Need to lose some flab? Sit your big butt down with a math book—and feel the burn. The human brain is a 24-hour workhorse.

While you’re thinking, millions of neurons fire messages back and forth to each other and to the various tissues in the body. These neurons need fuel, consuming a full 75 percent of the blood sugar from the liver and 20 percent of the body’s total used oxygen

Here’s how your neurons feed: Astrocytes—the cells near the capillary walls in your brain—suck energy-rich glucose from the bloodstream and convert it into a form that the neurons can soak up. The neurons then use it to fuel the production of neurotransmitters and, eventually, conscious thought.

“The more energy an area of the brain wants, the more glucose that part of the brain will break down,”

when you’re hunched over a crossword puzzle? Your brain is blasting through a respectable 1.5 calories a minute.