The Journey of a Bite

TL;DR. Follow an apple from your teeth to the toilet. Chewing and saliva start breaking down starch. The stomach is an acid tank that sterilises and dissolves, and absorbs almost nothing. The small intestine, which is where essentially all absorption happens, is about six metres long with an internal surface area of roughly a tennis court, and it works with pancreatic enzymes and bile that arrive on cue. The colon takes what your enzymes could not digest and hands it to thirty-odd trillion bacteria, who ferment it into compounds that feed your gut lining and talk to your immune system. Total transit: one to three days, of which the colon is most of it.

Key takeaways

• The stomach absorbs almost nothing except alcohol, water, and a few drugs. Its job is acid, mechanical breakdown, and pathogen control.
• The small intestine has an absorptive surface of roughly 200 to 300 square metres, produced by folds, villi, and microvilli stacking three levels of magnification.
• Nothing is absorbed as food. Starch must become glucose, protein must become amino acids, and fat must be emulsified, split, and reassembled, before anything crosses.
• Everything absorbed from the gut goes to the liver first, via the portal vein, which is why the liver is the body's chemical checkpoint and why it dominates drug metabolism.
• Transit time is 24 to 72 hours, and the colon accounts for most of it.

The bite

In short: Chewing multiplies surface area and mixes in the first enzyme; swallowing is a reflex you cannot fully control once it starts.

Take a bite of apple, about 20 grams. Roughly 17 g of that is water. Around 2.5 g is carbohydrate, mostly fructose, glucose, and sucrose, with some starch if the apple is not fully ripe. About 0.5 g is fibre, mostly cellulose, hemicellulose, and pectin. Traces of protein, almost no fat, some potassium, some vitamin C, and several hundred different polyphenols, most concentrated in the skin.

Chewing does two things. It multiplies surface area, so enzymes have somewhere to work. Halve a cube and you double its exposed area, and thorough chewing raises the available surface enormously. And it mixes in saliva, of which you make roughly 0.5 to 1.5 litres a day.

Saliva contains:

• Salivary amylase (ptyalin), which begins cutting starch into shorter chains. This is the reason bread tastes sweeter the longer you chew it: you are producing maltose in your mouth.
• Lingual lipase, a minor fat-splitting enzyme, more important in infants.
• Mucins, which lubricate.
• Lysozyme, lactoferrin, and immunoglobulin A, an antimicrobial set.
• Bicarbonate, which buffers acid and protects teeth.

Taste is happening simultaneously, and it is a chemical assay, not a decoration. Sweetness signals available energy, umami signals protein, salt signals electrolytes, sourness signals acid (unripe or fermented), and bitterness signals possible toxin. There is now good evidence for a fat taste (oleogustus) as well. Aroma matters more than taste: most of what you call flavour is volatile compounds reaching the olfactory receptors through the back of the nose, which is why food is bland with a blocked nose.

Swallowing is voluntary to start and reflexive after. The epiglottis folds over the airway, breathing stops for about a second, and the bolus enters the oesophagus. From there peristalsis, a travelling ring of muscular contraction, moves it down in about 8 seconds. Peristalsis is why you can swallow upside down.

The stomach: an acid tank that absorbs nothing

In short: Hydrochloric acid at pH 1.5 to 3.5 unfolds proteins, kills most microbes, and releases minerals from food; the stomach's main output is a controlled trickle into the small intestine.

The stomach is the most misunderstood organ in digestion. People assume it is where food is digested and absorbed. It is neither, mostly.

What it secretes, roughly 2 to 3 litres a day:

The acidity is genuinely extreme: pH 1.5 is roughly the acidity of battery acid diluted a little, and a hundred thousand times more acidic than blood. The stomach survives it because of a two-millimetre mucus-bicarbonate layer that maintains a near-neutral pH right at the cell surface. When that defence fails, through Helicobacter pylori infection or through NSAIDs suppressing the prostaglandins that maintain it, you get ulcers. That mechanism is why Chapter 66 spends time on stomach damage.

Mechanical work. The stomach's three muscle layers churn, grinding food against a closed pyloric sphincter until particles are under about 2 mm. The result is chyme, a semi-liquid acidic soup.

Gastric emptying is controlled and slow, and its speed determines a great deal:

Fat and protein in the small intestine trigger hormones (CCK and GLP-1) that slow the stomach down, which is why a fatty meal "sits heavy" and why adding fat or protein to carbohydrate flattens the resulting blood sugar rise. It is also the mechanism the GLP-1 weight drugs exploit (Chapter 76).

What the stomach absorbs: water, some minerals, alcohol (which is why drinking on an empty stomach hits fast, and why food genuinely slows intoxication), aspirin and a few other weak acids, and caffeine to a small degree. Everything else waits.

The small intestine: where almost everything happens

In short: About six metres long, folded and villous and microvillous, with an internal surface area of a couple of hundred square metres, and it does essentially all of the absorption.

The small intestine has three sections: the duodenum (about 25 cm, where the chemical work is set up), the jejunum (about 2.5 m, where most absorption happens), and the ileum (about 3.5 m, which mops up, plus the specific absorption of bile salts and vitamin B12).

The surface area trick

The intestine solves the absorption problem with three nested levels of folding:

1. Circular folds (plicae circulares), ridges running round the inside, multiply surface area about threefold.
2. Villi, finger-like projections about 1 mm long, roughly 20 to 40 per square millimetre, multiply it another tenfold.
3. Microvilli, microscopic projections on each absorptive cell forming the brush border, roughly a thousand per cell, multiply it another twentyfold.

The result is an absorptive surface commonly estimated at 200 to 300 square metres, about the area of a tennis court, packed into a tube you could coil into a bucket. That the surface is so large is why partial intestinal removal can be survived, and why diseases that flatten the villi (coeliac disease, most notably) cause malabsorption of everything at once.

Each villus contains a capillary network, which takes up sugars, amino acids, minerals, and water-soluble vitamins, and a lacteal, a lymphatic vessel that takes up fat.

The chemistry arrives on cue

When acidic chyme enters the duodenum, the duodenal lining releases hormones that summon the two support organs.

The pancreas delivers, through a duct into the duodenum:

plus a large volume of bicarbonate, which neutralises stomach acid so that the pancreatic enzymes, which work near pH 7 to 8, can function at all.

The protein-cutting enzymes are secreted as inactive precursors and activated only in the duodenum, by an enzyme anchored in the intestinal wall. This is not an accident: if trypsin activated inside the pancreas, it would digest the pancreas. That is exactly what happens in acute pancreatitis, and it is as bad as it sounds.

The gallbladder delivers bile, made continuously by the liver and stored and concentrated between meals. Bile contains no enzymes. Its active ingredients are bile salts, which are detergents: molecules with a water-loving end and a fat-loving end, so they surround fat droplets and break them into a fine emulsion. Since lipase can only work at the surface of a droplet, emulsifying a large droplet into a thousand small ones raises the working surface enormously. Without bile, fat digestion largely fails and fat-soluble vitamins (A, D, E, K) are not absorbed.

Bile is also an excretion route: bilirubin from broken-down red blood cells, excess cholesterol, and some drug metabolites leave the body this way. Bilirubin is what colours stool brown, which is why bile duct obstruction produces pale stools and yellow skin.

Final digestion happens on the cell surface

Some enzymes are not secreted into the cavity at all; they are embedded in the brush border membrane, so the last cut happens right at the point of absorption:

• Lactase splits lactose into glucose and galactose. Its production declines after weaning in most of humanity, which is lactose intolerance, and its persistence into adulthood is the mutation (Chapter 54).
• Sucrase splits sucrose into glucose and fructose.
• Maltase and isomaltase finish starch digestion.
• Peptidases finish protein digestion.

Absorption, molecule by molecule

Sugars. Glucose and galactose are pumped in actively by the SGLT1 transporter, using sodium as the driving force. Fructose enters passively via GLUT5, which is slower and has limited capacity; that limit is why large fructose loads can cause bloating and diarrhoea in susceptible people, and why fruit juice does this more readily than whole fruit.

A useful aside: the SGLT1 mechanism, glucose absorption coupled to sodium, is the basis of oral rehydration solution. Water follows sodium, sodium follows glucose, so a solution containing both is absorbed even by an inflamed gut that would reject plain water. That insight, published in the 1960s, has saved tens of millions of lives from diarrhoeal disease, and it is arguably the highest-impact single application of gut physiology in medicine.

Amino acids are taken up by a family of transporters, some sodium-coupled, and short di- and tripeptides are absorbed intact by the PepT1 transporter and broken down inside the cell. That peptide transporter is also how several oral antibiotics get in.

Fats take an entirely different route. Fatty acids and monoglycerides, packaged with bile salts into tiny aggregates called micelles, diffuse into the intestinal cell. Inside, they are reassembled into triglycerides, wrapped in a protein-and-phospholipid coat to make a chylomicron, and exported into the lacteal rather than into the blood. From the lymphatic system they eventually enter the bloodstream at the base of the neck.

That routing is important. Fat, and fat-soluble drugs and vitamins that ride with it, bypass the liver's first-pass metabolism, unlike everything else absorbed from the gut.

Water follows osmotic gradients: about 9 litres a day passes through the small intestine (2 litres consumed, 7 litres of secretions), and roughly 8 litres of it is reabsorbed before the colon.

The portal vein: everything goes to the liver first

In short: Blood from the entire gut drains to the liver before reaching the rest of you, which makes the liver the body's chemical checkpoint.

Blood leaving the intestines does not join the general circulation. It collects into the hepatic portal vein and goes straight to the liver.

The consequences are large:

• The liver sees every absorbed molecule first, at the highest concentration it will ever reach, and can act before anything reaches the brain or heart.
• First-pass metabolism: many drugs are substantially destroyed on this first pass through the liver, which is why oral doses are often much larger than injected ones, and why some drugs cannot be given orally at all. This is covered in Chapter 62.
• Glucose is buffered. After a meal the liver takes up a large share of incoming glucose and stores it as glycogen, blunting the rise reaching the rest of the body.
• Ammonia from protein metabolism and from gut bacteria is converted to urea before it can reach the brain. When the liver fails, it does not, and the result is hepatic encephalopathy.
• Fructose is handled almost entirely by the liver, unlike glucose, which is why very high fructose intake specifically stresses that organ (Chapter 11).

The large intestine: the fermentation vat

In short: What your enzymes could not digest arrives here for bacteria to ferment, producing short-chain fatty acids that feed your gut lining and influence your immune system.

By the time the residue reaches the ileocaecal valve, essentially all the digestible carbohydrate, protein, and fat is gone. What remains is fibre, resistant starch, some undigested protein, bile acids, sloughed-off intestinal cells, and roughly 1.5 litres of water.

The colon is about 1.5 metres long. It has three jobs.

1. Reabsorb water and electrolytes. Of the ~1.5 L arriving, about 1.3 L is reabsorbed, leaving roughly 100 to 200 mL in stool. When this fails, you get diarrhoea, and the reason severe diarrhoea kills is that the losses are fast and the volumes large.

2. Host the microbiome. Roughly 38 trillion bacterial cells, of hundreds of species, at densities of about 10¹¹ per gram of contents, in a near-oxygen-free environment. This is covered fully in Chapter 14, but the essential transaction is this: bacteria ferment fibre and resistant starch into short-chain fatty acids, principally acetate, propionate, and butyrate. Butyrate is the preferred fuel of the cells lining the colon, so a fibre-poor diet literally starves the gut lining. Fermentation also produces gas (hydrogen, carbon dioxide, methane), synthesises vitamin K and several B vitamins, and generates signals that shape immune development.

3. Form and store stool. Final composition is roughly 75 percent water and 25 percent solids, and of those solids, about a third is bacteria, a third is undigested fibre, and a third is sloughed cells, fats, and inorganic material.

Timing, end to end

In short: One to three days total, with wide normal variation, and the colon accounting for most of it.

Faster is not better and slower is not worse within that range. Very fast transit means poor absorption; very slow transit is associated with constipation and, in observational data, with higher levels of some bacterial metabolites of concern. Fibre, water, and physical activity all shorten transit; opioids, dehydration, low fibre, and inactivity lengthen it.

You can measure your own transit time with sweetcorn, beetroot, or blue food colouring, and it is a more informative experiment than most people expect.

The gut talks to the brain constantly

In short: The gut has its own nervous system, sends far more signals up than it receives down, and produces most of the body's serotonin.

The enteric nervous system contains something like 500 million neurons, more than the spinal cord, and can run peristalsis and secretion with no input from the brain. It is routinely called the second brain, which oversells it, but not by as much as you would think.

Communication runs mostly upward. Roughly 80 to 90 percent of the fibres in the vagus nerve carry signals from gut to brain, not the other way. The gut reports stretch, nutrient content, osmolarity, and irritation, and the brain responds with hunger, satiety, nausea, and mood.

Gut hormones are the other channel:

About 90 to 95 percent of the body's serotonin is made in the gut, by enterochromaffin cells, where it mostly regulates motility rather than mood. It does not cross into the brain. This is why "most of your serotonin is in your gut, so gut health controls your mood" is a real fact leading to an unsupported conclusion. The gut does influence mood, via the vagus nerve, immune signalling, and microbial metabolites, but not by exporting serotonin to the brain.

The bottom line

• Chewing and saliva start carbohydrate digestion; the stomach sterilises, unfolds proteins, and meters food out; the small intestine does essentially all the absorption; the colon ferments the leftovers.
• Nothing crosses the gut wall as food. Everything must be cut to its monomers first, and fat must additionally be emulsified, split, absorbed, reassembled, and shipped through the lymph.
• The small intestine's absorptive area is a couple of hundred square metres, produced by three nested levels of folding.
• Everything absorbed except fat goes to the liver first, which is why the liver dominates drug metabolism and why oral doses differ so much from injected ones.
• Total transit is one to three days, and the colon, where your bacteria live and work, is most of it.

Sources and notes

Digestive anatomy, secretion volumes, enzyme actions, transit times, and absorption mechanisms follow standard physiology texts; Guyton and Hall, Textbook of Medical Physiology, and Johnson, Gastrointestinal Physiology, are the usual references. Small intestinal surface area estimates have been revised downward from the often-quoted tennis court figure by Helander and Fandriks, Scandinavian Journal of Gastroenterology, 2014, who put it nearer 30 square metres; the larger figure remains in wide use and both are given here as an order of magnitude. SGLT1 glucose-coupled sodium absorption and the oral rehydration solution story follow the Lancet-era work of Hirschhorn, Cash, and colleagues in the 1960s and the WHO ORS programme. Gut hormone actions (ghrelin, CCK, GLP-1, PYY) follow endocrinology reviews. Enteric nervous system neuron counts and vagal afferent proportions follow Furness, The Enteric Nervous System, 2006. Gut serotonin production and its inability to cross the blood-brain barrier follow standard neuropharmacology.

Open questions. Small intestinal surface area is genuinely disputed by an order of magnitude depending on measurement method. How much the gut-brain axis influences mood in humans, as opposed to in animal models, remains an active and immature research area.

👉 Next: carbohydrates and blood sugar, the nutrient that generates the most argument and the most confusion.