Since our bodies cannot synthesize their own source of energy like plants do, we need to ingest the fuel that allows our cells to live and function. The food we eat contains 3 metabolic fuels: proteins, carbohydrates, and lipids (commonly referred to as fats). Most foods have some combination of these fuels, as shown in the Venn diagram.
I highly recommend watching the 15 minute Crashcourse video below that explains more about metabolic fuels. It’s very entertaining and succinctly explains what these fuels are made of.
When we eat, Metabolic fuels are digested in the stomach and small intestine into their smallest components (called monomers), then are absorbed from the small intestine into the bloodstream. Once in the bloodstream, the fuel flows to all the cells in the body. The fuel either is used by the cells for energy, or stored for later use.