What your poop says about your gut

Why Stool Consistency Reveals More Than People Realise

This is well-established science, built on studies directly comparing stool appearance with detailed gut bacteria testing.

Stool consistency isn't just a crude, embarrassing detail. It's a genuine, measurable proxy for how long food spends moving through the gut, which itself shapes the bacterial community living there.

The most widely used tool for this is the Bristol Stool Scale, a seven-point visual scale running from hard, separate lumps through to entirely liquid stool.

Stool consistency tracks closely with bacterial diversity

A study analysing stool samples from 53 healthy women found stool consistency, scored on the Bristol Stool Scale, correlated strongly with gut bacterial species richness, the number of different species detected.

The relationship held across nearly every other major microbiome marker measured too, including the balance between two dominant bacterial groups and the overall community type someone's gut settled into.

This matters because stool consistency is something almost anyone can observe about their own gut, without needing a laboratory test, and it turns out to carry real, meaningful information about the community living inside.

Source: Stool consistency is strongly associated with gut microbiota richness and composition, enterotypes and bacterial growth rates

Both extremes of transit time come with lower diversity

A more recent review pulling together data from thousands of people across multiple studies found a consistent pattern at both ends of the transit-time spectrum. People with very fast gut transit and people with very slow gut transit both tended to have lower microbial diversity than people in the middle range.

Fast transit favoured faster-growing bacterial species suited to a high-carbohydrate, lower-fat diet, while slow transit favoured different species better suited to fermenting protein. Each extreme created conditions where a narrower set of specialists could dominate.

This suggests something close to a middle ground being genuinely healthiest for microbial diversity, rather than either extreme, whether that's very hard, slow-moving stool or very loose, fast-moving stool, representing an ideal state.

Source: How Long Poop Stays in Your Body May Impact Your Health, Study Finds

Colour comes from bacteria doing chemical work, not just diet

Stool's typical brown colour isn't simply a byproduct of food. It's the result of gut bacteria chemically transforming bilirubin, a yellow pigment produced when the liver breaks down old red blood cells and releases it into bile.

Bile enters the gut still carrying this pigment in an earlier chemical form, and it's gut bacteria specifically that convert it step by step into stercobilin, the compound actually responsible for stool's characteristic brown colour.

When gut transit is very fast, this conversion sometimes doesn't finish in time, which is one of the more common, usually harmless explanations for green-tinged stool. This is a genuinely useful, everyday example of the gut's bacterial community doing active chemical work most people never think about.

Source: Why is my poo green?

What this means for you

Persistent changes in stool consistency, especially alongside pain, bleeding, or unexplained weight loss, need a doctor's assessment rather than self-diagnosis based on appearance alone. In addition, the research above shows stool consistency is a genuinely useful, easily observed marker of the gut community's overall state, with a middle-ground transit time consistently linked to the greatest bacterial diversity. A varied, plant-rich diet supports the same gut bacteria this research ties to a steadier, more typical transit time. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.