According to Popular Science, your brain requires a tenth of a calorie per minute, just to stay alive. Compare this to the energy used by your muscles. Walking burns about four calories a minute. Kickboxing can burn a whopping ten calories a minute. Reading and pondering this article? That melts a respectable 1.5 calories a minute. Feel the burn (but try the kickboxing if you're trying to lose weight).

While 1.5 calories per minute might not seem like very much, it's a rather impressive number when you take into account your brain only accounts for about 2% of your mass and that, when you add up these calories over the course of a day, this one organ uses 20% or 300 of the 1300 calories the average person needs per day.​

Where the Calories Go

It's not all to your gray matter. Here's how it works: The brain is comprised of neurons, cells that communicate with other neurons and transmit messages to and from body tissues. Neurons produce chemicals called neurotransmitters to relay their signals. To produce neurotransmitters, neurons extract 75% of the sugar glucose (available calories) and 20% of the oxygen from the blood. PET scans have revealed your brain doesn't burn energy uniformly. The frontal lobe of your brain is where your thinking takes place, so if you are pondering life's big questions, like what to have for lunch to replace the calories you are burning, that part of your brain will need more glucose.

Calories Burned While Thinking

Unfortunately, being a mathlete won't get you fit. In part, that's because you still have to work muscles to earn that six-pack, and also because pondering the mysteries of the universe only burns twenty to fifty more calories per day compared with lounging by the pool. Most of the energy used by the brain goes toward keeping you alive. Whether you're thinking or not, your brain still controls breathing, digestion, and other essential activities.

Calories and Mental Fatigue

Like most biochemical systems, the brain's energy expenditure is a complex situation. Students routinely report mental exhaustion following key exams, like the SAT or MCAT. The physical toll of such tests is real, although it's likely due to a combination of stress and concentration. Researchers have found the brains of people who think for a living (or for recreation) become more efficient as using energy. We give our brains a workout when we focus on difficult or unfamiliar tasks.

Sugar and Mental Performance

Scientists have studied the effect of sugar and other carbohydrates on the body and brain. In one study, simply rinsing the mouth with a carbohydrate solution activated parts of the brain that enhance exercise performance. But, does the effect translate into improved mental performance? A review of the effects of carbohydrates and mental performance yields conflicting results. There are evidence carbohydrates (not necessarily sugar) can improve mental function. Several variables affect the outcome, including how well your body regulates blood sugar, age, time of day, the nature of the task, and the type of carbohydrate.

If you're facing a tough mental challenge and don't feel up to the task, there's a good chance a quick snack is just what you need.