Glucose Is the Central Fuel in the Energy Metabolism of Most Animals

Glucose can be stored in the body as glycogen, a highly branched starch found in liver and skeletal muscle. Glycogen is the only direct storage form of glucose in the body, although glucose can be synthesized from other compounds.

The major means by which glucose is used as fuel is through the Embden-Meyerhof pathway, or glycolysis, the series of bio­chemical steps that initiate the oxidation of glucose. Glycolysis leads directly into the Krebs cycle, the site of complete fuel oxidation and the major energy-yielding metabolic pathway of the body. For the study of fuel homeostasis, you should appreciate that the process of glycolysis is reversible overall, meaning that glucose can be produced from the compounds that constitute the end products of glycolysis. Because of the close link between glycolysis and the Krebs cycle, any of the Krebs cycle intermediates can potentially move “backward” into the glycolytic pathway to produce glucose. The synthesis of glucose from end products of glycolysis and intermediates of the Krebs cycle is referred to as gluconeogenesis, a critical part of fuel homeostasis. Although Krebs cycle activity occurs in virtually all tissues except red blood cells (RBCs), the process of gluconeogenesis occurs only in liver and, to a limited extent, kidney.

Another pathway for glucose oxidation is the pentose­phosphate pathway. This is a quantitatively minor pathway that does not have great impact on fuel homeostasis. However, it is an important metabolic pathway in erythrocytes (RBCs), which have an absolute need for glucose, although these cells’ overall need for energy is small compared with the rest of the body.