Glucose Regulation as an Example of Homeostasis

When an animal takes in more calories than it needs to produce ATP, the excess can be used for biosynthesis. If the animal isn′t growing in size or reproducing, the body tends to store the surplus in energy depots. In humans, the liver and muscle cells store energy in the form of glycogen. Glucose is a major fuel for cells, and its metabolism, regulated by hormone action, provides an important example of homeostasis.

If the body′s glycogen depots are full and caloric intake still exceeds caloric expenditure, the excess is usually stored as fat.

When fewer calories are taken in than are expended—perhaps because of sustained heavy exercise or lack of food—fuel is taken out of storage depots and oxidised. This may cause an animal to lose weight. The human body generally expends liver glycogen first and then draws on muscle glycogen and fat. Most healthy people, even if they are not obese, have enough stored fat to sustain them through several weeks of starvation (an average human′s energy needs can be fueled by the oxidation of only 0.3 kg of fat per day).