What your stool tells you: reading colour, form and smell properly

You press the flush and hesitate for a second. Something was different. Paler perhaps. Or green. Or it floated, although it never usually does.

And then what almost always happens happens. You tell nobody. You search in bed that evening, find a colour chart without sources, read the word cancer and sleep worse.

Many people know this pattern. In my consulting room the sentence usually comes right at the end, in a lowered voice, after the actual matter. May I ask something embarrassing.

No, it is not embarrassing. It is physiology. And it is surprisingly readable once you know what to look for.

This article turns an everyday observation into a tool. It explains why stool is brown at all, what each of the seven Bristol forms reveals about the pace inside the gut, what colours can mean, why it sometimes smells of rotten eggs and how you can measure your own gut transit time at home.

And it starts with the part that cannot wait.

Before you interpret anything: these five pictures need medical assessment

I deliberately put this box right at the front. Not at the end, where nobody reads it any more.

The reason is simple. A colour chart almost always reassures. It delivers a harmless explanation for every observation, and that is exactly where its danger lies. Whoever reads first that black can come from blueberries stops searching at blueberries.

Why this selection and not a longer list? Because behind two of these points sit solid figures showing how seriously and how calmly you may take them at the same time.

And now you know why this box sits at the top and not at the bottom.

Why stool is brown

Ask ten people why stool is brown. Nine say: because of what you eat.

That is only partly true. The base tone does not come from food. It comes from your own blood. Food can overlay the colour, as we will see shortly with green, red and black. But the brown underneath arises elsewhere.

Red blood cells live for about 120 days. After that they are broken down, and the red blood pigment haem is converted into bilirubin, a yellowish molecule. The liver takes it up, makes it water soluble and sends it with the bile into the small intestine. There it is still greenish yellow. Only in the colon do bacteria turn it into urobilinogen and further into the brown stercobilinoids.

Brown is therefore no coincidence and no food colour. Brown is the joint product of three participants: liver, bile duct and microbiome.

Medicine knew the bacterial step for a long time only as an observation. People knew that it happens. They did not know what it happens with. That changed only recently.

Bile deserves a sentence of its own here. It is not only a pigment carrier, but the emulsifier for fat and a signal for gut movement. If this part interests you in more detail, it is covered at length in Bile, bile acids and TUDCA.

And now you know why we are not about to talk about twenty colours, but about three places where something can tear.

The Bristol scale, complete and with pace

If you remember only one thing from this article, let it be this: form says more than frequency.

That is not an opinion, that is measured. The Bristol stool form scale comes from the English city of Bristol and was developed in the early nineties on a population sample. It was then tested against actual gut transit time, so against the time the content needs from mouth to end.

In children the same relationship turns out even more clearly.

Here is the scale in full, with what each type says about pace.

The scale has arrived in professional practice. The German S3 guideline on irritable bowel syndrome uses stool form for the classification into subtypes, and the German S2k guideline on chronic constipation likewise works with it as the standard instrument for consistency.

And yet it is not a microscope. That belongs in the picture too.

Two demarcations, so that this article stays what it is. If types 1 and 2 are your lasting picture, this is about constipation, and the search for causes sits in Constipation: more than too little fibre. If types 6 and 7 are your lasting picture, this is about diarrhoea, and the overlooked causes sit in Chronic diarrhoea: the overlooked causes. Here we stay with the reading.

And now you know why form is the most honest value at the toilet.

How often is actually normal

Hardly any question produces so much quiet guilt. Once a day, somebody said that at some point, and the number has stuck ever since.

The corridor the professional world works with is considerably wider: three times a day to three times a week. That stands in many guides. What stands almost nowhere is the study behind it.

The difference between men and women is not a feeling here, it is measured. And it is large enough to skew a stool diary if you do not know about it.

So much for the reassuring news. Now the counterpoint I do not want to leave out, because it serves honesty.

And now you know why the number one was never a law.

The colours, one after another

Now comes the part most people are searching for. I deliberately place it behind the red flags and behind the physiology, because only then does it become readable.

And I say the framing sentence right away, because it is the most important one in the whole article: stool colour is not a diagnostic instrument. It is a reason to ask a question.

Green: mostly a question of pace

If you remember the path of the colour from earlier, green is almost self explanatory. Bile arrives greenish in the small intestine. It only turns brown through the bacterial conversion in the colon, and this conversion takes time.

If it goes faster than usual, for example after an infection, with nervousness, after a very large portion or during a bout of urgency, the conversion stays unfinished. What arrives still carries the colour of bile.

The second route is even simpler. Chlorophyll is a strong pigment and survives the passage. A green smoothie, a plate of spinach or a portion of algae powder can be visible a day later.

Important for honesty: for the explanation via fast passage there is no clinical study of its own, it follows from the breakdown pathway. That is mechanistically plausible, but not evidenced by a study in humans. I would rather mark that than keep quiet about it.

Yellow and greasy: when the fat does not arrive

Precision pays off here, because two things are constantly confused. Yellowish stool on its own is harmless and occurs with a fast passage. Steatorrhoea is something else, and you recognise it not by the colour but by the whole picture: pale, glossy, sticky, bulky, extremely foul smelling, hard to flush away, sometimes with an oil film in the water.

Disturbed fat digestion can sit behind it. Fat needs two things: bile, which breaks it into fine droplets, and enzymes from the pancreas, which split it. If one of the two drops out, the fat passes through undigested.

For the pancreas there is a stool test, elastase-1. And it is precisely around this value that most misunderstandings arise.

The other big cause of fat in stool sits not in the organ but in the lining of the small intestine. If the villi are damaged, fat does not get through the wall despite good enzymes. Coeliac disease is the best known example of this. And here stands the practically most important sentence of this whole subject area: whoever already eats gluten free on suspicion can make the diagnosis impossible, because antibodies and the mucosal finding regress on a gluten free diet. Test first, then leave out. More on this in Recognising coeliac disease and, for the grey zone without coeliac disease, in Gluten and gliadin without coeliac disease.

Pale to clay coloured with dark urine: the combination is what counts

On its own, pale stool is unspecific. After a very low fat meal, after contrast medium, with a very fast passage, it can look paler.

What counts is the combination. Pale, decoloured, almost putty coloured stool together with strikingly dark urine is a pattern, not a coincidence. It fits the situation in which too little bile reaches the gut while the bile pigment leaves via the kidney instead. The stool loses its colour source, the urine gains it.

This can have two quite different roots. Either it lies with the outflow, in which case gallstones in the duct, inflammation and narrowings come into question, benign as well as malignant. Or it lies with the liver itself, in which case a viral hepatitis can be behind it, or a strain from medicines, supplements or alcohol. Which of the two roots it is, is decided by laboratory tests and ultrasound and not by the colour. That belongs promptly in medical hands. If skin or the whites of the eyes turn yellow as well, promptly becomes immediately.

And one situation belongs here explicitly: in pregnancy, intrahepatic cholestasis of pregnancy can sit behind pale stool with dark urine, usually together with intense itching, often on hands and feet and worst at night. That is a reason to call the practice looking after you the same day, because it concerns the child as well.

Black: the relaxed cases and the one that cannot wait

Black is the colour that frightens most people, and in many cases, on closer inspection, not rightly so after all.

A second point belongs with iron, because in everyday life it often explains complaints that nobody connects with the tablet.

Red: beetroot or blood

Beetroot is the best known harmless red colourer, joined by large amounts of tomato, pepper skins, watermelon and red food colouring from drinks or sweets.

Two questions narrow the matter down in everyday life. Was there such a meal in the last one to two days. And does the colour disappear once the food is gone.

And now the point that many colour charts have exactly the wrong way round: reddish urine is not a sign of the all clear. Beetroot can tint it, but so can blood from the bladder or the kidney. If your urine is reddish and you cannot say with confidence that it comes from yesterday's meal, that belongs in a medical assessment, regardless of what the stool is doing. In the same analysis by Jones from which the figures on rectal bleeding come, visible blood in the urine was in fact the alarm symptom with the highest predictive value.

Blood looks different. Bright red streaks on an otherwise brown stool, red drops in the water or on the paper, or a darker red admixture in the stool itself. Colour from food runs evenly through the whole stool, blood usually lies on it or in it.

And even so, and this is the point: this distinction never ends at the toilet. It ends in a conversation. Visible blood belongs in a medical assessment, even when haemorrhoids are known, because the same bleeding can come from a fissure, a fistula, an inflammatory bowel disease or a tumour. Placing these complaints is covered in Haemorrhoids and anal complaints in context.

On the frequently quoted figure of how many people show any discolouration at all after beetroot: I deliberately leave it out. It circulates in many texts, and in this research I could not trace it back to a checkable original source. Better no figure than an uncovered one.

And now the sentence that stands above everything

One more thing: for infants and small children a completely separate set of rules applies, and there colour, and especially decoloured stool, carries a quite different weight. This article is written exclusively for adults.

And now you know why there are three serious colours here and not twenty.

Floating, mucus, undigested remains

Three observations that are dramatised particularly often online. All three are usually harmless, and all three have a border where that stops.

Floating stool: a question of gas

The widespread assumption goes: if it floats, there is fat in it. This assumption was already examined more than fifty years ago, and the work from that time does not support it.

Steatorrhoea looks different, and you recognise it by several features at once: pale, glossy, sticky, bulky, hard to flush away, extremely foul smelling, sometimes with an oil film. One is everyday life. The other is a reason for diagnostics.

If the gas side interests you in more detail, so where the air comes from at all and when it becomes a problem, that sits in Bloating: where the air comes from. And if certain sugars are the trigger, in Lactose, fructose and sorbitol.

Mucus: more common than you think

The gut lining constantly produces mucins. That is a lubricating layer and at the same time a protective layer that keeps bacteria at a distance. A little of it always comes along, usually invisibly.

Mucus becomes visible as a whitish or glassy film on the stool, sometimes in threads. That occurs frequently: in irritable bowel syndrome, after infections, in phases with very hard stool, with haemorrhoids, after strongly irritating meals.

What is interesting is how the professional world has dealt with it. Mucus was long regarded as a feature of irritable bowel syndrome and stood in older criteria catalogues. In the current version of the Rome criteria it is no longer part of the definition, because it is too unspecific. Too many healthy people have it, too little meaning sits in it.

The border can be drawn clearly. Mucus together with blood, with pus, with fever, with night time symptoms, with weight loss or with a new persistent change in bowel habit is something other than mucus alone. Then it is about inflammation, and for that there are laboratory tests and endoscopy. If the pattern as a whole points towards irritable bowel syndrome, you will find the search for causes in Irritable bowel: finding the causes.

Undigested remains: sweetcorn is normal, fat is not

The most common worry goes: I am not digesting anything any more, I see it all again.

Mostly you see again exactly those things that humans cannot split at all. Sweetcorn has a hull of cellulose, and for cellulose humans possess no enzyme. The same applies to pepper skins, tomato skin, berry pips and whole seeds such as linseed or sesame. Whoever does not chew them thoroughly or eat them ground sees them again.

That is not a weakness. That is the blueprint. And it is textbook physiology without a study base of its own, which is why I mark it exactly that way.

At this point the question of digestive enzymes from a capsule often comes up. My order in practice is a different one, and it is deliberately unspectacular: first the question of whether enough stomach acid is being produced at the top at all, then the enzyme question. Stomach acid can be the first switch of protein digestion and at the same time a signal that helps set pancreas and bile in motion. Whoever skips this question, in my experience, sometimes replaces something that is not missing at all. This is my clinical order and not a guideline recommendation, and I say that explicitly. The counterpart belongs with it just as much: betaine HCl is not a harmless self experiment. With a stomach ulcer, on acid blockers or on anti-inflammatory painkillers it can do harm. So this question belongs in a conversation and not in a shopping basket. The background sits in Low stomach acid and betaine HCl, the enzyme question itself in Digestive enzymes: when they make sense.

And now you know why these three observations create so much fear and so rarely mean anything.

Smell, and what it reveals about your food

Nobody likes talking about smell, and yet it is the part of this whole subject that can be grasped chemically best.

Stool never smells neutral. It becomes interesting when the smell changes markedly, especially towards rotten eggs. What sits behind it is not a character flaw, but a molecule.

And where does the sulphur come from? Very largely from sulphur containing amino acids, so from protein. That can even be dosed.

Now comes the bridge that connects smell with form. Because it depends not only on what arrives, but also on how long it lies there.

A word on alcohol, because it is often underestimated in this context. It changes passage, mucosa and microbiome by several routes at once. Factually and without moralising, that sits in Alcohol and the gut.

And what can push putrefaction back is fibre. The more fermentable material arrives in the colon, the more likely the bacteria are to run on carbohydrate fermentation instead of protein breakdown. That is one of the most plausible reasons why smell can change with diet. What is behind the number 30 and what is not sits in Fibre myths: what holds up, and which fibres actually feed the microbiome in Prebiotics and resistant starch.

And now you know why smell is the most honest feedback on your plate.

Measuring your transit time yourself, and what a diary really gives you

Up to here we have been estimating. Form and frequency are proxy measures for the pace inside the gut. It can also be done directly, at home, with a muffin.

To place your number: in the British study by Cummings the median transit time was 60 hours, in men 55 and in women 72. Your personal number somewhere in this order of magnitude is unremarkable. Very short and very long values are more a reason to look at the pattern over weeks than a diagnosis.

There is another very practical reason to know your own consistency. It influences what a microbiome test shows at all.

What shifts your pattern from outside

Before you draw conclusions from four weeks of a diary, a look at the confounders pays off. Most of them are everyday observations without a solid study base, and I name them exactly that way.

For the stress and sleep side there are articles of their own, because both measurably co-steer the gut: Gut brain axis and the vagus nerve as well as Sleep and the gut microbiome.

What a four week diary gives the consulting room

And with that we are at the actual use of this whole article.

A diary over four weeks turns a feeling into a description. Instead of somehow irregular, it then says: 21 entries, 14 times type 5 to 6, clustering at the weekend, two days with urgency, no blood, no fever, no weight loss, cycle day noted each time.

With a sheet like that, a conversation begins at a completely different place. And it has a second effect that I keep observing in the consulting room: some people discover a pattern themselves while writing it down, one they could not name before. How often that happens I cannot quantify, that is an observation and not a study result.

Two limits at the end, so that nothing slips. First: nothing in this text replaces a recommended examination. If a colonoscopy, an endoscopy or laboratory diagnostics has been advised to you, a diary is an addition and never a substitute. Second: the way from observation to answer runs via laboratory tests and endoscopy, not via colour. That is exactly how the international irritable bowel guideline puts it too.

If you want to put the whole thing into a larger context, so from observation to an ordered approach, the overall concept sits in Gut reset: the whole gut treatment concept.

And now you know why the embarrassing glance is worth it.

Frequently asked questions about bowel movements

Where this subject connects to the rest of the gut

What you see at the toilet is one section of the picture. It only becomes usable once you place it next to the question that actually occupies you.