The brain gobbles up 20 percent of the body’s energy, even though it’s only about 2 percent of the body’s weight. When the brain is fully working, it uses more energy per unit of tissue weight than a fully exercising quadricep. In fact, the human brain cannot simultaneously activate more than 2 percent of its neurons at any one time. More than this, and the brain’s energy supply becomes so quickly exhausted that you will faint.