Final answer:
Carbohydrates are the main source of energy for the body, with glucose being the most efficient fuel for ATP production. Excess glucose is stored as glycogen or converted to fat.
Step-by-step explanation:
Carbohydrates, including sugars and starches, serve as the primary source of energy for the body's cells. When carbohydrates are consumed, they are digested and broken down into smaller sugar molecules such as glucose, which is the most efficient fuel for ATP production in cells. In the event of excess glucose, it can be stored as glycogen in the liver and muscle tissues, or it can be turned into fat in adipocytes for later use. Glucose is especially vital as it is quickly mobilized and used by the body during high-energy-demand situations like running a marathon, where glycogen reserves play a crucial role.