muscle lacks the enzyme glucose-6-phosphatase needed to release free glucose. Glycogen is built up after meals (stimulated by insulin) and broken down during fasting and exercise (stimulated by glucagon and adrenaline).

    MetabolismGlucoseLiver and muscle
    Liver stores≈ 100 g (approx.)
    Muscle stores≈ 400 g (approx.)
    Liver glycogen lastsroughly a day of fasting
    Built byinsulin; broken down by glucagon, adrenaline
    Glycogen cycle: after meals insulin drives glucose into liver and muscle, where it is stored as glycogen; during fasting liver glycogen is broken down to glucose for the blood, during exercise muscle glycogen is used inside the muscle Glycogen: storage and release Blood glucose ≈ 4–6 mmol/L fasting Liver glycogen releases glucose to blood Muscle glycogen used by the muscle itself meal: insulin fasting: glucagon meal: insulin exercise: adrenaline
    Figure 1. Glycogen is stored after meals and broken down during fasting (liver) or exercise (muscle).

    Clinical relevance

    • Diabetes: with too little insulin, the liver stores less glycogen and releases too much glucose; with excessive insulin, glycogen accumulates. In poorly controlled type 1 diabetes, excess glycogen can enlarge the liver (glycogenic hepatopathy, Mauriac syndrome in children). Restoring glucose control usually normalises glycogen metabolism.
    • Hypoglycaemia occurs when liver glycogen is exhausted (prolonged fasting, alcohol, liver failure) or cannot be released (some glycogen storage diseases).
    • Glycogen storage diseases are inherited enzyme defects. Liver forms (for example von Gierke disease, type I) cause fasting hypoglycaemia, enlarged liver, high lactate and lipids. Muscle forms (for example McArdle disease, type V) cause exercise intolerance, cramps and myoglobinuria. Pompe disease (type II, lysosomal α-glucosidase deficiency) affects heart and skeletal muscle.
    • Sport: muscle glycogen limits endurance; its depletion causes fatigue ("hitting the wall"), and carbohydrate intake replenishes it.
    When to see a doctor

    Repeated low blood sugar, an enlarged liver, muscle cramps and dark urine after exercise, or poor growth in a child need medical evaluation for a disorder of glycogen metabolism.

    References

    1. Adeva-Andany MM, González-Lucán M, Donapetry-García C, Fernández-Fernández C, Ameneiros-Rodríguez E. Glycogen metabolism in humans. BBA Clin. 2016;5:85–100. doi:10.1016/j.bbacli.2016.02.001

    This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.

    Published on 30 July 2024