It is my personal experience that the use of these three terms, glucose, glycogen, and glucagon, especially when we are talking about type 1 and type 2 diabetes, may lead to some confusion since somehow the terms are similar among themselves.
However, their functions, although related to glucose, are very different. To clarify them and their roles in our body, I have put together a brief explanation of each one.
Glucose is a form of carbohydrate known as a simple sugar and the primary source of energy for cell function. The body transforms the carbohydrates we eat into glucose which is the main type of carbohydrate that the body can use instantly for energy and the main form of energy that can be used by the nervous system and the brain. Once the body has transformed the carbohydrates into glucose, where that glucose came from, either from bread or candy, is irrelevant to the body.
The Uses of Glucose
Glucose can be used immediately. If energy is not needed immediately, the liver and the muscles convert glucose into glycogen which is stored in the liver and can be converted back into glucose when necessary. After the glucose stored in the liver has been expended, the body uses fat deposits as an alternative source of energy.
If our body has a surplus of glucose and glycogen reserves are full, the liver and other organs convert excess glucose into fat. The resulting fatty acids may be used eventually as energy but are a far less efficient source of energy than glucose and are used when glucose stores become depleted.
Glycogen is the form in which carbohydrate energy is stored in the muscles and the liver. When glucose (sugar) is not needed for immediate energy, it is converted into glycogen by the liver or the muscles and remade into glucose when necessary.
Glucose released from glycogen produced by the liver can be used anywhere in the body. Glycogen produced by the muscles is released later.
The liver can store only half a day’s supply of glycogen. After 24 hours without food, the carbohydrate reserves of the liver are usually exhausted and the body must use fat or protein stores for energy.
An Interesting Feature of Glycogen
Glycogen loves water, meaning it binds as much as three times its weight in water. A person starting a weight-loss program, especially one that includes very little carbohydrates, will use up glycogen stores within the first couple of days. As these stores disappear, the water associated with them will disappear as well, in the form of increased output of urine. This accounts for the rapid initial weight loss of a few pounds that is so common for new dieters.
Of course, those who fall off the wagon before they have mobilized a substantial amount of fat can expect to welcome back their glycogen and its associated water very quickly.
Glucagon is a hormone that converts glycogen into glucose. Cells in the pancreas that secrete hormones are located in clusters the size of pinheads, called islets of Langerhans, which are scattered throughout the pancreas. Two of those hormones are insulin and glucagon.
During digestion, carbohydrates are broken down into glucose, which the body either uses for energy or stored as glycogen. When glucose levels in the blood rise, the pancreas releases insulin which causes the cells to take up glucose. When food does not enter the body, such as between meals, during sleep, and during periods of starvation, blood glucose levels drop. The alpha cells of the pancreas then secrete glucagon, which converts glycogen into glucose. This glucose then travels back into the bloodstream where it is used for energy.