Glucose is a form of simple sugar that your body uses to generate energy inside many different types of cells. Sometimes you need this energy immediately, while at other times you need to store fuel for future use. The first phase of glucose breakdown varies according to your current energy requirements.
Glucose Basics
Glucose is itself a breakdown product of foods in your diet that contain carbohydrates. When you eat these foods, your digestive system processes them and passes glucose to your bloodstream through the wall of your small intestine. Inside your bloodstream, the increasing presence of glucose makes your pancreas release the hormone insulin, which acts as a chemical go-between and tells your cells to open and let glucose enter. Cells in your body that rely on this process include those located in your muscles and fatty tissues.
Cellular Breakdown and Storage
If you have immediate energy needs, the presence of glucose in your cells triggers a breakdown process called glycolysis. During glycolysis, glucose is put through a chemical conversion that produces two distinct substances, called pyruvic acid and adenosine triphosphate, or ATP. To continue the creation of cellular energy, both pyruvic acid and ATP then undergo additional transformations that vary with your cells' specific ongoing requirements. If your cells don't have any immediate energy needs, their glucose content gets broken down or converted into a related substance called glycogen, which remains on hand to meet future demands.
Liver Storage and Conversion
Much of the glucose that forms in your small intestine doesn't circulate in your bloodstream or pass into the cells in your muscles or fatty tissue. Instead, it gets pulled directly from your small intestine to your liver. Once it's inside your liver, this glucose also gets converted into glycogen and stored to meet your future needs. When your energy and blood glucose levels fall during exercise or between meals, your pancreas releases another hormone, called glucagon, that tells your liver to take its glycogen supply and reconvert it into glucose.
This glucose then enters your bloodstream and the interiors of your cells, where it triggers the start of another round of glycolysis. Glycogen stored in your cells also gets reconverted into glucose when needed.
Considerations
After glycolysis, pyruvic acid is normally converted into a substance called acetyl CoA; in turn, acetyl CoA gets converted into a substance called citric acid, which helps your body produce additional ATP. However, when you engage in activities that stress your muscles' capacities, pyruvic acid is turned into a substance called lactic acid instead of acetyl CoA. When the moment of muscular stress passes, lactic acid gets reconverted back into pyruvic acid, which in turn gets reconverted back into glucose. If you don't need this glucose for another purpose, it gets stored for future use in the form of glycogen.
