Digestion

A meal is made of things your body cannot use. Bread, meat and vegetables are built from huge molecules, starches, proteins and fats, far too large to pass into the blood. Digestion is the process of taking them apart into small pieces, sugars, amino acids and fatty acids, that can be absorbed and then rebuilt into you. It happens along a tube about nine metres long, with a chemical plant attached, and it takes a day or more.

Mouth

Digestion starts before you swallow. Teeth crush and grind food into smaller pieces, increasing the surface area for chemicals to work on. Saliva, about a litre of it a day, wets the food and contains an enzyme that begins breaking starch into sugar; this is why bread held in the mouth starts to taste sweet. The tongue rolls the mixture into a ball and pushes it to the back of the throat, where swallowing takes over automatically. A wave of muscle contraction carries it down the oesophagus to the stomach in about eight seconds. This squeezing, called peristalsis, moves food along the entire tract, which is why you can swallow upside down.

Stomach

The stomach is a muscular bag that can stretch from about 50 millilitres when empty to over a litre after a large meal. Its lining pours out hydrochloric acid strong enough to dissolve a nail, which kills most bacteria in food and unravels proteins so that enzymes can begin cutting them into pieces. The stomach protects itself with a thick layer of mucus and by replacing its entire lining every few days; when the protection fails, the acid burns a hole, an ulcer, most often caused by a bacterium that can survive the acid.

For two to four hours the stomach churns the food into a thick soup, releasing it a little at a time through a ring of muscle into the intestine. Liquids pass quickly; a fatty meal lingers longest.

The chemical plant

As the soup enters the small intestine, two organs add their contributions. The pancreas secretes a cocktail of enzymes that finish breaking down starch, protein and fat, together with alkali to neutralise the stomach acid. The liver produces bile, a green fluid stored in the gallbladder and squirted into the intestine, which breaks fat into tiny droplets so the enzymes can reach it, much as washing-up liquid lifts grease from a plate.

The liver does far more than this. It is the largest internal organ and the body's chemical refinery. All the blood from the intestine passes through it first, and it stores sugar and releases it between meals, builds proteins, breaks down drugs and alcohol, and removes toxins. It performs more than 500 known functions and can regrow if part of it is removed.

Small intestine: where food becomes you

The small intestine is about six metres long but only a few centimetres wide, and it is where almost all absorption happens. Its lining is folded, and the folds are covered in millions of finger-like projections called villi, each covered in turn with microscopic bristles, so that the total surface area is about 30 square metres. Across this surface, over about four hours, the digested sugars, amino acids, fatty acids, vitamins and minerals pass into the blood and lymph and are carried away to the liver and the rest of the body. What remains, water, fibre and whatever could not be digested, moves on.

Large intestine: the bacterial garden

The large intestine, about one and a half metres long, absorbs most of the remaining water, turning the liquid residue into solid waste over a day or two. It is also home to the densest population of microbes on Earth: trillions of bacteria of hundreds of species, weighing a kilogram or more. They ferment the fibre that human enzymes cannot digest, producing gases, which is where wind comes from, and short fatty acids that feed the gut lining and may influence everything from immunity to mood. They also make vitamin K and some B vitamins. A newborn's gut is sterile; its microbiome is acquired from its mother and surroundings in the first years of life, and it is reshaped throughout life by diet. Antibiotics can devastate it for weeks.

Control

The gut has its own nervous system, about 500 million neurons, more than in the spinal cord, which coordinates the movement and secretions of the whole tract largely without the brain's involvement. It communicates with the brain constantly, which is why anxiety causes a churning stomach and why hunger is felt as much in the head as in the belly. Hormones released by the stomach and intestine tell the brain when you are full, and when they misfire, appetite goes wrong.

Why it matters

Digestion is where the outside world becomes you. Every protein in your muscles, every cell membrane, every molecule of energy you burn came in through this tube. Its health shapes your weight, your immunity, your mood and your risk of many diseases, and the bacteria living in it, which outnumber your own cells, are only beginning to be understood.