Science Facts

The adult human brain weighs on average about 1.2–1.4 kg (2.6–3.1 lb) which is about 2% of the total body weight. Although the human brain represents only 2% of the body weight, it receives 15% of the cardiac output from the heart, 20% of total body oxygen consumption, and 25% of total body glucose utilization. The brain consumes up to 20% of the energy used by the human body, more than any other organ. Brain metabolism normally relies upon blood glucose as an energy source, but during times of low glucose (such as fasting, endurance exercise, or limited carbohydrate intake), the brain uses ketone bodies for fuel with a smaller need for glucose. Bonus fact: The adult human brain is estimated to contain 86±8 billion neurons.