Why Is Faeces Brown? Science Explained

The short answer is that faeces are brown because of a pigment called stercobilin. This pigment is a waste product created when your body breaks down old red blood cells. The entire process happens in your liver and intestines, and the final brown colour is a sign that your digestive system is working as it should.

Where Does the Brown Colour Come From?

The story starts with red blood cells. These cells live for about 120 days. When they get old, your body breaks them down and removes them from circulation. This happens mostly in the spleen and liver.

During this breakdown, the red blood cells release a yellow-orange pigment called bilirubin. Bilirubin travels to the liver, where it is processed and mixed into bile. Bile is a digestive fluid that helps you break down fats.

Bile moves from the liver into the gallbladder and then into the small intestine. Here, bile helps digest your food. But the bilirubin in the bile is not done yet. It continues moving through the intestines, where bacteria that live naturally in your gut start to work on it.

These gut bacteria change the bilirubin into a new substance called urobilinogen. Some of this urobilinogen is absorbed back into the blood and eventually leaves your body in urine. That is why urine has a yellow tint. The rest of the urobilinogen stays in the intestines, where it is converted into stercobilin. Stercobilin is a dark brown pigment, and it gives your stool its characteristic colour.

Does Diet Change the Colour?

Your diet can change the shade of your stool, but it does not usually change the fact that it is brown. The brown base colour comes from stercobilin. Food can add to that colour or change it temporarily.

Foods that are high in iron can make stool look darker, sometimes almost black. Foods with strong natural dyes can also have an effect. Beets are a good example. Eating beets can turn stool a reddish or dark maroon colour. This is harmless and usually passes within a day or two.

Green leafy vegetables like spinach or kale contain chlorophyll, which is the green pigment in plants. If you eat a lot of them, your stool can look greenish. This happens because the chlorophyll is not fully broken down by the time it reaches the end of your digestive tract.

Artificial food colouring can also change stool colour. Blue or purple dyes, for example, can make stool look blue or green. These changes are temporary and not a cause for concern.

What Does Pale or Clay-Coloured Stool Mean?

If your stool loses its brown colour and becomes pale, grey, or clay-coloured, that is a different situation. This is not caused by diet. It usually means that bile is not reaching your intestines properly.

Bile is what carries bilirubin into the digestive tract in the first place. If bile cannot flow from the liver or gallbladder into the intestines, the bilirubin never gets a chance to be converted into stercobilin. Without stercobilin, your stool has no brown pigment.

A blockage in the bile ducts is the most common cause. Gallstones, tumours, or inflammation of the bile ducts can all block bile flow. Liver conditions such as hepatitis or cirrhosis can also reduce bile production.

Pale stool is not something to ignore. If your stool is persistently pale or clay-coloured, you should see a doctor. It can be an early sign of a problem with your liver, gallbladder, or pancreas.

Other Stool Colours and What They Can Signal

Most stool colour changes are harmless and temporary. But some colours deserve more attention than others. Knowing the difference can help you decide when to seek medical advice.

• Black stool: Can be caused by iron supplements, bismuth medications like Pepto-Bismol, or black licorice. It can also be a sign of bleeding in the upper digestive tract, such as the stomach or the first part of the small intestine. If you have not taken iron or bismuth, black stool should be evaluated by a doctor.
• Red stool: Bright red blood in stool usually comes from bleeding in the lower digestive tract, such as the rectum or the large intestine. Haemorrhoids and anal fissures are common causes. Red stool can also come from eating beets or foods with red dye. If the redness is from blood and not food, it needs medical attention.
• Green stool: Often caused by eating green vegetables, green food colouring, or food passing through the intestines too quickly. When transit time is fast, bile does not have enough time to turn brown. This is usually harmless.
• Yellow stool: Can appear greasy and foul-smelling. This can happen when your body does not absorb fat properly, which may be related to conditions like coeliac disease or problems with the pancreas.

If a stool colour change lasts more than a few days and is not linked to something you ate, it is worth discussing with a healthcare provider.

Why Is Faeces Brown in Different Animals?

The brown colour of stool is not unique to humans. Most mammals produce brown faeces for the same reason. They break down red blood cells, produce bilirubin, and gut bacteria convert it into stercobilin.

There are exceptions. Birds, reptiles, and amphibians excrete waste differently. They combine solid waste with uric acid, which is a white paste-like substance. Their faeces often look like a dark centre surrounded by white. The white part is the uric acid, not the digested food.

Some animals, like rabbits, produce two types of faeces. One type is the familiar brown pellet. The other is a softer form called caecotrophs, which they eat to get more nutrients from their food. The colour is still brown, but the composition is different.

Even among mammals, the shade of brown can vary. Herbivores like cows and horses tend to have greener or more olive-toned stool because their diet is so high in plant material. Carnivores tend to have darker, more uniform brown stool because their food is more easily digested.