Here's why:

* Brain's primary fuel source: The brain has a high energy demand and relies heavily on glucose for its energy production. It utilizes glucose through aerobic respiration, a process that uses oxygen to break down glucose and generate ATP (adenosine triphosphate), the energy currency of cells.

* Limited energy storage: The brain has very limited glycogen stores (a form of stored glucose), so a constant supply of glucose from the blood is essential for its function.

* Ketone bodies as alternative: In prolonged periods of fasting or during periods of low blood glucose, the brain can utilize ketone bodies as an alternative energy source. However, glucose remains the primary fuel source.

While other molecules can be utilized to a limited extent, glucose is the brain's main respiratory substance, making it crucial for normal brain function.