system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 What is actually happening when you burn fat?

This might be a stupid question, I might indeed be stupid. But what is the actual process going on inside my body at 290lbs when I go hiking for example?
gpt
Your body needs energy to do things.

When you go on a hike, fibers made of muscle cells must expand and contract. Where do they get the energy to do this?

That energy is bound up as chemical bonds between atoms in various molecules in your cells. The main energy source for much of your body are sugars, particularly glucose. Every cell in your body is capable of performing a complicated process of breaking glucose down to make use of the energy stored in its chemical bonds. This reaction is, fundamentally, no different from burning any organic material. You're taking some complicated molecule and breaking it down into water and CO2 while releasing the energy stored in all the bonds you broke.

Other molecules sitting in your cells, such as fats and proteins, also contain energy in their bonds and can be burnt or broken down to release energy. Fats, among other uses, serve as a long-term energy storage system when you have much more energy than you need. When you start running out of sugar to burn, your cells will start breaking down this excess fat.