Stool colour: what matters and what is just food
The colour key
Brown, in all its shades
NormalBile pigment is broken down by gut bacteria into stercobilin, which is brown. The exact shade shifts constantly with diet and transit speed and carries almost no information on its own.
Green
Usually harmlessTwo ordinary explanations. Either you ate something green (leafy vegetables, food colouring, iron supplements), or things moved through fast enough that bile did not have time to finish breaking down from green to brown. Persistent green with persistent diarrhoea is worth mentioning to a clinician, but green alone is not an alarm.
Yellow, greasy, foul-smelling, hard to flush
Get it checkedThis combination, not yellow alone, suggests fat is not being absorbed properly. It is called steatorrhoea. Causes worth excluding include coeliac disease and pancreatic exocrine insufficiency, which is diagnosed with a faecal elastase test and treated with prescribed enzymes rather than by cutting foods out. See which gut tests are worth it.
Pale, clay-coloured or chalky white
See a doctorStool gets its colour from bile. Persistently pale stool can mean bile is not reaching the gut, which points at the liver, gallbladder or bile ducts, particularly alongside dark urine, yellowing of the eyes or skin, or itching. One pale stool after a barium study or certain medicines is different. Persistent pale stool needs assessment.
Red or bloody
See a doctorBeetroot, tomato and red food colouring do this, and that is the common explanation. But bright red blood can come from haemorrhoids, a fissure, inflammation, polyps or cancer, and you cannot tell which from looking. Do not assume it is piles because you have piles. Get it looked at, especially if it is new, repeated, or comes with a change in bowel habit.
Black and tarry, with a sticky texture and strong smell
UrgentIron tablets, bismuth medicines such as Pepto-Bismol, black liquorice and a lot of blueberries all turn stool black, and that is usually what it is. The one that matters is melaena, which is digested blood from higher up the gut, and it is a medical emergency. It is characteristically tarry and sticky rather than simply dark. If you are not taking iron or bismuth and your stool is black and tarry, seek urgent care the same day.
Appearance matters more than colour
Colour gets all the attention, but three other observations carry more clinical weight and are routinely ignored.
- Greasy, oily, hard to flush. The combination that suggests fat malabsorption. Worth a faecal elastase test and coeliac serology rather than an elimination diet.
- Mucus. A small amount of clear or white mucus is normal, since the bowel is lined with it. Consistently large amounts, or mucus with blood, is worth reporting.
- Narrow, pencil-thin stool that persists. Occasional thin stool means little. A sustained change in calibre is one of the things a clinician wants to hear about.
Why colour is a poor tracking target
It is tempting to log colour daily. In practice it produces a lot of noise and almost no signal, because the variation is dominated by what you ate one to two days ago rather than by anything about your gut. Stool form on the Bristol scale is the more informative thing to record routinely, because it tracks transit time and it responds to the things you are actually trying to test.
Colour is better treated as a flag: something you note when it is one of the three that matter, and otherwise leave alone. That is how Minthe handles it. Colour is there to catch a red flag and route you out of the app, not to be graded every morning.
Minthe is a wellness and self-tracking tool, not a medical device. It does not diagnose or treat any condition and isn't a substitute for professional medical advice. If you have red-flag symptoms such as blood in your stool, unintentional weight loss, difficulty swallowing, persistent vomiting or a fever alongside gut symptoms, see a clinician rather than tracking them.
Frequently asked
Which stool colours actually need a doctor?
Three. Black and tarry stool when you are not taking iron or bismuth, because it can be digested blood from the upper gut and that is urgent. Red or bloody stool that is not explained by beetroot, because you cannot tell haemorrhoids from something more serious by looking. And persistently pale, clay-coloured stool, which can mean bile is not reaching the gut. Everything else is usually diet or transit speed.
Why is my stool green?
Almost always either something green you ate, including leafy vegetables, food colouring and iron supplements, or fast transit. Bile starts out green and gut bacteria turn it brown over time. If stool moves through quickly, that conversion does not finish. Green on its own, in someone who feels well, is not a warning sign.
Does floating stool mean something is wrong?
Usually not. Floating is mostly caused by trapped gas, which is common and unremarkable, particularly after a high-fibre or high-FODMAP meal. It becomes worth investigating when it comes with the full picture of fat malabsorption: pale or yellow, greasy or oily, foul-smelling, difficult to flush, and often alongside weight loss.
I take iron and my stool is black. Should I worry?
Iron tablets very commonly turn stool black or dark green, and so does bismuth subsalicylate. That is expected. The distinction that matters is texture and smell: iron-related black stool looks like normal stool that is dark, whereas melaena is sticky, tarry and has a distinctive strong odour. If you are unsure, that is a reasonable thing to ask a pharmacist or GP about rather than guess.
Can food really change stool colour that much?
Yes, and it is the single most common explanation. Beetroot turns it red, which alarms people every year. Heavy leafy greens turn it green. Blueberries, black liquorice and dark food colourings darken it. Turmeric-heavy meals can yellow it. Food colouring used in iced drinks and sweets is a frequent culprit in children.