Why Does the Brain Use So Much of the Body’s Energy?

Edited and published by Wellness Monster Mark

It’s fairly common knowledge that the brain uses more energy than any other part of the body. For example, an average adult brain weighs approximately three pounds and burns about 327 calories each day which averages about 109 calories for each pound of the brain. Another way to interpret this is to say the brain burns about 20% of the body’s total calorie resting energy expenditure (REE) each day which amounts to about 400-500 calories for a man and 350-400 calories for a woman. Basically, REE is how many calories the body burns just to stay alive which does not include any calories burned during movement. When looked at this way, it can be easily observed that the brain actually burns quite a high percentage of expenditure, especially when compared to skeletal muscle which burns only 6% at rest.

It can be helpful to imagine the human brain as a computer where all files are sent to, stored, and retrieved from. It takes a certain amount of energy to run a computer. It also takes a certain amount of energy to run the brain even when it is at rest. But first, there are other ways the body burns calories while at rest as well.


The more muscle the body has, the faster its metabolism will be, and the faster the metabolism is the more calories the body will burn even when it is not moving.

Temperature Regulation

The body burns more energy and, therefore, more calories to stay warm in cooler temperatures and it even grows more tissues to up-regulate the process.

The Thermic Effect: Digestion

The process of digestion and elimination requires a lot of energy which is called the “thermic effect”. The thermic effect burns approximately 10 percent of all calories eaten.


Bodily processes continue during sleep with some processes becoming more active. While fewer calories are used during sleep, the body is still constantly burning fats and sugars to supply energy for the constant needs of the brain and body.

Increased Heart-Rate

increased-heart-rateAerobic exercise burns calories and fat by increasing the heart-rate. Anything that even slightly increases an individual’s heart-rate will burn off extra calories.

Getting back to how the brain expends calories, it’s important to note calories are always burned more significantly by the brain during movement, particularly during exercise. Otherwise, calories expended by the brain remain fairly consistent, regardless of energy expenditures made throughout the rest of the body. Meaning, the brain is challenged to work harder when the body is engaging in physical exercise and, as a result, the brain itself burns even more calories. Why? Because complex exercise involves physical balance, direction, and various speeds which forces the brain to work harder to fire off messages to help the body to perform these functions.

So, does the brain expend more energy and calories when an individual is studying or thinking really hard? Yes and no. Yes, the brain does burn slightly more calories while it is extremely busy mentally but not enough to notice. Thinking will never replace exercise. In fact, a couple of experiments were carried out in the United Kingdom that measured glucose levels in the brains of two groups of volunteers. One group was asked to perform simple, repetitive tasks that required no thought what-so-ever, such as pushing (over and over again). The other group was asked to solve a difficult math question or a puzzle. The researchers discovered that the uptake levels of glucose to the brain increased among those who were trying to solve a difficult problem which meant it needed the extra energy to work out the answer. No changes were noted in those who were simply pushing buttons. Meaning, the brain was not working harder and expending no additional energy to push buttons. It did, however, work harder to work out a problem. Still, even when the brain is working very hard mentally it only uses up about one and a half calories every minute which isn’t really all that impressive. Furthermore, while studying and thinking hard can leave an individual feeling tired and even exhausted it’s not because there is a deficit of brain energy (lost calories). In actuality, any eating or sleeping a person feels they need related to thinking and study is all in the “mind” and one does so purely for self-comfort in order to feel happier.

The Brain’s Source of Energy

Adenosine triphosphate (ATP) molecule

So, what is the source of the energy that powers the brain? The answer? Sugar in the form of glucose. Because the brain expends such high amounts of energy, foods consumed have to be able to supply “sugar” in order for it to be delivered to every cell in the brain and the body by means of the blood system. However, too little or too much sugar can alter brain function dramatically and negatively affect learning, recall, emotions, and the body’s ability to function physically. Additionally, the quality of the foods containing sugar affects the structure of the cells in the brain. There’s a whole “science” behind how adenosine triphosphate (ATP), the actual energy found in cells, begins with a combination of glucose and oxygen and ends with the brain electrically firing messages from one cell to another in itself and to and from the rest of the body.

Until very recently, scientists believed the brain used most of its energy to fuel electrical impulses that enabled neurons to communicate with each other but this is only partially accurate. New studies reveal that while two-thirds of the brain’s energy expenditure is used to help neurons fire messages the remaining third is used for maintaining healthy nerve cells and cleaning up unhealthy ones. This was discovered by the radiologist, Wei Chen, at the University of Minnesota Medical School. Chen, along with other researchers, imaged the brain with Magnetic Resonance Spectroscopy (MRS) to measure its output of energy during shifts of activity. Chen said this technology could be used to track the products of metabolism in different brain tissues. He and his colleagues used the MRS to track the rate of adenosine triphosphate (ATP) in the brains of rats. ATP is the primary source of energy not just in the brain but in all body cells. The goal was to determine whether levels of ATP (energy) being expended during various levels of consciousness changed at all and, if so, how? They found ATP levels definitely appeared to vary with brain activity. For example, when the rats were sleeping, ATP expenditure declined up to 50% and the ATP produced when the brain was inactive went mostly toward cell maintenance and clean up while ATP in the more alert rats helped to fuel other brain functions. Chen speculated that about one-third of all ATP produced in a fully awake brain is used for maintaining healthy cells and scientists from the University of Minnesota Medical School are in agreement. They have also suggested that up to a third of the brain’s energy goes towards the cleanup and maintenance of healthy cells in the brain. As technology continues to advance and improve, questions regarding how the brain works and expends energy will, hopefully, be answered.

One thing is certain. Maintaining cells is important for keeping the brain alive and for making sure the biological processes in the brain continue to work, such as supplying the energy for minerals (products of metabolism) to traverse cell membranes. Proper mineral balance is key to reducing inflammation as well as the prevention of strokes, Alzheimer’s, Parkinson’s, and other neurodegenerative diseases.

The Selfish Brain

selfish-brainAccording to a study by evolutionary anthropologists at the University of Cambridge, the human brain chooses to take care of its own requirements before allowing organs and muscles involved in physical activities to receive what they need to perform. A larger brain requires a higher energy expenditure and, genetically speaking, the human brain has evolved in size to the point of ranking its own cerebral power needs over the requirements necessary for peak physical performance. Researchers suggest this is due to the ever increasing and preferable need of homo sapiens to think fast, rather than run fast, in order to survive and thrive. They believe this change in preference caused a preferential shift of glucose supplying the brain first.

During vigorous exercise, muscles compete with the brain for available glucose and oxygen much as they would if an individual was running and sprinting while hunting down prey in the hunter-gatherer days. However, the longer and harder an individual continues to “exercise” vigorously (run, sprint, or jump) the harder it becomes to continue to do so. This is because the brain expends more energy during exercise and, at some point, mental performance is going to take precedence over physical strength and endurance. The cognitive function takes priority which is why a hard sprint can’t last forever. In every way, this is the brain’s survival mechanism kicking in. While it has long been known that there are energy-related “trade-offs” between organs and tissues, as far as energy deficits go, it is still not completely clear on how the brain has developed a built-in safety gauge that will not allow it to ignore its own energy needs over physical output requirements even while the body wastes away from long-term malnutrition or starvation.

Brain Loops

brain-loopsJames Kozloski, a computer neuroscientist at IBM’s Thomas J. Watson Research Centre, has been quoted as saying, “The brain consumes a great amount of energy doing nothing. It’s a great mystery of neuroscience.”

Kozloski says the brain repeatedly loops through well-established pathways, continually exploring the senses (what’s being sensed physically or energetically in the present moment), the emotions (what something means emotionally), and behaviors (what can be done) constantly repeating this process or cycle even while the body is at rest. He calls this the “Grand Loop” and believes this constant exploration explains why the brain uses so much energy. In fact, he ran his hypothesis through a neural tissue stimulator at IBM which has the ability to simulate an accurate model of the way nerve cells fire in the human brain and, after doing so, suggested the “Grand Loop” could explain how humans draw from past experiences to form a reaction to new ones by consulting “memory” in these three areas. By consulting memory the brain will then have a means of knowing what to decide to do next.

Kozloski believes degenerative brain diseases, such as Huntington’s, are caused by defective genes knocking the “Grand Loop” off course. In studies unrelated to Kozloski’s research, Tinnitus, a constant ringing in one or both ears, is thought to be a symptom or syndrome of a faulty brain loop sending improper messages to the hearing center of the brain.

Additionally, Kozloski believes approximately 90 percent of the energy used by the brain still remains unaccounted for. So far, there are many who would agree with him.









Image Credits:

3d Brain connects the green energy [ID 45919285 © 3dalia | Dreamstime.com]

Heart rate rhythm [ID 66811942 © Sasinparaksa | Dreamstime.com]

Ball-and-stick model of the adenosine triphosphate molecule, also known as ATP. [By Jynto – Own work, Public Domain, https://commons.wikimedia.org/wiki/File:Adenosine-triphosphate-3D-balls.png]

Brain Energy [ID 41043261 © Kts | Dreamstime.com]

Brain with infinity symbol 3d rendering [ID 83957279 © Petrovv | Dreamstime.com]