As you already know, the brain is one of the most complex and important organs in the human body. It controls most bodily functions and is responsible for thought, feelings, memory, and many other processes.
And to ensure this function it has to spend a lot of energy, in relation to the total size, but not in functional ones. The main efficiency of the brain – processing and transmitting information through electrical signals – is very, very expensive in terms of energy use.
The brain is a very active organ that consumes a significant amount of energy. Although it makes up about 2% of body weight, it uses about 20% of the body's total energy.
In an equivalent explanation, this energy our brain consumes is about 0,3 kilowatt hours (kWh) per day for the average adult, or 100 times more than your smartphone uses per day. And it is equal to 260 calories or 1,088 kilojoules (kJ) per day (the average adult's total energy consumption is about 8,700 kJ per day).
The main energy currency of our brain cells is a molecule called adenosine triphosphate (or ATP), which our body produces from sugar and oxygen.
Explanation of the Process
Basal Brain Metabolism (BMR): Most of the calories the brain consumes are used to maintain basal functions, such as maintaining the membrane potential of nerve cells and communicating between them through synapses.
Neuronal Processes: Activities like thinking, learning, memory, and information processing require energy. Even when we are at rest, the brain is constantly active.
Glucose as an Energy Source: The brain primarily uses glucose for energy. Glucose is transported into the brain across the blood-brain barrier. In the event of a lack of glucose, such as during starvation, the brain can also use ketones.
Factors Affecting Energy Consumption: Level of mental activity: More mental activity (intensive study, complex problem solving) can increase energy consumption.
Stress and emotions: Stress and strong emotions can also increase the brain's calorie consumption.
The brain is an energy-intensive organ that requires a significant amount of calories to function optimally. To support this demand, it is important to have a balanced diet that provides enough carbohydrates, fats, and proteins, as well as a lifestyle that includes physical activity and sufficient rest.
Like all other parts of your body, your brain needs energy to function. Simon Laughlin, professor in the department of zoology at the University of Cambridge, explains why the brain needs so much energy, how it uses energy, and how these needs have influenced our evolution.
For the average adult at rest, the brain consumes about 20 percent of the body's energy. The brain's primary function—processing and transmitting information through electrical signals—is very, very expensive in terms of energy use.
The exact percentages are hard to pin down, but we have pretty good estimates of where that energy is going, although it varies by brain region. In the cerebral cortex of mice, about a quarter of the brain's energy goes to maintaining the neurons and glial cells themselves—the processes that all cells go through to stay alive. The remaining 75 percent is used for signaling—the sending and processing of electrical signals across brain circuits. These numbers appear to be very similar in humans.
Most of this energy is consumed in the synapses – the tiny gaps between brain cells where signals are sent and received. There, cells are constantly pumping ions into the gap between cells – exchanging potassium and sodium to create electrical charges. This pumping action is essential for the functioning of brain circuits, but they are very energy-intensive.
Do certain parts or areas of the brain require more energy than others?
Of the two main types of tissue in the brain—gray matter and white matter—gray matter requires much more energy than white matter. White matter, made up of bundles of axons, contains large amounts of myelin, the fatty substance that wraps around axons to insulate them and prevent electrical current from flowing. Because of this insulation, white matter uses about 20 to 25 percent as much energy as gray matter, which is made up of dendrites, cell bodies, and synapse sites.
Some functions require more energy than others. The areas of the brain responsible for processing hearing require more energy than the olfactory system or the areas of the brain responsible for memory. Hearing requires very fast and precise signaling – it would not be beneficial for the sound of danger to be delayed in any way. Relatively slow processes such as smell do not have the same intense energy needs.
Do you use more energy when you think hard or complete complex tasks?
There are two important factors to keep in mind. The brain requires this expensive electrical energy to function. And your brain never shuts down. Even when you're sleeping at night, your brain uses about the same amount of energy as it does during the day. While you're resting, your neurons are constantly communicating, updating each other on what's going on. Their constant alertness is where most of the energy is consumed.
If a specific task demands a particular area of the brain, the area’s energy needs increase. You can see the increase in an fMRI scan – the area will be bright red where the circuits are particularly active. If you’re talking to another person, Broca’s area – the area associated with producing speech – will become more active. Despite what you might assume from the bright colors, the increase in energy is small – about eight percent at most. Unlike the muscles in your leg, where energy consumption can increase three to four times when you’re walking compared to when you’re sitting, the brain requires a relatively steady amount – whether you’re doing something complex like solving a difficult math problem or staring into space. The increase is small, compared to the huge amount of energy it needs as a baseline.
There’s a myth that you only use 10 percent of your brain and a magic pill can unlock the remaining 90 percent. Most of your neurons are relatively silent for long periods of time, waiting to spring into action when they’re triggered. But they’re doing this to stay energy efficient. If you somehow tripled the number of neurons firing at once, your brain’s oxygen needs would increase dramatically, perhaps using as much as your leg muscles do when you sprint.
Did the brain's energy needs influence human evolution?
The brain doesn’t have a reserve of energy to store when it needs it. Unlike muscles, which can store excess carbohydrates, the brain needs a constant supply of oxygen and energy in order to function properly. If the blood supply to the brain is interrupted or cut off — as during a stroke or head injury — neurons quickly begin to shut down. This may seem like a flaw, but it’s an integral part of how the brain works. If the brain contained cells that stored reserve power, those cells would take up space between neurons. This would increase the distance that electrical signals would have to travel, and they would need more energy to do so. Early organisms may have had nervous systems that included these kinds of vaults, but over millions of years of evolution, we sacrificed this reserve power for efficiency. It makes us vulnerable to injury, but it also allows us to take advantage of the brain’s complex circuitry.
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