Glucose is the primary energy source normally used by the human brain.
Brain cells require a continuous supply of energy because neurons have limited fuel reserves. Under ordinary dietary conditions, glucose crosses the blood–brain barrier and is metabolized to produce ATP, the chemical energy used to maintain electrical signaling and cellular functions.
The brain also requires oxygen, but oxygen is not the fuel itself; it enables efficient aerobic metabolism of glucose. This distinction is a common source of confusion. During prolonged fasting or starvation, the liver produces ketone bodies, which the brain can use as an alternative fuel. Red blood cells cannot use ketones because they lack mitochondria.
Although the brain accounts for only a small fraction of body mass, it consumes a substantial share of the body's resting energy. Neurons and supporting glial cells use this energy to maintain ion gradients, transmit signals, recycle neurotransmitters, and preserve cellular function.