Colon Cancer and Thin Stools

Bowel Movement Changes and Colon Cancer

When it is caught early, colon cancer rarely causes symptoms. Over time, if a tumor is allowed to grow, it can start to cause problems within the bowel. Although there are many causes of thin stools, this symptom can be caused by a large tumor in the colon.

What Is a Thin Stool?

Thin stools are described as "ribbon-like," "pencil-like," "thin" or "narrow" bowel movements. If you normally pass stools the size of bananas, starting to pass stools the size of pencils would indicate a bowel habit change.

Referring to the anatomy of the colon, your stool should be completely formed (solid) as it reaches your descending and sigmoid colon, which lie on the left side of your abdomen. Left-sided colon cancers most frequently cause bowel movement related symptoms of colon cancer, such as thin or bloody stools.

Normal Bowel Movements

At the risk of being indelicate, there is no such thing as a universally normal bowel movement. Clinically speaking, your doctor only cares about what is normal for you. Everyone’s stool size, color, smell and consistency are different. The characteristics of your bowel movements are determined by your fiber intake, water consumption, bowel motility, exercise, and the size of your anal sphincter (anus) and rectum. Bowel movements are typically brown in color due to bile breakdown during digestion. Your bowel movements should be smelly – this is a sign that you have good bacteria in your digestive tract, which your body uses to break down ingested foods.

You may have a bowel movement twice a day or twice a week – only you will know if your bowel movements have changed. Sudden changes in bowel movement frequency, color, shape or consistency could have many causes, many of them benign (non-cancerous). Some common causes of non-cancerous bowel habit changes include:

  • Diarrhea
  • Constipation
  • Dietary changes
  • Bacteria in food
  • Gastroenteritis
  • Dehydration
  • Sedentary lifestyle
  • Illness
  • Irritable Bowel Syndrome
  • Prescription or over-the-counter medication use

If you pass a bowel movement once daily for the last 10 years, decreasing stool frequency to four bowel movements in one month is a major personal bowel habit change that should be reported to your doctor. Similarly, if you normally pass very large, firm bowel movements and begin passing small, thin or watery stools, this bowel habit change should be investigated.

Why Can Colon Cancer Cause Thin Stools?

In an otherwise healthy person, thin stools related to colon cancer are usually caused by a narrowing within the colon, also called a partial blockage of the colon. Think of your colon as a hollow tube, like a drinking straw. If a tumor is growing inside the colon, it can start to obstruct the flow of stool through your colon, just like a pea inside a drinking straw would block or slow the flow of fluids.

Cancers of the rectum can also cause partial and complete blockages of stool passage, as well as an uncomfortable feeling of urgency. This symptom can be caused by a growing tumor within the small rectal space, which places pressure on the nerves making you feel as if you need to move your bowels.

Linked Symptoms

If you have noted bowel habit changes and are waiting to see your doctor, pay special attention to other symptoms to discuss them with your doctor. In addition to thin stools, advanced colon cancer can cause:

Risk Factors

Several risk factors may increase your chances of having a partial or complete intestinal blockage with colon cancer. Two treatment modalities for colon cancer, abdominal surgery and radiation, may increase your chances of a bowel obstruction due to the formation of adhesions or scar tissue around the colon.

Hernias, diverticulitis, and cancers of the stomach and ovary may also contribute to the formation of a bowel obstruction, resulting in thin or absent stools.

What Now?

Once you have identified bowel habit changes, the first step is to report these and any related symptoms to your doctor. He or she can order screening tests to rule out colon cancer and other, less serious, causes of thin stools. If you have concerns about your bowel habits it is best to discuss them with your doctor. When it comes to your stool – you are the expert.


American Cancer Society. (2006). American Cancer Society’s Complete Guide to Colorectal Cancer. Clifton Fields, NE: American Cancer Society.

National Cancer Institute. (n.d.). Bowel Obstruction. Accessed: January 4, 2012.

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