The metabolically insatiable organ. Two percent of body weight, the brain spends a fifth of the body's oxygen and glucose — and it will not tolerate a supply interruption of even minutes.
The brain's metabolic geography matches its work: gray matter burns brighter than white, and firing costs ATP at the synapse — pumps restoring ion gradients after every spike. The organ is closed behind the blood-brain barrier, whose endothelial tight junctions force fuels and nutrients through specific transporters. Under starvation the brain adapts to burn ketone bodies, easing but not ending its glucose demand.
Metabolic Pathways
Glucose is the default fuel: glycolysis plus oxidation, with lactate from astrocytes shuttled to neurons as supplementary fuel.
Glutamate cycling: the main excitatory transmitter doubles as a metabolic intermediate, handed between astrocytes and neurons.
No fuel store: glycogen sits mainly in astrocytes, a buffer of minutes, not days.
Regulation
Local blood flow tracks activity — the basis of fMRI's BOLD signal.
Glucose transport (GLUT1, GLUT3) sets the ceiling on supply; hypoglycemia confuses then comas.
Appetite circuits in the hypothalamus — ghrelin, leptin — make metabolism a brain function too.