{"schemaVersion":1,"id":"en-mechanism-digestive-system","type":"mechanism","pillar":"body","locale":"en","translationGroupId":"mechanism-digestive-system","slugPath":"body/systems/the-digestive-system","title":"The digestive system","shortTitle":"Digestive system","summary":"A tube several metres long whose inside is, technically, outside the body. Three pages of this corpus describe what becomes of nutrients without any of them describing the route they take.","level":"beginner","estimatedMinutes":9,"synonyms":["digestive system","digestion","intestine","stomach","liver","absorption","gut microbiota"],"blocks":[{"id":"b1","type":"heading","level":2,"text":"The route three pages assumed","anchor":"the-route-three-pages-assumed"},{"id":"b2","type":"paragraph","content":[{"kind":"text","value":"This corpus describes what becomes of "},{"kind":"link","value":"protein","href":"/en/adaptations/what-happens-to-dietary-protein","external":false},{"kind":"text","value":", "},{"kind":"link","value":"carbohydrate","href":"/en/energy/what-happens-to-dietary-carbohydrate","external":false},{"kind":"text","value":" and "},{"kind":"link","value":"fat","href":"/en/energy/what-happens-to-dietary-fat","external":false},{"kind":"text","value":" once eaten. Each of those pages begins with \"digestion continues in the small intestine\" without anything saying what that tube is or how it works."}]},{"id":"b3","type":"paragraph","content":[{"kind":"text","value":"Here is the backbone the three of them share."}]},{"id":"b4","type":"heading","level":2,"text":"An inside that is an outside","anchor":"an-inside-that-is-an-outside"},{"id":"b5","type":"paragraph","content":[{"kind":"text","value":"This is the most counter-intuitive fact on the subject, and it illuminates all the rest."}]},{"id":"b6","type":"paragraph","content":[{"kind":"text","value":"The digestive tract is a continuous conduit running right through the body. What is inside it is not inside the body: it is in a tunnel crossing it. A swallowed food really enters the organism only at the moment it crosses the wall of that tube."}]},{"id":"b7","type":"definition","term":"Absorption","definition":"The crossing of the digestive tract wall by a molecule, which then joins the blood or the lymph. It is the only moment when a food genuinely enters the body.","alsoKnownAs":["entry into the body"]},{"id":"b8","type":"paragraph","content":[{"kind":"text","value":"The consequence is direct: "},{"kind":"strong","value":"eating is not absorbing"},{"kind":"text","value":". What is not absorbed comes back out, and that is true of a share of what is swallowed."}]},{"id":"b9","type":"heading","level":2,"text":"The stages, in order","anchor":"the-stages-in-order"},{"id":"b10","type":"timeline","title":"From a mouthful to absorption","steps":[{"id":"t0","label":"Mouth","timing":"seconds","description":"Mechanical grinding and the start of starch digestion."},{"id":"t1","label":"Stomach","timing":"thirty minutes to a few hours","description":"Churning in a very acid medium, the start of protein digestion. Nothing is absorbed there in quantity."},{"id":"t2","label":"Small intestine","timing":"a few hours","description":"The essential happens here: enzymes finish the cutting, and almost all absorption takes place along this stretch."},{"id":"t3","label":"Large intestine","timing":"ten to several tens of hours","description":"Recovery of water and salts; the microbiota transforms part of what was not digested."},{"id":"t4","label":"Elimination","timing":"afterwards","description":"What did not cross the wall comes out, with the residues of the microbiota."}]},{"id":"b11","type":"paragraph","content":[{"kind":"text","value":"The small intestine deserves the emphasis. It is where this corpus places all its key stages, and its wall is built for one thing: offering the largest possible surface. Folds, villi, microvilli — three nested levels of pleating, for the same reason as the branching of the "},{"kind":"link","value":"lungs","href":"/en/body/systems/the-lungs","external":false},{"kind":"text","value":": an exchange is proportional to its surface."}]},{"id":"b12","type":"heading","level":2,"text":"The liver sees almost everything pass","anchor":"the-liver-sees-almost-everything-pass"},{"id":"b13","type":"paragraph","content":[{"kind":"text","value":"The blood leaving the intestine does not join the general circulation directly. It is collected and led to the liver, which filters it before letting it go on."}]},{"id":"b14","type":"paragraph","content":[{"kind":"text","value":"This is what the "},{"kind":"link","value":"dietary carbohydrate","href":"/en/energy/what-happens-to-dietary-carbohydrate","external":false},{"kind":"text","value":" page calls \"the compulsory pass through the liver\". Amino acids follow the same route. "},{"kind":"link","value":"Fats","href":"/en/energy/what-happens-to-dietary-fat","external":false},{"kind":"text","value":" are the exception: too bulky for the blood capillaries, they take the lymph and avoid that filter on the first pass."}]},{"id":"b15","type":"comparison","title":"Three nutrients, three routes","columns":["Nutrient","Cut into","Route after absorption"],"rows":[{"label":"Protein","cells":["Amino acids","Blood, then liver"]},{"label":"Carbohydrate","cells":["Simple sugars","Blood, then liver"]},{"label":"Fat","cells":["Fatty acids, reassembled","Lymph, then blood, liver afterwards"]}]},{"id":"b16","type":"heading","level":2,"text":"What the tube lets through, and how fast","anchor":"what-the-tube-lets-through-and-how-fast"},{"id":"b17","type":"paragraph","content":[{"kind":"text","value":"Absorption is not instantaneous, and its speed is capped by the number of transporters available in the wall."}]},{"id":"b18","type":"evidence","level":"established","statement":"The speed at which a given carbohydrate can be absorbed is limited by the intestinal transporters able to handle it.","detail":"This is established by work on carbohydrate use during exercise, in trained subjects. It concerns rates measured in the laboratory; it sets no amount to consume and does not transfer directly to an ordinary meal.","referenceIds":["burke2011"]},{"id":"b19","type":"paragraph","content":[{"kind":"text","value":"It is also what makes the digestive tract sensitive to effort. Blood is partly diverted from it towards the working muscles, as the "},{"kind":"link","value":"blood and circulation","href":"/en/body/systems/blood-and-circulation","external":false},{"kind":"text","value":" page describes, and a less well-supplied wall absorbs more slowly."}]},{"id":"b20","type":"heading","level":2,"text":"The microbiota, with caution","anchor":"the-microbiota-with-caution"},{"id":"b21","type":"paragraph","content":[{"kind":"text","value":"The large intestine houses a considerable microbial population, which transforms part of what human enzymes cannot digest."}]},{"id":"b22","type":"callout","tone":"caution","title":"A field where the gap is wide","content":[[{"kind":"text","value":"The existence of this population and its role in transforming fibre are agreed. Beyond that, many claims circulate — about mood, weight, immunity, performance — whose level of evidence is far below their circulation."}],[{"kind":"text","value":"Body Lab sticks here to what is established and recommends no diet, no supplement and no practice aimed at changing that population."}]]},{"id":"b23","type":"heading","level":2,"text":"Why no scene accompanies this page","anchor":"why-no-scene-accompanies-this-page"},{"id":"b24","type":"callout","tone":"limit","title":"A system under a licence that cannot be reused","content":[[{"kind":"text","value":"The reason differs here from the other systems pages. The digestive tract does exist in the anatomical atlas Body Lab derives its scenes from — but its visceral system is published under a non-commercial licence, incompatible with the project's use."}],[{"kind":"text","value":"So this is not an absence of data, it is a limit of rights, and it is declared as such. No other available source supplies these organs."}]]},{"id":"b25","type":"heading","level":2,"text":"What this page does not do","anchor":"what-this-page-does-not-do"},{"id":"b26","type":"callout","tone":"caution","title":"No diet, no disease","content":[[{"kind":"text","value":"This page describes a route and its stages. It addresses no digestive disease, no intolerance, no particular diet and proposes no food."}],[{"kind":"text","value":"A digestive disturbance that lasts, involuntary weight loss, blood in the stool, difficulty swallowing or severe abdominal pain belongs to a health professional."}]]},{"id":"b27","type":"heading","level":2,"text":"Sources","anchor":"sources"},{"id":"b28","type":"sourceList","title":"Main sources","referenceIds":["hall2020","standring2020","burke2011"]},{"id":"b29","type":"shapierAction","actionId":"nutrition-around-training","label":"Organize your nutrition around training","description":"Choosing what to eat before and after a session is done in Shapier; Body Lab only explains the route those foods will take.","webTarget":"https://shapier.app/en/dossiers/nutrition-autour-entrainement","appTarget":"shapier://dossiers/nutrition-autour-entrainement"},{"id":"b30","type":"quiz","title":"Check my understanding","questions":[{"id":"q1","prompt":"Has a food that has just been swallowed already entered the organism?","choices":[{"id":"q1c1","label":"No, it genuinely enters the body only when it crosses the wall of the digestive tract","correct":true,"explanation":"That is the moment of absorption: the digestive tract is a tunnel crossing the body, and its contents stay technically outside until a molecule crosses the wall to join the blood or the lymph."},{"id":"q1c2","label":"Yes, as soon as it reaches the stomach","correct":false,"explanation":"It feels that way because the food now seems inside, but the stomach only churns it in a very acid medium and the page states that nothing is absorbed there in quantity: being in the tube does not mean being in the body."},{"id":"q1c3","label":"Yes, as soon as it has been chewed","correct":false,"explanation":"Chewing is only a mechanical grinding that starts starch digestion; no molecule has joined the blood or the lymph yet at that stage."}]},{"id":"q2","prompt":"Where does most of nutrient absorption take place?","choices":[{"id":"q2c1","label":"In the small intestine","correct":true,"explanation":"Enzymes finish the cutting there and almost all absorption happens along that stretch, whose wall is pleated in three nested levels (folds, villi, microvilli) to offer the largest possible surface."},{"id":"q2c2","label":"In the stomach","correct":false,"explanation":"That is where food sits and is churned, which makes it plausible, but the page states that nothing is absorbed there in quantity."},{"id":"q2c3","label":"In the large intestine","correct":false,"explanation":"The large intestine mainly recovers water and salts and houses the microbiota; it comes after the small intestine, where the essential has already happened."}]},{"id":"q3","prompt":"Which nutrient escapes the liver's filter on its first pass?","choices":[{"id":"q3c1","label":"Fat","correct":true,"explanation":"Too bulky for the blood capillaries, it takes the lymph then joins the blood, and only reaches the liver afterwards."},{"id":"q3c2","label":"Carbohydrate","correct":false,"explanation":"The simple sugars from carbohydrate follow the blood then liver route, which the page calls the compulsory pass through the liver."},{"id":"q3c3","label":"Protein","correct":false,"explanation":"Amino acids follow the same route as simple sugars: blood, then liver."}]}]},{"id":"b31","type":"relatedContent","title":"Read next","targetIds":["en-mechanism-dietary-carbohydrate","en-mechanism-dietary-fat","en-mechanism-blood-circulation"]}],"relations":[{"type":"explains","targetId":"en-mechanism-dietary-protein"},{"type":"explains","targetId":"en-mechanism-dietary-carbohydrate"},{"type":"explains","targetId":"en-mechanism-dietary-fat"},{"type":"next-step","targetId":"en-mechanism-blood-circulation"}],"authors":["equipe-editoriale-shapier"],"reviewers":["Thanh Chau"],"references":[{"id":"hall2020","authors":"Hall JE, Hall ME","year":2020,"title":"Guyton and Hall Textbook of Medical Physiology, 14th edition","source":"Elsevier","kind":"book","url":"https://shop.elsevier.com/books/guyton-and-hall-textbook-of-medical-physiology/hall/978-0-323-59712-8"},{"id":"standring2020","authors":"Standring S","year":2020,"title":"Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition","source":"Elsevier","kind":"book","url":"https://shop.elsevier.com/books/grays-anatomy/standring/978-0-7020-7705-0"},{"id":"burke2011","authors":"Burke LM, Hawley JA, Wong SHS, Jeukendrup AE","year":2011,"title":"Carbohydrates for training and competition","source":"Journal of Sports Sciences","kind":"review","doi":"10.1080/02640414.2011.585473"}],"review":{"publishedAt":"2026-08-02","reviewedAt":"2026-08-02","reviewDueAt":"2027-08-02","evidenceLevel":"established"},"limitations":["No scene accompanies this page: the visceral system of the atlas Body Lab uses carries a licence that cannot be reused, and no other available source contains it.","This page describes a route and its stages. It addresses no digestive disease, no particular diet and no intolerance.","The gut microbiota is an active research field where many conclusions remain provisional; this page sticks to what is agreed.","A digestive disturbance that lasts, involuntary weight loss, blood in the stool or severe abdominal pain belongs to a health professional."],"seo":{"title":"The digestive system — the route nutrients take","description":"From mouth to absorption, the role of the liver and what the gut lets through: the backbone shared by the three pages on nutrients.","canonicalPath":"/en/body/systems/the-digestive-system","image":"/og/en/mechanism-digestive-system.png"},"app":{"offlineEligible":true},"version":1}