Ikigai Daily
Menu
WonderBrain fact

An adult brain weighs about 1.3 kg, roughly 2% of body weight, yet it uses about 20% of the energy the body burns at rest. Most of that energy pays for the brain's constant background activity, not for the extra effort of thinking hard.

Neuroscience

MindScience

The story behind the card

The brain's main fuel is glucose, the sugar carried in the blood; during long fasts it can also run on ketones, which the liver makes from fat. Much of its energy powers tiny pumps that move charged particles in and out of nerve cells, keeping each one ready to send a signal. Brain scans by the American neurologist Marcus Raichle and others showed that a hard mental task raises the brain's total energy use only slightly, often by less than 5%, and he called the constant background activity the brain's “dark energy”. In most vertebrates the brain takes 2 to 8% of resting energy, so humans are unusual. The share peaks in early childhood: a 2014 study estimated that at about four years old the brain uses around two thirds of the body's resting energy.

Türkçe oku
Yetişkin bir insanın beyni yaklaşık 1,3 kilogramdır, yani vücut ağırlığının %2'si kadar; ama vücudun dinlenirken harcadığı enerjinin yaklaşık %20'sini tek başına kullanır. Bu enerjinin çoğu, zor bir konuyu düşünmenin getirdiği ek yüke değil, beynin hiç durmayan arka plan etkinliğine gider.

Beynin asıl yakıtı kandaki şeker, yani glikozdur; uzun süre aç kalındığında karaciğerin yağlardan ürettiği ketonları da kullanabilir. Enerjinin önemli bir kısmı, sinir hücrelerinin zarındaki minik pompalara gider. Bu pompalar elektrik yüklü parçacıkları hücrenin içine ve dışına taşır, böylece her hücre sinyal göndermeye hazır kalır. Amerikalı nörolog Marcus Raichle ve başka araştırmacılar beyin taramalarıyla, zor bir zihinsel görevin beynin toplam enerji tüketimini çok az, çoğu zaman %5'ten bile az artırdığını gösterdi. Raichle, beynin hiç durmayan bu arka plan etkinliğine “karanlık enerji” adını verdi. Omurgalıların çoğunda beynin payı, dinlenirken harcanan enerjinin %2'si ile %8'i arasındadır; bu açıdan insan bir istisnadır. Bu pay en yüksek düzeyine erken çocuklukta ulaşır: 2014'te yayımlanan bir çalışmaya göre dört yaş civarında beyin, vücudun dinlenirken harcadığı enerjinin yaklaşık üçte ikisini kullanır.

Sources and further reading

Image details

Illustration of a brain; not a medical scan.

Background reading: Brain metabolism
Get the mobile appAbout our cards

Related cards

These cards share a subject or reading mode with the card above.

WonderScience

Deliberately smiling can make people feel slightly happier. In 2022 a study of nearly 4,000 people in 19 countries found that posing a smile, or copying a smiling face, raised reported happiness a little; a subtler method, holding a pen between the teeth, gave less clear results.

The idea that the face can feed back into feelings, called the facial feedback hypothesis, goes back to Charles Darwin and William James. In a famous 1988 experiment, Fritz Strack and colleagues had people hold a pen in their teeth, which forces a smile, or between their lips, which prevents one, and those in the first group rated cartoons as funnier. In 2016, seventeen laboratories repeated that experiment exactly and found no effect. A 2019 review of 138 studies concluded that the effect is real but small and varies from study to study. The 2022 Many Smiles Collaboration, led by Nicholas Coles and designed jointly by supporters and sceptics of the idea, found that deliberate smiling could both strengthen and start feelings of happiness, while the pen method remained inconclusive.

Ikigai DailyRead card
WonderScience

Your gut and brain communicate through nerves, hormones, immune signals and microbes, which is why stress and digestion can affect each other.

Scientists call this link the gut–brain axis. The gut has its own network of about 500 million nerve cells, the enteric nervous system, sometimes called the “second brain”, which can run digestion largely by itself. It is connected to the brain mainly through the vagus nerve, a long nerve running from the brainstem down to the abdomen, and most of its fibres carry messages upwards, from the gut to the brain. More than 90 per cent of the body's serotonin, a chemical messenger, is found in the gut. Gut bacteria play a part too: in a 2004 Japanese study, mice raised entirely free of germs overreacted to stress. Stress can speed up or slow down the gut, one reason it often worsens irritable bowel syndrome, a common gut disorder. Most research on the microbes' role has been in animals, and human evidence is still limited.

Ikigai DailyRead card
WonderScience

A placebo is a dummy treatment, such as a sugar pill. In pain studies, placebos can reduce the pain people report and also dampen activity in the brain areas that process pain, even though they contain no active drug.

The word placebo is Latin for “I shall please”. One of the first clues that the effect is physical came in 1978, when Jon Levine and colleagues studied patients recovering from wisdom tooth removal. They gave them a placebo and naloxone, a drug that blocks the effect of endorphins, the body's own painkilling chemicals. In patients who had responded to the placebo, naloxone increased the pain, which suggested that the placebo relief came from endorphins. In 2004 Tor Wager and colleagues used brain scans to show that a placebo painkiller reduced activity in pain-processing regions during a painful stimulus. Expectation works the other way too: in the nocebo effect, from the Latin for “I shall harm”, expecting harm makes symptoms worse. None of this means the pain was imaginary, or that placebos can replace effective treatment.

Ikigai DailyRead card
WonderScience

Memory does not work like a recording. Each time we recall an event, the brain rebuilds it from stored fragments, filling gaps with what we know and what we have heard since, so a memory can change without our noticing.

Psychologists call this reconstructive memory. In his 1932 book Remembering, the British psychologist Frederic Bartlett described asking people to retell “The War of the Ghosts”, a Native American folk tale unfamiliar to them; each retelling drifted towards what felt familiar in their own culture. In 1974 Elizabeth Loftus and John Palmer showed people films of car accidents and asked how fast the cars were going when they “hit” or “smashed” into each other. Those asked with “smashed” gave higher speeds, and a week later were more likely to remember broken glass that was never there. This is known as the misinformation effect. It matters in court: according to the Innocence Project, mistaken eyewitness identification played a part in most US convictions later overturned by DNA evidence.

Ikigai DailyRead card
WonderScience

Predictive processing, an influential theory in neuroscience, says the brain does not simply wait for information from the senses. It keeps guessing what is out there and uses the incoming signals mainly to check and correct those guesses.

The idea goes back to the German physicist Hermann von Helmholtz, who argued in the 1860s that seeing involves “unconscious inference”: the brain fills in what the eyes cannot fully tell it. In 1999 the researchers Rajesh Rao and Dana Ballard turned this into a computer model of vision. Higher brain areas send predictions down, and lower areas send back only the difference between prediction and input, called the prediction error. The philosopher Andy Clark and the neuroscientist Karl Friston later helped develop it into a broad theory of the mind. A well-known illustration is the hollow-mask illusion: the inside of a mask still looks like a face bulging outwards, because the brain's expectation of a normal face outweighs the evidence. The theory is widely used but still debated, and critics ask how some of its claims could be tested.

Ikigai DailyRead card
Next card in the archiveCyclic sighing, breathing in twice through the nose and then out slowly and fully, lifted mood more than mindfulness meditation in a 2023 Stanford study. Of four methods that 108 adults practised for five minutes a day for a month, it gave the biggest rise in good mood and the biggest drop in resting breathing rate.Read next →