Final answer:
Fat is the body's main long-term energy source with large reserves not quickly metabolized, providing more energy per mass than carbs but metabolized slower, suitable for long-term energy needs.
Step-by-step explanation:
The energy source in the body that has the largest reserve but is not quickly metabolized is fat.
While glycogen serves as a short-term energy storage, especially in the liver and skeletal muscle cells, fat is the main form of long-term energy storage.
Fatty acids from adipose tissue are broken down within the mitochondria to release energy in the form of ATP, a process that is slower and less immediate compared to the breakdown of glycogen for energy.
This is especially important for long-term energy supply in situations such as fasting, hibernation, or extended periods of physical exertion where immediate energy sources are not sufficient.
Moreover, fat has more energy per mass or volume as compared to carbohydrates and glucose, making it a vital energy reserve for the body.
However, because of its high energy content and the complex process required for its mobilization and metabolism, it is not the first choice when quick energy is required, such as during high-intensity activities where glycogen is preferentially metabolized to provide a quick source of glucose.