All living things require a constant supply of energy to facilitate the various chemical reactions in the cells. Many of these metabolic processes occur within the liver and depend mostly upon the sugar glucose for fuel. Glucose is not the only source of energy. Most cells in your body can also use fats and the amino acids from proteins. However, glucose is the most efficient source; the brain relies upon glucose exclusively for energy.
Glucose
The most basic unit of carbohydrates that still retains the characteristics of a carbohydrate is known as a monosaccharide. Glucose is by far the most common monosaccharide and a plentiful source of fuel for the cells. Most carbohydrates are made from multiple glucose units bound together. For example, starches from grains and beans, and lactose from milk, all contain varying levels of glucose. When you consume carbohydrates, the digestive system breaks them down carbohydrates and absorbs the glucose into the blood. The circulating glucose molecules are the provenance of the term "blood sugar."
Carbohydrate Stores
The primary role of the liver in the metabolism of carbohydrates is to take up about one-third of the absorbed glucose from the blood and manufacture glycogen --- one of the few types of carbohydrates that the human body can actually store. The formation of glycogen from glucose is known as glycogenesis. Both the liver and the muscles store glycogen and release it during energy intensive tasks when dietary glucose isn't immediately available to the cells. Long distance athletic competitions, such as marathons, cross-country skiing and cycling, tend to deplete glycogen stores the most.
Glycogen Regulation
As blood glucose levels rise, the pancreas produces and releases a hormone known as insulin. At its most basic level, insulin ushers the glucose into cells for energy production. It also stimulates the synthesis of glycogen in the liver. In the post-meal "fed state," the liver takes in more glucose from the blood than it releases. The breakdown of glycogen --- a process known as glycogenolysis --- occurs if blood glucose levels fall too low. In this state, the pancreas releases a hormone called glucagon. It inhibits the synthesis and stimulates the breakdown of glycogen.
Synthesis of Glucose
If carbohydrates are not immediately available for energy, then the liver has one other important task. It can synthesize glucose de novo from non-carbohydrate sources such as amino acids and fat found in the diet or body tissue. This is a process known as gluconeogensis. The liver works to maintain glucose levels in the blood to meet the demands of other organs such as the brain and skeletal muscles.
