This quotation is from the book I’m reading, Other Minds. The Octopus, The Sea, and The Deep Origins of Consciousness:

Neurons cost a great deal of energy to run and maintain. Creating their electrical spasms is like the continual charging and discharging of a battery, hundreds of times each second. In an animal like us, a large proportion of the energy taken in as food, nearly a quarter in our case, is spent just keeping the brain running.

Is that right? A quarter of our food? I found it corroborated here: Why Does the Brain Need So Much Power?, Scientific American, April 2008

It is well established that the brain uses more energy than any other human organ, accounting for up to 20 percent of the body’s total haul.

That article went on to say that electrical impulses aren’t the only thing the brain does with those calories:

Until now, most scientists believed that it used the bulk of that energy to fuel electrical impulses that neurons employ to communicate with one another. Turns out, though, that is only part of the story.

Two thirds of the brain’s energy budget is used to help neurons or nerve cells “fire” or send signals. The remaining third, however, is used for … “housekeeping,” or cell-health maintenance.

Housekeeping includes things like ATP production (ATP, adenosine triphosphate, is a chemical storage form of energy), movement of ions across membranes, cell division, production of neurotransmitters and receptors. Lots of metabolic processes.

The brain is a hungry and demanding organ. And you know what it eats? Glucose. Almost exclusively:

Glucose is virtually the sole fuel for the human brain, except during prolonged starvation. The brain lacks fuel stores and hence requires a continuous supply of glucose. It consumes about 120 g daily, which corresponds to an energy input of about 420 kcal (1760 kJ), accounting for some 60% of the utilization of glucose by the whole body in the resting state.

Fatty acids do not serve as fuel for the brain, because they are bound to albumin in plasma and so do not traverse the blood-brain barrier. In starvation, ketone bodies generated by the liver partly replace glucose as fuel for the brain.
– Biochemistry. 5th edition. Berg JM, Tymoczko JL, Stryer L. 2002.

The brain doesn’t store fuel like muscles and other organs do. It needs a constant supply of glucose, always flowing in. Other organs will sacrifice to keep the brain running. If carbs (which supply glucose) are limited in the diet, the liver can produce some, along with ketones, but eventually the body will begin to break down its muscle to supply glucose for the brain.