The substance that provides humans with their main source of energy is glucose.

Here's why:

* Glucose is a simple sugar that is readily absorbed into the bloodstream.

* It is the primary fuel source for cellular respiration, the process that breaks down glucose to generate ATP (adenosine triphosphate), the body's energy currency.

* Glucose is derived from carbohydrates we eat, like bread, pasta, fruits, and vegetables.

While other substances like fats and proteins can also be used for energy, glucose is the body's preferred fuel source and the one that is most readily available.