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Mechanism
Beginner
9 min read

The digestive system

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.

The route three pages assumed

This corpus describes what becomes of protein, carbohydrate and fat 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.
Here is the backbone the three of them share.

An inside that is an outside

This is the most counter-intuitive fact on the subject, and it illuminates all the rest.
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.
Absorption
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.
entry into the body
The consequence is direct: eating is not absorbing. What is not absorbed comes back out, and that is true of a share of what is swallowed.

The stages, in order

From a mouthful to absorption

  • 1
    Mouth
    seconds
    Mechanical grinding and the start of starch digestion.
  • 2
    Stomach
    thirty minutes to a few hours
    Churning in a very acid medium, the start of protein digestion. Nothing is absorbed there in quantity.
  • 3
    Small intestine
    a few hours
    The essential happens here: enzymes finish the cutting, and almost all absorption takes place along this stretch.
  • 4
    Large intestine
    ten to several tens of hours
    Recovery of water and salts; the microbiota transforms part of what was not digested.
  • 5
    Elimination
    afterwards
    What did not cross the wall comes out, with the residues of the microbiota.
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 lungs: an exchange is proportional to its surface.

The liver sees almost everything pass

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.
This is what the dietary carbohydrate page calls "the compulsory pass through the liver". Amino acids follow the same route. Fats are the exception: too bulky for the blood capillaries, they take the lymph and avoid that filter on the first pass.

Three nutrients, three routes

Nutrient
Cut into
Route after absorption
Protein
Amino acids
Blood, then liver
Carbohydrate
Simple sugars
Blood, then liver
Fat
Fatty acids, reassembled
Lymph, then blood, liver afterwards

What the tube lets through, and how fast

Absorption is not instantaneous, and its speed is capped by the number of transporters available in the wall.
Certainty level · Established
The speed at which a given carbohydrate can be absorbed is limited by the intestinal transporters able to handle it.
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.
Burke LM, Hawley JA, Wong SHS, Jeukendrup AE (2011)
It is also what makes the digestive tract sensitive to effort. Blood is partly diverted from it towards the working muscles, as the blood and circulation page describes, and a less well-supplied wall absorbs more slowly.

The microbiota, with caution

The large intestine houses a considerable microbial population, which transforms part of what human enzymes cannot digest.
Caution
A field where the gap is wide
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.
Body Lab sticks here to what is established and recommends no diet, no supplement and no practice aimed at changing that population.

Why no scene accompanies this page

Limit
A system under a licence that cannot be reused
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.
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.

What this page does not do

Caution
No diet, no disease
This page describes a route and its stages. It addresses no digestive disease, no intolerance, no particular diet and proposes no food.
A digestive disturbance that lasts, involuntary weight loss, blood in the stool, difficulty swallowing or severe abdominal pain belongs to a health professional.

Sources

Main sources

  • Hall JE, Hall ME (2020). Guyton and Hall Textbook of Medical Physiology, 14th edition. Elsevier.
  • Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
  • Burke LM, Hawley JA, Wong SHS, Jeukendrup AE (2011). Carbohydrates for training and competition. Journal of Sports Sciences.
Put it into practice in Shapier

Organize your nutrition around training

Choosing what to eat before and after a session is done in Shapier; Body Lab only explains the route those foods will take.
Organize your nutrition around training
Body Lab explains; Shapier lets you act and track.

Check my understanding

Has a food that has just been swallowed already entered the organism?
Where does most of nutrient absorption take place?
Which nutrient escapes the liver's filter on its first pass?
Choose an answer

Read next

  • What happens to dietary carbohydrate
    A carbohydrate you eat is taken apart into simple sugars, absorbed, then sorted. Some is burnt at once, some stored in the liver, some in the muscle — and those two stores do not do the same job at all.
    With a 3D scene
  • What happens to dietary fat
    Fat does not dissolve in water: its route out of the intestine resembles that of no other nutrient. It joins an enormous store, and its share during effort follows a curve that rises and then falls.
    Described without a scene
  • Blood and circulation
    Thirty-two pages of this corpus mention blood without any of them saying what it carries or how it travels. Everything that matters happens in the smallest vessels, and the return journey poses a problem the outward one does not.
    Described without a scene

Available offline

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Trust and method

Author
equipe-editoriale-shapier
Editorial review
Thanh Chau
Scientific review
Pending
Published on
August 2, 2026
Reviewed on August 2, 2026
Next review due August 2, 2027
Limits of this page
  • 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.
Sources
  • Hall JE, Hall ME (2020). Guyton and Hall Textbook of Medical Physiology, 14th edition. Elsevier.
  • Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
  • Burke LM, Hawley JA, Wong SHS, Jeukendrup AE (2011). Carbohydrates for training and competition. Journal of Sports Sciences.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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