Here's why:

* Glucose is the brain's preferred energy source: The brain has a high energy demand and glucose is the most readily available and efficient fuel. It can be easily transported across the blood-brain barrier, which protects the brain from harmful substances.

* Brain cells can't store glucose: Unlike other tissues, brain cells can't store significant amounts of glucose. They need a constant supply from the bloodstream.

* Ketones can be used as an alternative fuel source: During prolonged fasting or starvation, the body can produce ketones from fat. Ketones can be used as an alternative fuel source by the brain, although they are not as efficient as glucose.

Other important factors:

* Oxygen is also essential for brain function: Glucose needs oxygen to be broken down and produce energy.

* Insulin helps regulate glucose uptake: Insulin, a hormone produced by the pancreas, facilitates glucose uptake into brain cells.

* Other nutrients are important for brain health: Although glucose is the primary fuel, other nutrients like amino acids, fatty acids, and vitamins are also crucial for brain function and development.

In summary, glucose is the primary fuel source for the brain, and it's essential for optimal brain function. However, other nutrients are also important for maintaining brain health.