In this article we explore glycogenesis, the formation of glycogen. We also examine its structure, formation, functions, and role in energy production.
WHAT IS THE STRUCTURE, FUNCTION AND ROLE OF GLYCOGEN?
Just as plants store starches, humans also store complex carbohydrates. Glycogen is a polysaccharide of glucose, meaning it is made up of many glucose molecules bonded together. While glycogen is made up of glucose and has many branches in its structure, starches are made up of amylose or amylopectin, which are less branched and therefore more difficult to break down.
Glycogen is produced and broken down by several enzymes, available wherever glycogen is stored. The vast majority of glycogen is found in muscles and the liver, although very small amounts are also found in the kidneys, brain, and some other tissues. The amount of glycogen stored in muscle depends on a person's lean body mass and what they have eaten. But typically, it can range from 400 up to 900 grams of glycogen stored in your muscles when your carbohydrate stores are full. A much smaller amount is stored in the liver, probably around 80 g. This total amount of stored carbohydrate would be enough for 1.5 to 2 hours of intense exercise, if this were the only energy source used. Athletes who are highly aerobically trained may store more glycogen than those who are not. Muscle glycogen can be increased beyond normal amounts by “carbohydrate loading,” which can maximize the energy available for exercise.
DIGESTION AND STORAGE OF CARBOHYDRATES
When carbohydrates are ingested, they are digested and absorbed as individual molecules of glucose (or fructose or galactose, depending on the carbohydrate source). Once absorbed, they enter the blood and can be used in a variety of tissues, including the heart and brain. However, when you consume large amounts of carbohydrates, most of the carbohydrates go into glycogen
GLYCOGEN IS STORED WITH WATER
When glycogen is stored in muscle tissue, water is also absorbed along with it. Roughly in a ratio of 3:1; this means that for every g of glycogen, 3 g of water is stored. This is why glycogen loading programs can lead to relatively large increases in body weight, despite relatively small amounts of carbohydrates being stored. This water will become available once glycogen is broken down and can help with hydration of the body. If an athlete stores an additional 400 grams of glycogen as a result of glycogen loading, this will mean a weight gain of approximately 1.6 kg.
HOW DOES GLYCOGEN ACT AS A SOURCE OF ENERGY?
Glycogen is basically a source of energy; it is broken down to provide glucose very quickly when it is needed for fuel, such as during exercise. Carbohydrates are an important source of energy during exercise, especially high-intensity exercise. If no carbohydrates are ingested during exercise, glycogen is practically the only source of energy. This is true for moderate- and high-intensity exercise, which not only includes traditional resistance exercises like marathons, but also many intermittent sports like soccer or rugby. Glycogen stores can actually become depleted or severely depleted as exercise progresses. When glycogen stores drop below a critical level, this is often known as "hitting the wall" or "getting high." We have many examples in professional sports where athletes lose competitions because of this.
LIVER GLYCOGEN VS MUSCLE GLYCOGEN
The glycogen stored in the muscles and liver, although the same in terms of structure, have different roles. Muscle glycogen is broken down when energy is needed and used by the muscle in which it is stored. All muscles can store glycogen, and exercise that uses those muscles will use the glycogen from those muscles. For example, the glycogen stored in the biceps and triceps muscles of the upper arm will not be used during running at the same rate as the glycogen stored in the thigh and calf muscles. Liver glycogen is used to control blood sugar, which can be used by any tissue that needs it, especially the brain and heart. This means that even at rest, small amounts of liver glycogen are broken down to keep blood glucose levels constant.
Blood sugar is tightly controlled, which is essential for good health. If blood glucose levels are too low (hypoglycemia), tremors, dizziness, or even seizures may occur. If they are too high for prolonged periods (hyperglycemia), this can lead to cardiovascular disease, nerve or kidney damage. The worst effects of poor blood sugar control are limited to people with diabetes, but hypoglycemia can easily occur during prolonged exercise or fasting or during long periods of fasting without food.
GLYCOGEN IS A SENSOR OF ENERGY AVAILABILITY
Since thousands of calories can be stored in the body in the form of glycogen, the amount stored is monitored by the body. The amount of stored glycogen can act as an “energy sensor,” which can inform other aspects of the body's metabolism. Low glycogen stores can act as a signal to increase your ability to use fat for energy, which is part of the theory behind low-intensity training, or fasted training, to improve aerobic metabolism.
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