The history
From an inner fire to 38 trillion helpers: 2,000 years of working out what happens to a meal.
For centuries people pictured digestion as cooking or an inner fire. Then scientists swallowed tubes, fed metal cages to a kite and watched a living stomach through a gunshot wound. They found acid, enzymes, the first hormone and the gut’s own nerves, and then learned to look inside, to save lives with salt and sugar, and to count the microbes that live in us.
1752
Digestion by juice, shown in a bird
René-Antoine de Réaumur, France
1824
Stomach acid identified as hydrochloric acid
William Prout, UK
1836
First digestive enzyme named (pepsin)
Theodor Schwann, Germany
1902
First hormone (secretin)
William Bayliss and Ernest Starling, UK
1971
Oral rehydration used in a mass emergency
Dilip Mahalanabis, India
200 BCE – 1790
Digestion as an inner fire or cooking, until birds and brave scientists swallowed tubes to test it.
100 BCE
c. 2nd century BCE to 2nd century CE (dates uncertain)
The Charaka Samhita, a founding text of Ayurveda compiled over several centuries, pictures digestion as the work of agni, a fire inside the body. Its main fire, jatharagni, burns in the belly and turns food into the body’s tissues.
Why it mattered. It is one of the oldest ideas of digestion as a process that transforms food, not just a place where food sits.
170
2nd century CE
Galen, a Greek doctor in the Roman Empire, taught that the stomach “concocts” food like a cooking pot, and that the liver turns what it absorbs into blood. His books guided medicine for well over a thousand years.
Why it mattered. It linked the gut and the liver, but as cooking rather than chemistry.
1622
23 July 1622 (published 1627)
Aselli opened a dog that had just eaten and saw white threads on its intestines that oozed a milky liquid. He named them lacteals, milky veins. His book appeared in 1627, after his death.
Why it mattered. Lacteals are how the gut absorbs fat, and they were the first part of the lymph system to be found.
1752
Réaumur fed a tame kite (a bird of prey) small metal tubes with holes, filled with meat. The tubes stopped the meat being crushed, yet when the bird coughed them up the meat had partly dissolved. He also had it swallow sponges on threads to collect its stomach juice.
Why it mattered. It showed that digestion is more than grinding: a juice dissolves food.
1780
The Italian priest and scientist swallowed small linen bags and pierced wooden tubes holding food, and got them back to see what had happened. He collected gastric juice and showed it could digest meat in a glass, outside the body.
Why it mattered. Digestion in a glass proved it is chemistry, not a life force or simple cooking.
1790 – 1860
A window into a living stomach, hydrochloric acid, pepsin and the pancreas.
1822
6 June 1822; book 1833
A musket went off by accident and wounded Alexis St. Martin, a young French-Canadian fur trader. The army surgeon William Beaumont saved him, but the wound healed with an opening from the stomach to the skin. From 1825 to 1833 Beaumont dangled food on silk threads through it and drew off gastric juice, and in 1833 he published his results.
Why it mattered. For the first time a living human stomach could be watched at work, though St. Martin, a poor and dependent patient, had little real choice.
1824
Read 11 December 1823, published 1824
The English doctor and chemist Prout analysed stomach contents and showed that the acid in them is hydrochloric acid, the same acid chemists made from sea salt.
Why it mattered. It named the stomach’s strongest weapon and explained its very low pH.
1836
Schwann found a substance in gastric juice, different from the acid, that breaks down egg white and meat. He named it pepsin, from the Greek word for digestion. It was one of the first enzymes ever described.
Why it mattered. It showed that molecular “scissors”, not acid alone, do the cutting.
1848
1848 to 1856
Bernard, a French physiologist, found that pancreatic juice turns fat into a milky mix and splits it. Without this juice, fat was not digested. His memoir on the pancreas came out in 1856.
Why it mattered. It showed that most of the real work of digestion happens in the small intestine, with juice from the pancreas.
1857
1848 to 1857
Bernard found that the liver makes sugar even in animals fed no sugar, and in 1857 he isolated the starch-like store it comes from, glycogen. Blood from the gut reaches the liver first, which stores and releases what the meal provided.
Why it mattered. It showed that absorbed food is managed by the liver, not simply burned.
1855 – 2000
The gut’s own nerves, the first hormone, Pavlov’s dogs and the fullness hormone.
1862
Meissner 1857; Auerbach 1862 to 1864
Meissner described a net of nerve cells under the gut lining, and Auerbach a second net between the gut’s muscle layers. Together they form the enteric nervous system, which runs the gut’s movements.
Why it mattered. It was the first sign that the gut has its own nervous system.
1898
1896 to 1911
Months after X-rays were discovered, the medical student Cannon fed animals food mixed with bismuth salts, which show up on X-ray film. He watched waves of squeezing push food along, and saw that fear could stop them.
Why it mattered. It was the first time people could watch peristalsis inside a living body.
1899
In dogs, Bayliss and Starling showed that squeezing the intestine at one point makes the gut above it contract and the gut below it relax. This happens even when the nerves from the brain are cut.
Why it mattered. It explained how peristalsis always pushes food the right way.
1902
1902 (word “hormone” coined 1905)
Bayliss and Starling showed that when stomach acid reaches the duodenum, its lining releases a substance into the blood that makes the pancreas pour out juice. They called it secretin. In 1905 Starling coined the word hormone for chemical messengers like this.
Why it mattered. It showed that organs can talk to each other through the blood, and founded endocrinology.
1904
Pavlov showed that the sight, smell and taste of food start gastric juice flowing, through nerves including the vagus nerve. He won the Nobel Prize for his work on the physiology of digestion, years before his famous work on conditioned reflexes.
Why it mattered. It showed that the brain switches digestion on before food even arrives.
1905
Edkins found that an extract of the lower stomach, injected into the blood, made the stomach produce acid. He called the messenger gastrin. Many doubted it until gastrin was purified in 1964.
Why it mattered. It added the stomach’s own acid switch to the list of gut hormones.
1928
Ivy and Oldberg found that an extract of the upper intestine made the gallbladder of dogs contract. They named the messenger cholecystokinin, “the gallbladder mover”. It was later shown to call for pancreatic enzymes too.
Why it mattered. It explained how bile arrives just when fat does.
1987
1986 to 1987
Mojsov and Habener identified the active form of GLP-1, a hormone from the lower gut, and with Drucker showed it boosts insulin. Holst’s team in Copenhagen found the same independently. Drugs that copy it now treat diabetes and obesity.
Why it mattered. It showed that a gut hormone helps control blood sugar and appetite, and led to a new family of medicines.
1998
The anatomist Michael Gershon, who studied serotonin in the gut, wrote a popular book, The Second Brain, about the enteric nervous system. Its hundreds of millions of neurons can run the gut on their own.
Why it mattered. It brought the gut’s own nervous system and the gut–brain axis to a wide public.
1900 – today
Friendly bacteria, a map of the microbiome, and correcting old numbers.
1907
1905 to 1907
The Bulgarian student Grigorov identified the bacterium that sours yoghurt. Metchnikoff, a Nobel prize-winning immunologist at the Pasteur Institute, argued that such friendly bacteria could keep the gut healthy.
Why it mattered. It was an early step towards seeing gut microbes as partners, not just germs.
2007
Launched 2007; first results June 2012
The US National Institutes of Health launched the Human Microbiome Project in 2007. Scientists sampled 242 healthy adults at many body sites and read the genes of their microbes. In 2012 they published a map of the normal microbes of healthy people, with the gut among the richest places.
Why it mattered. It turned the gut’s bacteria from a curiosity into a major field of science.
2014
Textbooks often said the inside of the gut had the area of a tennis court. Helander and Fändriks measured it carefully under the microscope and found about 30 to 40 square metres, about 32 on average, almost all of it in the small intestine.
Why it mattered. It fixed a famous wrong number, though the gut’s folds, villi and microvilli still make its surface huge.
2016
People had long said bacteria outnumber our cells ten to one, a rough guess from 1972. Sender, Fuchs and Milo worked it out again: about 38 trillion bacteria and 30 trillion human cells in a typical adult, most of the bacteria in the colon.
Why it mattered. We are roughly half microbe by cell count, not ninety percent.
1950 – 1975
Flexible fibre-optic scopes for the stomach and the colon.
1957
February 1957
With the physicist Wilbur Peters and the student Larry Curtiss, the South African-born doctor Hirschowitz made a flexible scope from a bundle of thin glass fibres that carry light round bends. He passed it down his own throat first.
Why it mattered. The flexible endoscope let doctors look inside the gut without surgery.
1969
June and September 1969
Using a long fibre-optic colonoscope made in Japan, the surgeons Wolff and Shinya examined the whole colon. Three months later Shinya used a wire snare he designed to remove a polyp through the scope.
Why it mattered. Finding and removing polyps early is how colonoscopy prevents colon cancer.
1960 – 2006
Salt and sugar against diarrhoea, fibre, and a germ that causes ulcers.
1960
August 1960
At a meeting in Prague, the biochemist Robert Crane proposed that gut lining cells pull in glucose and sodium together, through the same doorway. Many experts doubted him, but he was right.
Why it mattered. It is why a drink of salt and sugar can rehydrate a person even during severe diarrhoea.
1971
June to August 1971 (published 1973)
During the Bangladesh Liberation War, cholera broke out in refugee camps and drips ran short. The Kolkata doctor Dilip Mahalanabis and his team handed out packets of salt, sugar and baking soda to mix with water. About 3.6 percent of about 3,700 patients died, against about 30 percent usually.
Why it mattered. It proved oral rehydration works in a real disaster, and ORS has since saved millions of lives.
1971
Burkitt, a surgeon who had worked in Uganda, noticed that bowel cancer and other bowel diseases were rare in rural Africa. Gathering reports from rural hospitals in Africa and India, he suggested that fibre-rich food, which makes bigger, faster-moving stools, might protect the bowel.
Why it mattered. It started decades of research linking fibre to gut health.
1982
1979 to 1982
The pathologist Robin Warren saw curved bacteria in inflamed stomach linings, where experts said no germ could live. The young doctor Barry Marshall joined him, and in 1982 the cultures finally grew after plates were left over the long Easter weekend. The germ was named Helicobacter pylori.
Why it mattered. It opened the way to curing most ulcers with antibiotics.
2005
Marshall and Warren won the Nobel Prize in Physiology or Medicine for discovering Helicobacter pylori and its role in gastritis and ulcers. Marshall had even drunk a culture of the germ in 1984 to prove it could make a healthy stomach ill.
Why it mattered. It honoured an idea most experts had laughed at twenty years earlier.
By the numbers
Estimates rebuilt with modern methods by Black et al. (Journal of Global Health, 2019): a fall of about 80 percent as oral rehydration, zinc, clean water and vaccines spread. WHO’s latest figure is about 444,000 a year.
Did you know?
About 9 litres of water go into your gut every day, but only about 2 of them come from food and drink. Nearly all of it is taken back.
The inside of your gut is about 32 square metres, not the “tennis court” of old textbooks.
A meal usually takes about a day or two to go from mouth to toilet, and most of that time is spent in the colon.
Your colon holds about 38 trillion bacteria, roughly as many as the cells of your own body.
Secretin, a gut hormone found in 1902, was the first hormone ever discovered.
The people
1802 – 1880 · Fur trader and experimental subject · Quebec, Canada
Lived with a hole in his stomach for 58 years; much of what we first learned about gastric juice came from his body.
1785 – 1853 · Army surgeon · Connecticut, USA
Ran hundreds of experiments through St. Martin’s stomach opening and published them in 1833.
1813 – 1878 · Physiologist · France
Showed that the pancreas digests fat and that the liver stores sugar as glycogen.
1849 – 1936 · Physiologist · Russia
Won the 1904 Nobel Prize for showing how nerves switch on the digestive glands.
1934 – 2022 · Paediatrician · West Bengal, India
Proved in a 1971 refugee camp that oral rehydration could save lives on a huge scale.
born 1947 · Chemist · Skopje, North Macedonia (worked in the USA)
Identified the active form of the gut hormone GLP-1; shared the 2024 Lasker Award.
Where it happened
Sources
Dates marked “c.” are approximate, and historians sometimes disagree about who was first. If you spot a mistake, tell us.
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