Here's why:

* Catabolism: This is the process of breaking down complex molecules (like carbohydrates, fats, and proteins) into simpler ones.

* Energy Release: This breakdown process releases energy that is stored in the chemical bonds of the complex molecules.

* ATP: This is the primary energy currency of cells. Catabolism generates ATP by transferring energy from the broken down molecules to ADP (adenosine diphosphate), converting it to ATP.

So, when you eat food, your body catabolizes the nutrients, releasing energy that is used to make ATP. This ATP is then used to power all the functions of your cells, including muscle contraction, nerve impulse transmission, and protein synthesis.