Our brains have an upper limit on how much they can process at once due to a constant but limited energy supply, according to a new UCL study using a brain imaging method that measures cellular metabolism.
The study, published in the Journal of Neuroscience, found that paying attention can change how the brain allocates its limited energy; as the brain uses more energy in processing what we attend to, less energy is supplied to processing outside our attention focus.
Explaining the research, senior author Professor Nilli Lavie (UCL Institute of Cognitive Neuroscience) said: “It takes a lot of energy to run the human brain. We know that the brain constantly uses around 20% of our metabolic energy, even while we rest our mind, and yet it’s widely believed that this constant but limited supply of energy does not increase when there is more for our mind to process.
“If there’s a hard limit on energy supply to the brain, we suspected that the brain may handle challenging tasks by diverting energy away from other functions, and prioritising the focus of our attention.
“Our findings suggest that the brain does indeed allocate less energy to the neurons that respond to information outside the focus of our attention when our task becomes harder. This explains why we experience inattentional blindness and deafness even to critical information that we really want to be aware of.”
The research team of cognitive neuroscientists and biomedical engineers measured cerebral metabolism with a non-invasive optical imaging method. In this way they could see how much energy brain regions use as people focus attention on a task, and how that changes when the task becomes more mentally demanding. They used broadband near-infrared spectroscopy to measure the oxidation levels of an enzyme involved in energy metabolism in brain cells’ mitochondria, the energy generators that power each cell’s biochemical reactions.
The researchers employed their technique to measure brain metabolism in different regions of the visual cortex in the brains of 18 people as they carried out visual search tasks that were either complex or simple, while sometimes also presented with a visual distraction that was irrelevant to the task.
They identified elevated cellular metabolism in the brain areas responsive to the attended task stimuli as the task was more complex, and these increases were directly mirrored with reduced cellular metabolism levels in areas responding to unattended stimuli. This push-pull pattern was closely synchronised, showing a trade-off of limited energy supply between attended and unattended processing.
Co-author Professor Ilias Tachtsidis (UCL Medical Physics & Biomedical Engineering) said: “By using our in-house developed broadband near-infrared spectroscopy, an optical brain monitoring technology we developed at UCL, we were better able to measure an enzyme in the mitochondria (the power factory of the cells) that plays an integral part in metabolism.”
First author, PhD student Merit Bruckmaier (UCL Psychology & Language Sciences) said: “Using these methods, our conclusions about brain energy usage are more direct and telling than in past studies using fMRI imaging methods that measure cerebral blood oxygenation levels instead of an intracellular marker of metabolism.”
Professor Lavie said: “In this way, we have managed to connect people’s experience of brain overload to what’s going on inside their neurons, as high energy demands for one purpose are balanced out by reduced energy use related to any other purpose. If we try to process too much information we may feel the strain of overload because of the hard limit on our brain capacity.
“During recent months, we’ve heard from a lot of people who say they’re feeling overwhelmed, with constant news updates and new challenges to overcome. When your brain is at capacity, you are likely to fail to process some information. You might not even notice an important email come in because your child was speaking to you, or you might miss the oven timer go off because you received an unexpected work call. Our findings may explain these often-frustrating experiences of inattentional blindness or deafness.”
The study was supported by the Economic and Social Research Council, Toyota Motor Europe, and Wellcome.
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- Four stills from the study's visual search task, including swirls and a checkerboard pattern used as visual distractions.
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Chris Lane
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Email: chris.lane [at] ucl.ac.uk
Medically reviewed by Danielle Hildreth, RN, CPT — Written by Kirsten Nunez on October 28, 2020
After a long day at work or school, it’s common to feel tired. This is especially true if you do a lot of mental tasks, which can zap your physical energy.
In turn, you may wonder if the act of thinking burns more calories than mindless activities like watching television. After all, if you’re tired after thinking, doesn’t that mean you’re burning energy?
Yes, but only to a certain extent. Although thinking hard uses calories, the energy burn is minimal. It’s not enough to burn fat and cause weight loss.
The brain is also an organ, not a muscle. Exercise can grow your muscles, which makes them burn more calories. This concept doesn’t apply to the brain.
Still, working out your brain has other benefits. It can boost cognitive functions like memory and focus, and slow down cognitive decline.
To learn how thinking affects your calorie burn, read on.
First, it’s important to understand how your body uses calories.
You burn energy even when you’re sedentary. For example, as you read this article, your body is using calories to support:
- breathing
- blood flow
- body temperature
- digestion
Your brain also uses energy to perform normal functions. Although it makes up 2 percent of your body weight, it burns 20 percent of the energy you consume. This metabolic activity is generally consistent, regardless of what you’re doing.
In fact, your brain burns more energy at rest than a human thigh while running.
Specifically, your brain gets energy from glucose. Glucose comes from the food you eat. The glucose enters your bloodstream, then travels to your brain.
Here, it’s used to make adenosine triphosphate (ATP), a form of energy. Your nerve cells need ATP to communicate and carry out cognitive tasks.
When you mentally exert yourself, your brain uses more energy to do the task. But it’s not much more than normal brain activities.
To put things into perspective, a 155-pound person burns about 23 calories per 30 minutes of sleep. Your brain performs the most basic functions during this time. Here’s how other activities compare:
Does studying burn calories?
If a 155-pound person studies on a computer, they’ll burn about 51 calories per 30 minutes. If they study at a desk or sit in class, they’ll burn about 65 calories per 30 minutes.
Does reading burn calories?
The same 155-pound person burns about 42 calories in 30 minutes of reading while sitting. If they read while standing, they’ll burn about 47 calories per 30 minutes.
Though thinking burns calories, it’s not enough to cause weight loss. The increase in calorie burn is small.
You need to burn 3,500 calories to lose 1 pound. In 1 week, this equals out to 500 calories a day.
If you were to solely depend on mental tasks, you’d need to exert yourself for many hours.
For example, as mentioned earlier, a 155-pound person burns about 42 calories in 30 minutes of reading. This person would have to read for almost 6 hours to burn 500 calories in a day.
Granted, occasionally moving around will increase calorie burn per hour. But it still wouldn’t be enough to cause weight loss.
It’s still beneficial to work out your brain. Examples of brain exercises and their benefits include:
- Brain teasers. A 2013 study found that brain-training games boost executive functions and memory in healthy young adults. Brain puzzles, like Sudoku or crossword puzzles, can challenge your brain.
- Learning something new. Reading, learning a new hobby, or watching documentaries can keep your brain active.
- Playing or listening to music. According to a 2013 article, these activities help boost memory and executive function.
- Socializing. Maintaining healthy social relationships can support cognitive performance, according to a 2017 study. Consider joining a book club, fitness class, or volunteer group.
- Regular exercise. Physical activities like walking or gardening are beneficial for your body and brain. In a 2018 study, researchers found that aerobic exercise can support neuronal health and cognitive function.
All brain workouts are not equal, though. It’s important to continuously challenge yourself. If you frequently repeat the same activities, your brain won’t be as active.
Also, it’s a myth that age-related cognitive decline is inevitable. According to a 2015 article, lifestyle factors can protect and boost brain function. This includes the activities above, along with eating well and avoiding smoking.
It’s normal to feel tired after going to the gym. Likewise, it’s typical to get tired after work or school, but it’s not the same thing.
Glucose is stored as glycogen in your muscles. When you exercise, your muscles break down glycogen and use glucose to move.
Eventually, your muscles run out of energy and you feel tired. It also becomes difficult for your central nervous system to contract your muscles. The result is physical fatigue.
Mental activity also uses glucose, but it doesn’t require your muscles to break down glycogen. Instead, it causes mental exhaustion, which increases your perceived exertion of physical tasks, according to a 2017 review.
Your brain burns calories to perform basic functions. It burns a bit more if you think really hard, but it’s not enough to make you lose weight.
That doesn’t mean exercising your brain has no benefits. Activities like listening to music, doing puzzles, and learning new hobbies can all boost your cognitive function.
If you want to burn more calories, focus on exercise and healthy eating. Exercising your largest muscles, like your glutes and biceps, will increase your calorie burn.
Last medically reviewed on October 28, 2020
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