Foods that are high in fiber or have hard shells, like sweetcorn, often pass through the body undigested. However, other undigested foods in stool could indicate an underlying condition, such as Crohn’s disease.

When undigested food in the stool occurs with a change in bowel habits, weight loss, or diarrhea, this may signal an underlying condition that needs medical attention.

In this article, we look at reasons why undigested food may appear in stool and when a person should speak with a doctor.

Share on Pinterest
Antenore/Getty Images

There are some reasons why the body might not completely digest foods that do not cause concern.

Undigested food can appear in the stool if there is material in food that is indigestible, such as cellulose in some high-fiber foods. This could be due to a person not chewing the food well or the food containing shells or skins that the body’s natural enzymes cannot break down.

It could also happen if food passes too quickly through the digestive system, for example, if the person has a condition such as Crohn’s disease, which causes inflammation in the digestive system. It can also happen if someone has gastric surgery or a bowel resection.

High-fiber foods

Fiber refers to the indigestible part of plant foods. A high-fiber diet can have many benefits, including keeping the digestive system healthy and keeping bowel movements regular.

When a person eats high-fiber foods, it is common for some undigested material to appear in the stool because the body cannot fully break down the tough material.

Fiber also speeds up a person’s bowel movements by adding bulk to the stool, which encourages the intestinal walls to move. This movement pushes food through the digestive tract.

If food moves too quickly through the digestive system, a person may not fully digest some foods.

Certain foods are more likely than others to be difficult to fully digest. These foods include:

Foods such as corn often remain partially undigested. Corn has an outer shell made of an indigestible material called cellulose. The body digests the material inside of the corn and expels the hard outer casing in the stool.

High-fiber foods are beneficial for most people. Learn more here.

Eating too quickly

Another harmless culprit of undigested food appearing in stool is eating too quickly. When a person eats too fast and swallows their food without fully chewing it, food is more likely to pass through the digestive tract without being completely broken down.

Eating too quickly may force digestion to take place too rapidly, resulting in more food not being fully broken down. To avoid this, a person can slow down their eating during meals and chew their food for longer.

The most common cause of undigested food appearing in stool is fibrous foods, but some medical conditions can result in whole or partially digested food appearing in stool. In these cases, a person often notices other symptoms, such as diarrhea or stomach pain.

Medical conditions that may cause undigested food to appear in the stool include the following:

Crohn’s disease

This condition is a type of inflammatory bowel disease that causes inflammation in the digestive tract.

Symptoms include:

In some cases, undigested food may appear in a person’s stool.

Learn more about Crohn’s disease.

Celiac disease

This is an autoimmune disorder where the body cannot digest the protein known as gluten, which occurs in wheat, barley, and other grains.

Symptoms include:

Some people may notice undigested food in their stool.

Learn more about celiac disease.

Pancreatic insufficiency

A person with pancreatic insufficiency lacks enzymes in the pancreas, making it hard to break down food. This could lead to undigested food appearing in stool.

Symptoms of this condition include:

  • abdominal discomfort and bloating
  • weight loss
  • fatty stools
  • nutritional deficiencies, leading to muscle spasms, bone disease, and a higher risk of cardiovascular problems

What is exocrine pancreatic insufficiency?

Lactose intolerance

If a person’s digestive system cannot break down the protein in milk and dairy, it may indicate lactose intolerance.

Symptoms include:

  • bloating and gas
  • diarrhea
  • nausea and vomiting
  • abdominal pain
  • “growling” in the stomach

Undigested food could appear in the stool of people with lactose intolerance.

What is lactose intolerance?

Irritable bowel syndrome (IBS)

IBS is a common condition that affects the large intestine.

Symptoms include:

Some people may also notice undigested food in their stool.

Find out more about IBS.

Gastroenteritis, or a “stomach bug”

A stomach bug, stomach flu, or gastrointestinal virus may cause undigested food in the stool due to food passing quickly through a person’s system. Other symptoms of a stomach bug include:

In most cases of a stomach virus, symptoms will resolve in a few days when the virus has passed. However, dehydration can quickly become serious. People should seek medical help at once if they or someone else has signs of dehydration or experiences:

  • severe pain or worsening symptoms
  • changes in mental state or consciousness
  • six or more loose stools in a day
  • diarrhea lasting more than 2 days
  • blood or pus in stool

There is a possibility that undigested food could appear in the stool.

Find out more about gastroenteritis.

A person who notices a lot of undigested food in their stool should not worry most of the time, as it is likely to be due to undigested fiber or eating too quickly.

If someone notices the following symptoms plus undigested food in their stool, they should speak with a doctor:

  • unexplained weight loss
  • blood in stool
  • changes in bowel habits
  • loss of control of bowels
  • chronic diarrhea
  • persistent abdominal pain or cramping
  • persistent bloating or gas
  • signs of dehydration, such as increased thirst or reduced urination

If a doctor suspects that a person has a condition affecting their digestive system, they may order the following diagnostic tests:

  • a stool sample to look for blood and other abnormalities
  • blood tests to look for nutritional deficiencies or inflammation markers
  • endoscopy to look inside the upper digestive tract
  • colonoscopy to examine the lower gastrointestinal tract
  • biopsy to check for microscopic inflammation

A high-fiber diet is a healthy option for most people, and there is no need for concern if some food particles remain undigested.

However, if the body cannot digest food due to an underlying condition, a doctor will recommend treatment to address the problem.

Depending on the cause, they may recommend medication, dietary changes, or surgery in some cases.

Most of the time, seeing undigested food in the stool is not a cause for concern. It may be the result of eating too fast or eating food with high fiber content.

In situations where a person has other symptoms, including abdominal pain, weight loss, or changes in bowel habits and movements, they may have an underlying medical condition.

A person who has concerns about certain symptoms or changes in their bowel movements should speak with a doctor.