Final answer:
We switch to fat during prolonged exercise because of the depletion of glycogen stores which allows for sustained energy production through fat metabolism for extended periods of activity.
Step-by-step explanation:
During prolonged exercise, the body switches to fat as a primary energy source due to the depletion of glycogen stores. Initially, muscles use glycogen to produce glucose, which then generates ATP through glycolysis and the aerobic pathways, such as the Krebs cycle, with the help of oxygen. However, as the exercise continues, these glycogen stores start to dwindle, leading to an increase in fat metabolism to maintain ATP production for muscle contraction. With fat reserves being considerably large compared to glycogen, this switch allows for sustained energy production during extended periods of activity.