Our bodies put in a lot of work when it comes to continually supplying us with energy from the food we eat.
When we eat food is broken down into macro and micronutrients via digestive enzymes. Carbohydrates like rice and starchy vegetables get broken down into glucose to be absorbed into our bloodstream and utilized for immediate energy with the help of insulin.
When our cells don't use all of the glucose from the food we eat it gets stored in our liver and muscles as glycogen and any leftover gets store in our fat cells, as fat.
In-between meals, and when we first start fasting as long as we aren't snacking, our body will utilize glycogen for energy. But once our glycogen stores are depleted (typically between 12-36 hours after our last meal) our body will begin to break down fat for fuel (1).
Prolonged fasting will eventually prompt our body to experience ketosis and metabolicly switch to breaking down fat as fuel; using stored fatty acids instead of glycogen for energy (1).