Ikigai Daily
Menu
WonderFact

At rest, a kilogram of skeletal muscle burns about 13 kilocalories a day, while a kilogram of body fat burns about 4.5, according to estimates for younger adults. That is a difference of about 8.5 kcal per kilogram. The figures describe tissue at rest, not energy used during exercise.

Resting-energy-expenditure studies (Elia; Wang et al.)

HealthScience

The story behind the card

Muscle cells keep working even at rest, while fat cells mostly store energy. Swapping a kilogram of fat for a kilogram of muscle would therefore add only about 8.5 kilocalories (the “calories” on food labels) to a day's resting energy use, a tiny share of a typical daily need of around 2,000. The heavy users are the organs: per kilogram, the liver and brain burn roughly 200 to 240 kcal a day, and the heart and kidneys about 440. Together they make up only around 5 per cent of body weight, yet they account for well over half of the energy the body uses at rest. The figures come from work by the nutrition scientist Marinos Elia, later refined by Zimian Wang and colleagues.

Türkçe oku
Genç yetişkinler için yapılan tahminlere göre bir kilo iskelet kası dinlenirken günde yaklaşık 13 kilokalori, bir kilo yağ dokusu ise yaklaşık 4,5 kilokalori harcar. Aradaki fark kilo başına günde 8,5 kcal kadardır. Bu rakamlar spor sırasında harcanan enerjiyi değil, dokuların dinlenirken harcadığını gösterir.

Kas hücreleri dinlenirken de çalışmayı sürdürür; yağ hücreleri ise çoğunlukla enerji depolar. Bu yüzden bir kilo yağın yerini bir kilo kasın alması, günlük dinlenme harcamasına yalnızca 8,5 kilokalori kadar ekler (besin etiketlerindeki “kalori” budur). Günde ortalama 2.000 kilokalori harcayan biri için bu çok küçük bir paydır. Asıl büyük tüketiciler organlardır: Kilogram başına karaciğer ve beyin günde kabaca 200-240, kalp ve böbrekler ise yaklaşık 440 kilokalori harcar. Bu organlar vücut ağırlığının yalnızca yüzde 5 kadarını oluşturur, ama vücudun dinlenirken harcadığı enerjinin yarısından çoğunu kullanır. Bu rakamlar beslenme bilimci Marinos Elia'nın çalışmalarına dayanıyor; Zimian Wang ve ekibi daha sonra hesapları geliştirdi.

Sources and further reading

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

A 2016 estimate found that a typical adult man weighing 70 kg has about 30 trillion human cells and 38 trillion bacteria. That overturned the old claim that bacteria outnumber our own cells ten to one: the two numbers are roughly equal.

The estimate comes from Ron Sender, Shai Fuchs and Ron Milo of the Weizmann Institute of Science in Israel. Their reference man is 20 to 30 years old, 170 cm tall and weighs 70 kg. The ten-to-one figure, they found, went back to a single rough calculation from 1972 that had been repeated for decades. Almost all the bacteria live in the colon, the last part of the gut, and together they weigh only about 0.2 kg. Among human cells, red blood cells dominate the count, making up about 84 per cent, while fat and muscle cells, far fewer but much larger, make up most of the body's cell mass. The balance also shifts through the day: a single trip to the toilet empties a quarter to a third of the colon's contents.

Ikigai DailyRead card
WonderScience

An adult body replaces about 330 billion cells every day, around 80 grams of cells, according to a 2021 estimate by scientists at the Weizmann Institute of Science. Close to nine in ten of those cells are blood cells.

Ron Sender and Ron Milo combined the number, mass and lifespan of every major cell type in the body. Short-lived cells dominate the count. Red blood cells live about four months, and neutrophils, the most common white blood cells, only days, so the bone marrow must make vast numbers of them every day. By mass, the largest shares come from blood cells and from the cells lining the gut, which are shed and replaced within days. At this pace, a number of cells equal to all the roughly 30 trillion human cells in the body is replaced in about three months. That does not mean the whole body is renewed: most nerve cells in the brain last a lifetime, and heart muscle cells are replaced only very slowly.

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
Next card in the archiveThe appendix may be a “safe house” for helpful gut bacteria, researchers at Duke University proposed in 2007. After severe diarrhoea flushes out the intestine, bacteria sheltering there could help to re-seed it.Read next →