doodledigest00WHAT THE SCIENCE SAYS
A meal travels about 5 metres before it is done.
On the way, it is chewed, churned, soaked in digestive juices and passed across a lining that is folded many times over. Here is what each stop does, using US government health guidance and published measurements.
Follow a meal01Principles
02THE SHORT ANSWER
Digestion is two jobs at once.
Breaking food into small parts, and absorbing them.
THE MOUTH
Saliva moistens food so it slips down, and it carries an enzyme that starts breaking down starch before you have even swallowed.
Follow a mealTHE SMALL INTESTINE
Juices from the pancreas, liver and intestine finish breaking down proteins, carbohydrates and fats here, and the small intestine absorbs most of the nutrients.
See the numbersTHE LARGE INTESTINE
The large intestine absorbs water and turns what is left into stool. Its bacteria break down remaining nutrients and make vitamin K.
The journeyGeneral information only, not medical advice. For advice about your own health, speak to a doctor, pharmacist or other qualified health professional.
03FOLLOW A MEAL
Choose a part of the digestive tract to see what it does to your food. The bar shows how much of the breaking down and absorbing happens at that stop.
Based on the NIDDK guide to the digestive system. This is general information, not medical advice.
04TWO NUMBERS WORTH KNOWING
One figure on the size of the gut's lining,
one on the bacteria that live with us.
≈ 32 m²
Inner surface of the digestive tract
Textbooks often said 260 to 300 square metres, about a tennis court. A 2014 recalculation put it at about 32, roughly half a badminton court, with about 2 in the large intestine.
SURFACE AREA · SCAND J GASTROENTEROL 2014
≈ 1:1
Bacteria to human cells
The old claim was 10 bacteria for every human cell. A 2016 estimate for a 70 kg man found about 38 trillion bacteria and 30 trillion human cells, with the bacteria weighing about 0.2 kg.
BACTERIA · PLOS BIOLOGY 2016
The same 2014 review found the gut from mouth to anus is about 5 metres long, two thirds of it small intestine, though it varies considerably between people.
05HOW LONG IT TAKES
Transit times are measured with a swallowed sensor capsule that records where it is as it travels. They vary a lot from person to person.
Peristalsis, the squeeze-and-relax wave of gut muscle, moves food and mixes it at every stage after you swallow.
The stomach mixes food with its juices, then slowly empties the mixture, called chyme, into the small intestine.
In one capsule study, whole-gut transit in 26 healthy adults was about 36 hours.
A study of 215 healthy volunteers found transit times differed with sex, age, country and the way the test was run.
06NERVES AND HORMONES
Hormones made in the lining of the stomach and small intestine tell the body when to release digestive juices and send signals to the brain that you are hungry or full.
The gut wall also holds its own network of nerves, the enteric nervous system. When food stretches the walls, these nerves speed up or slow down the movement of food and the release of juices. Seeing or smelling food can make your mouth water before you take a bite.
07Questions
A 2014 review of measurements put the length from mouth to anus at about 5 metres, two thirds of it small intestine, with considerable variation between people.
No. That figure appears in textbooks, but a 2014 recalculation found the inner surface averages about 32 square metres, roughly half a badminton court.
A 2016 estimate found the numbers are of the same order: about 38 trillion bacteria and 30 trillion human cells in a 70 kg reference man.
The NIDDK says the small intestine absorbs most of the nutrients in food. The large intestine mainly absorbs water and turns waste into stool.
It varies a lot. In one sensor-capsule study, whole-gut transit in 26 healthy adults was about 36 hours; a larger study found times differ with sex, age and test method.
08Sources
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