The human brain uses 20 percent of body energy

The human brain is an energy-intensive organ with demands disproportionate to size. While comprising 2% of adult body weight, it accounts for approximately 20% of resting energy expenditure. This means maintaining conscious thought, memory formation, and baseline neural activity requires substantial glucose and oxygen. The brain has extremely dense vascularization; oxygen deprivation causes brain damage within minutes. Energy demand varies with neural activity: complex cognitive tasks increase consumption, while relaxed states use less. Energy supports maintaining resting membrane potential (electrical gradient allowing neuron function), neurotransmitter synthesis, synaptic plasticity, and cellular maintenance. This outsized consumption reflects evolution's prioritization of neural function—the metabolic cost justifies computational power gained.