How digestion works: the journey from mouth to large intestine

Whenever you eat, you set off a process that involves a long, muscular tube, three solid organs, a network of nerves and hormones, and the bacteria that live in your gut. This guide follows food on its journey, explains what each organ adds, and looks at what happens when part of the system does not work as it should. Most of it draws on the US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).

The parts of the digestive system

The digestive system is made up of the gastrointestinal (GI) tract and three solid organs: the liver, the pancreas and the gallbladder, NIDDK explains. The GI tract is a series of hollow organs joined in a long, twisting tube from the mouth to the anus: the mouth, the food pipe (oesophagus), the stomach, the small intestine, the large intestine and the anus.

The small intestine has three parts: the duodenum, the jejunum and the ileum. The large intestine includes the appendix, the caecum, the colon and the rectum. Nerves, hormones, blood and the bacteria that live in the gut all help too.

Why digestion matters

Your body needs the nutrients in food and drink to work properly: proteins, fats, carbohydrates, vitamins, minerals and water. Digestion breaks these into parts small enough to absorb and use for energy, growth and cell repair. According to NIDDK:

• proteins break down into amino acids;
• fats break down into fatty acids and glycerol;
• carbohydrates break down into simple sugars.

The journey, step by step

Food moves through the GI tract by peristalsis: the muscle behind the food contracts and squeezes it forward, while the muscle in front relaxes to let it through. Along the way, each organ adds something.

Mouth. Chewing breaks food up, and saliva moistens it so it moves easily. Saliva also contains an enzyme that starts to break down starches. When you swallow, a small flap of tissue, the epiglottis, folds over the windpipe so you do not choke.

Food pipe. Once you start to swallow, the process becomes automatic. At the bottom, a ring of muscle relaxes to let food into the stomach, then usually stays closed to stop the stomach's contents flowing back. Our guide to eating late and digestion explains what happens when they do.

Stomach. Glands in the stomach lining make stomach acid and enzymes that break down proteins, and the stomach muscles mix them with the food. The stomach then empties its contents, now called chyme, slowly into the small intestine.

Pancreas. The pancreas makes a digestive juice containing enzymes that break down carbohydrates, fats and proteins. It delivers this juice to the small intestine through small tubes called ducts.

Liver and gallbladder. The liver makes bile, a digestive juice that helps digest fats and some vitamins. The gallbladder stores bile between meals and, when you eat, squeezes it into the small intestine.

Small intestine. This is where most of the work is finished. The small intestine makes its own digestive juice, which mixes with bile and pancreatic juice to complete the breakdown of proteins, carbohydrates and fats. Bacteria in the small intestine make some of the enzymes needed to digest carbohydrates. The walls of the small intestine absorb most of the nutrients in food, along with water, into the bloodstream.

Large intestine. Here, more water is absorbed, and bacteria break down remaining nutrients and make vitamin K. What is left, including parts of food that are too large to digest, fluid and old cells from the lining of the gut, becomes stool. The rectum stores it until a bowel movement.

Once absorbed, sugars, amino acids, glycerol and some vitamins and salts travel in the blood to the liver, which stores, processes and delivers nutrients to the rest of the body. Fatty acids and some vitamins are absorbed by the lymph system, a network of vessels that also carries white blood cells.

How long does it take?

It varies a lot, from person to person and with how it is measured. A 2023 systematic review in the Journal of Clinical Medicine brought together 22 studies of 1,885 healthy adults who swallowed small capsules that track their own progress through the gut. Across those studies:

• passing through the small intestine took between about 3.3 and 7 hours;
• passing through the colon took between about 16 and 29 hours;
• the whole journey took between about 23 and 37 hours.

The time food stays in the stomach varied the most, from under half an hour to about 15 hours, and depended strongly on how each study was run.

Nerves, hormones and gut bacteria

Digestion is controlled by hormones and nerves working together, NIDDK says. Cells lining the stomach and small intestine release hormones that tell the body when to make digestive juices and send signals to the brain that you are hungry or full. The pancreas makes hormones that matter for digestion, too.

Nerves connect the brain and spinal cord to the digestive system. That is why the sight or smell of food can make your mouth water. The gut also has its own network of nerves in its walls, the enteric nervous system. When food stretches the walls of the gut, these nerves release substances that speed up or slow down the movement of food and the release of digestive juices.

The bacteria in your gut, also called gut flora or the microbiome, help with digestion, from making enzymes in the small intestine to making vitamin K in the large intestine.

When digestion does not work as it should

Many digestive problems come down to one step in this process not working as it should. In lactose intolerance, the small intestine cannot break down all the lactose, the natural sugar in milk, that you eat or drink, NIDDK explains. In coeliac disease, the immune system attacks the body's own tissues when you eat gluten, damaging the small intestine, according to the NHS. And when carbohydrates are not fully digested, bacteria in the large intestine break them down and create gas, one of the two ways gas normally builds up, as our guide to gas and bloating explains.

When the pancreas does not make enough enzymes

Exocrine pancreatic insufficiency (EPI), also called pancreatic exocrine insufficiency (PEI), is a medical condition in which the pancreas cannot make enough digestive enzymes, or they cannot reach the small intestine, explains the UK charity Guts UK. Food is then not digested properly.

Any disease that affects the pancreas can cause it. In the UK, the most common causes are cystic fibrosis, acute and chronic pancreatitis (inflammation of the pancreas) and pancreatic cancer, and it can also follow surgery on the pancreas. Guts UK says that 8 in 10 people with chronic pancreatitis will eventually develop it, and that more people are diagnosed with it as they get older.

Symptoms can be mild at first and can include:

• tummy discomfort or pain, bloating and wind;
• fatty poo (steatorrhoea) that may look pale, oily and bulky, smell very bad, float, and be difficult to flush away;
• diarrhoea;
• weight loss.

Over time, poor absorption of fat can lead to low levels of vitamins that are absorbed with fat, such as vitamins A, D and K. The NHS lists very smelly poo that floats and is difficult to flush away, and weight loss without trying, among the symptoms of chronic pancreatitis.

EPI is diagnosed by doctors, Guts UK explains, starting with questions about your symptoms and an examination. Tests usually include scans of the pancreas, such as an ultrasound, CT or MRI scan, and a stool test that measures how much of a pancreatic enzyme reaches the poo. If you are diagnosed with it, your doctor will advise on treatment. It is not a condition to diagnose or manage on your own.

When to see a doctor

See a GP if you have symptoms like those above, such as fatty, pale or oily poo that floats and is hard to flush, ongoing diarrhoea, or weight loss you cannot explain. Guts UK and the NHS advise talking to a doctor if you think you might have EPI or chronic pancreatitis.

Get medical advice urgently, the same day, if you get sudden, severe pain in your tummy. The NHS warns that this could be a sign of acute pancreatitis or another condition that needs treatment in hospital as soon as possible.

Seek medical care right away, NIDDK advises, if you have any of these symptoms of severe pancreatitis: tummy pain or tenderness that is severe or getting worse; nausea and vomiting; fever or chills; a fast heartbeat; or shortness of breath.

For other warning signs, such as blood in your poo or changes in your bowel habits, see our guide to digestion after 40.