How much energy do we expend thinking and using our brain?

• But does our brain burn more energy when engaging in mental athletics than it does during other activities, such as watching TV?
• Tracing brain energy consumption can be done using both sugar and oxygen, but oxygen is the more accessible option.
• Tracing oxygen consumption, the brain accounts for about 20% of the body’s energy consumption, despite only representing 2% of its weight.

How do we know?

• We can’t measure brain energy consumption in humans in this way, but we can follow the oxygen, as increased brain activity requires more oxygen.
• Research indeed shows increased mental load (such as performing mental arithmetic, reasoning, or multitasking) is linked to increased oxygen consumption (measured via CO₂ release).

Can we measure oxygen use just in the brain?

• That extra supply of oxygen-rich blood is region specific and can be (literally) channelled with micrometre precision to active neurons.
• Since blood and its oxygen are weakly attracted by magnetic fields, we can use MRI (magnetic resonance imaging), a radiation-free tool, to obtain an, albeit indirect, measure of brain activity.
• But unfortunately, we can’t use MRI to tell us how much energy our brain uses for different mental activities.

So, how much does brain activity increase?

• MRI studies have shown attentively monitoring moving objects compared to passively watching them increases brain activity in our visual cortex by around 1%.
• But interestingly, processing visual information leads to a reduction of activity in auditory areas, meaning we spend less energy processing the sounds in our environment.
• This works the other way around as well: when we attend to auditory information, we reduce our visual processing activity.

Then why do we feel exhausted after too much mental activity?

• Complex mental tasks are typically also emotionally challenging and lead to increased activation of our sympathetic nervous system, ultimately leading to mental and physical fatigue.
• The good news is we don’t have to worry that too much mental activity will drain our brain energy.