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The Secret of Glycogen: Why is This Multi-branched Polysaccharide Our Body's Energy Reserve?
Glycogen is a multi-branched polysaccharide composed of glucose that plays a role as an energy reserve in animals, fungi and bacteria. It is the main form of glucose storage in the body. As one of the three commonly used energy reserves, glycogen is mainly used for short-term energy supply, while creatine phosphate is used for short-term rapid energy, and triglycerides in adipose tissue are used for long-term reserves. Under normal circumstances, protein is not generally used as a major source of energy and is only utilized in times of famine or physiological crisis.
In the human body, glycogen is mainly produced and stored in the liver and skeletal muscle cells.
In the liver, glycogen can account for 5-6% of the fresh weight of the organ. The liver of an adult weighing about 1.5 kg can store about 100-120 grams of glycogen. In skeletal muscle, glycogen concentration is lower, accounting for about 1-2% of muscle mass. An adult weighing 70 kg can store approximately 400 g of glycogen in skeletal muscle. Glycogen is also stored in small amounts in many other tissues and cells, including the kidneys, red blood cells, white blood cells, and glial cells in the brain. During pregnancy, the uterus also stores glycogen to nourish the embryo.
The amount of glycogen stored mainly depends on oxidative type 1 fiber, physical training, basal metabolic rate and eating habits. Different resting muscle glycogen levels are achieved by changing the number of glycogen granules, rather than the size of the existing granules. It is worth noting that about 4 grams of glucose are always present in the human blood; in the fasting state, blood sugar levels are maintained stable using glycogen reserves in the liver, because glycogen in skeletal muscle is mainly used by this muscle. , but not involved in the regulation of blood sugar levels.
Glycogen in the muscles serves as the muscle's own energy reserve, while glycogen in the liver is used by the whole body, especially the central nervous system.
In fact, the human brain consumes about 60% of its blood sugar in the fasting state. Glycogen is similar to starch in plants and is a polymer of glucose used for energy storage. Although its structure is similar to amylopectin, a component of starch, glycogen's branches are more abundant and more compact. This efficient storage method allows glycogen to be released quickly to meet sudden energy needs.
Structure and function of glycogen
Glycogen is a polymer with a linear chain and a complex branched structure, usually composed of chains of 8-12 glucose units, the number of which ranges from 2,000 to 60,000 per glycogen molecule. The sugar molecules are linked by α(1→4) glycosidic bonds, and the branched parts are linked by α(1→6) glycosidic bonds. In summary, the structure of glycogen is like the ball of a glucose tree, with glycogen protein at the core.
In the liver, when blood sugar levels rise, insulin promotes liver cells to absorb glucose and convert it into glycogen; conversely, when blood sugar levels decrease, glucagon promotes glycogen degradation to release glucose.
The main function of glycogen in the liver is to regulate blood sugar levels. After a meal, blood sugar rises and insulin secretion increases, promoting the synthesis and storage of glycogen. However, once blood sugar begins to drop, insulin secretion decreases, glycogen synthesis stops, and the enzyme glycogen phosphorylase then converts glycogen back into glucose to meet the body's energy needs. In muscles, glycogen is used as a source of rapid energy, especially during high-intensity exercise.
Clinical Relevance and Exercise Impact
The most common disease with abnormal glycogen metabolism is diabetes. In this condition, the liver's glycogen storage increases or decreases abnormally due to abnormal insulin. During prolonged endurance sports, such as marathons, cross-country skiing, or cycling, athletes often experience glycogen depletion, a phenomenon known as "hitting the wall" or "crashing." To avoid this phenomenon, athletes can choose to continue consuming high-glycemic index carbohydrates to replenish energy during exercise.
Scientific research suggests that consuming carbohydrates with caffeine can help replenish glycogen stores more quickly after high-intensity exercise, but effective doses have not been determined.
In addition, glycogen nanoparticles have also been studied as potential drug delivery systems in recent years, showing the potential application of glycogen in the medical field. This versatile role has led to a renewed interest in the value of glycogen, both as an energy store and as an indicator of health. We can’t help but wonder, what new discoveries and applications will future research on glycogen bring?
