The Hungry Human Brain
in 1995 Leslie Aiello, then of University College London, and Peter Wheeler of Liverpool John Moores University offered the first possible reason for the rarity of big brains. Neurons, they pointed out, have a voracious appetite. They require lots of energy to produce their voltage spikes and to release neurotransmitters. They get that energy from oxygen and food, mostly glucose. A three-pound human brain burns up to 20 times as many calories as three pounds of muscle.
[...] The brain also requires a constant supply of food. Twenty-five percent of all the calories you eat each day end up fueling the brain. For a newborn infant, with its little body and relatively large and fast-growing brain, that figure leaps to 87 percent.
-- Carl Zimmer
Quoted on Tue Aug 2nd, 2011