One of the body’s smallest organs, the human brain, makes up around 2% of our body weight. Yet it consumes approximately 20% of the body’s energy. This makes the brain one of the most energy hungry organs in the body.
These facts are derived from studies that measure how much oxygen and glucose the brain and body consume. Using metabolic and brain imaging techniques, researchers can track how much fuel is used by different parts of the body.
Why does the brain need so much energy?
So where does all this energy go? A large part of the answer lies in how neurons communicate. The brain contains about 86 billion neurons. These specialised cells send electrical signals and communicate with other neurons, muscles, and gland cells using chemical messengers called neurotransmitters.
Each time a neuron sends an electrical signal, tiny, charged particles move across its cell membrane. To keep this system working, neurons must constantly maintain and restore delicate differences in these charged particles, known as ion gradients, across the membrane.
Maintaining these gradients is surprisingly metabolically expensive. After each electrical signal, the neuron must actively pump these ions back across the membrane to reset itself for the next message. These pumps run continuously and consume large amounts of energy.
The hidden cost of neural signalling
Electrical signalling is just one part of the story. When an electrical signal reaches the end of a neuron, it triggers the release of chemical messengers called neurotransmitters. These molecules cross a tiny gap between neurons known as the synapse, allowing one neuron to pass information to the next.
This chemical communication is also metabolically expensive. Neurons must produce neurotransmitters, package them into tiny vesicles, release them at the synapse, and then recycle or break them down after the signal has been transmitted.
Multiply these processes across 86 billion neurons and more than 100 trillion synapses, working continuously even when you are resting, the brain’s enormous energy demands begin to make sense.
Why evolution favoured such an expensive organ
This raises an interesting question: if the brain is so metabolically expensive to run, why did evolution favour such a costly organ?
The answer is that brains provide enormous advantages for flexible behaviour. Organisms with relatively simple nervous systems can rely on reflexes, or automatic responses to stimuli. But organisms with larger and more complex brains can do much more. They can learn from past experiences, anticipate future events, plan actions, and adapt to changing environments.
In other words, the brain’s main role is not just thinking in the abstract. Its job is to guide behaviour in ways that increase the chances of survival and reproduction. By integrating information from the senses and predicting what might happen next, the brain helps organisms make better decisions about how to act.
Energy efficiency and the predictive brain
Interestingly, the brain’s high energy cost may also help explain how it operates. Because neural activity is metabolically expensive, the brain cannot afford to process every piece of information in full detail. Instead, many scientists believe the brain has evolved strategies to be efficient.
One influential idea in neuroscience is that the brain constantly predicts incoming sensory information, allowing it to process the world more quickly and with less energy.
This link between brain activity and energy use is also important for neuroscience research. Many brain imaging methods, such as PET and fMRI, do not measure neural activity directly. Instead, they detect changes in metabolic activity, glucose consumption, blood flow, and oxygen use, which serve as indirect indicators of how active different parts of the brain are.
Thinking isn’t free
So the next time you feel mentally tired, it may help to remember that thinking is not free. Even when you are resting, your brain is quietly consuming a remarkable share of your body’s energy – about one in five calories – to keep billions of neurons ready to process the world.
Bradley Jack was a 2024 ACT Young Tall Poppy recipient.
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