How is food transformed into energy in the body?

The specifics of energy production and other stuffs that are difficult to understand has already been stated by Juan F R, so this one will be the layman’s term version.

Man eats the food. Food gets broken down during the digestive process and gets to be stored on specific body parts in accordance to their form - Carbohydrates, Fats, and Proteins. (I’m sure you already know this part).

So let’s fastforward to the part where the food is used as an energy source. Basically, the Glucose (most basic form of Carbohydrates) is the main energy source of the body. Think of it as the main fuel tank of a rocket ship. The glucose mostly doesn’t stay long inside the body. We expend glucose with just about every activity that we do, for every movement that we make. Now, not all glucose are used up, especially with not-so-active people. So the excess glucose is converted by the liver as glycogen (think of it as a spare fuel tank of our rocket ship). Now, everytime we it, the rocket ship replenishes it main fuel tank, and all the unused fuel is stored as glycogen. In the event that out rocket was unable to re-fuel (meaning, we missed a meal), the Liver automatically converts the glycogen into glucose (in a process called Glycogenolysis - breaking down of the glycogen into glucose), thus using the spare tanks just so that we could survive until the next re-fueling. The usage of the glycogen reserves have no known side-effects to the human body. Now, in the unfortunate event that the glycogen reserves (the spare tanks) have been exhausted, and yet, the body was still unable to receive its food supply (ergo, still no fuel/energy to support activities), the body will start producing fuel out of its own, starting with the fat deposits. The liver will begin the breakdown of fats and turn it into glucose to be used as energy supply. The breakdown of fats increases the amount of free fatty acids in the blood, which then causes heart and kidney ailments. (so basically, the body trades having a heart condition in exchange for energy). In the MOST unfortunate event that the fat deposits have been exhausted and the body still did not receive any energy supply, the Liver will do a final step for survival, that is converting the muscles (proteins) into glucose (a process known as Gluconeogenesis). The Muscles are broken down into Carbohydrates and are used as energy fuel for survival. As we all know, proteins contain Nitrogen. So when proteins are broken down, Nitrogen in the form of ammonia is released in the blood stream. Combine that with a bum kidney, and the person may go into comatose or die instantly.

hope this helps. =)