The human brain weighs only about three pounds, or roughly 2 percent of the average adult body mass. Yet its cells consume 20 to 25 percent of the body's total energy. In the process, inordinate amounts of potentially toxic protein wastes and biological debris are generated. Each day, the adult brain eliminates seven grams of worn-out proteins that must be replaced with newly made ones, a figure that translates into the replacement of about half a pound of detritus a month and nearly six pounds, twice the brain’s own weight, over the course of a year.*