Colon Cancer and Poop: Signs to Watch Out For

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Colon cancer, often described collectively as colorectal cancer, is the fourth leading cause of cancer and the second leading cause of cancer-related deaths in the United States, according to the National Cancer Institute.

Part of the reason for the high mortality rate is the relative lack of symptoms in the early stages of the disease. Because of this, more than 60% of cases are diagnosed when the malignancy has already spread regionally or to distant organs.

One of the earliest signs of colon cancer—and one that is frequently missed—are changes in stool and bowel habits. Arguably more than any other symptom, unexplained changes in the consistency, color, or movement of stool should raise concerns about colon cancer, particularly if the symptoms persist or worsen.

older man on toilet googling on his smart phone

Srinrat Wuttichaikitcharoen / EyeEm / Getty Images

Even so, changes like these are frequently missed or attributed to other causes, including age. Given that the median age of colon cancer diagnosis is 67 in men and 71 in women, it is not uncommon to dismiss bowel problems as being "aging-related" rather than investigating cancer as a cause.

By understanding your risk factors and recognizing the tell-tale signs—including those in stool—you will be better equipped to act and seek a diagnosis of this serious yet highly treatable form of cancer.

Early Signs

Colon cancer is a progressive disease in which a local tumor can spread over time and invade nearby tissues or distant organs. The progression of the disease is classified in stages from 1 to 4, with each advancing stage representing a worsening of the disease.

During the early stages of colon cancer—namely, stage 1 (when the tumor is localized) and stage 2 (when cancer has spread to nearby tissue but not the lymph nodes)—the signs can often be absent or non-specific.

The symptoms, if any, tend to be more apparent if there is cancer in the lower part of the digestive tract (such as the rectum and sigmoid colon) than in the parts of the proximal colon (such as the ascending colon or cecum).

Despite these concerns, there may be subtle clues that suggest the onset of a malignancy, including:

  • Narrow "pencil" stools: Caused by the narrowing of the intestinal passage as the tumor starts to grow
  • Pain with defecation: Also known as dyschezia, a symptom commonly associated with rectal cancer
  • A sensation of being unable to empty the bowel: Also known as tenesmus, a symptom commonly seen with early-stage rectal cancer
  • Rectal bleeding: More common with rectal cancer or cancer in the lower bowel.
  • Abdominal pain: Caused when localized inflammation triggers pain receptors called visceral nociceptors
  • Fatigue: Due to intestinal bleeding, the loss of red blood cells, and the development of anemia

Regional Disease

As colon cancer progresses to stage 3, wherein the malignancy has spread to nearby lymph nodes, changes in stool and bowel habits tend to become more apparent as the intestinal passageway (called the lumen) further narrows and the tumor starts to grow into deeper layers of the intestinal tissues.

The development of scar tissue and strictures can eventually cause bowel obstruction, either partial or complete. Bleeding tends to be greater, although it is not always seen in visual inspection.

Left-Sided vs. Right-Sided Colon Cancer

Changes in bowel habits are more typical with left-sided colon cancer, which involves the descending colon, sigmoid colon, and rectum. By contrast, right-sided colon cancer, which involves the ascending colon and cecum, may only manifest with fatigue, anemia, and other non-specific symptoms.

The progression of colon cancer can often be recognized by the following additional symptoms:

  • Constipation: Due in part to increasing bowel obstruction
  • Diarrhea: Caused by an overflow of intestinal fluid around a partial obstruction
  • Alternating constipation and diarrhea: A more tell-sign of colon cancer
  • Blood in stool: Also known as hematochezia, which can be bright red if the tumor is near the rectum or dark red or maroon if higher up in the colon
  • Tenesmus: Which can occur as colon cancer progresses due to the persistent and increasing inflammation
  • Abdominal bloating and cramps: Due in part to the inability to pass gas when the bowel is obstructed
  • Symptoms of iron deficiency anemia: Which occurs in around 48% of people with colon cancer

Advanced Disease

Stage 4 colon cancer is characterized by the spread of cancer to distant organs (also referred to as metastasis). It is the most advanced stage of cancer and one that can manifest with more overt symptoms as the obstruction and bleeding of the bowel worsens.

In some cases, bowel perforation can occur as the tissues are stressed and eventually rupture. Other symptoms can develop due to the spread of cancer to the liver, chest, bone, and other common sites of metastasis.

Although people with stage 4 colon cancer are more likely to develop symptoms, not everyone does. In fact, in the absence of obstruction, perforation, or active bleeding, many people with metastatic colon cancer can be entirely asymptomatic.

Among those who develop symptoms, some of the more common include:

  • Black, tarry stools: Also known as melena, more often seen with right-sided cancer as hemoglobin in the blood gradually darkens during its long passage toward the rectum
  • Nausea and vomiting: Generally seen when obstruction becomes severe
  • Unexplained weight loss: A common consequence of advanced cancer in which systemic inflammation triggers muscle and weight loss as well as the loss of appetite

When to Call 911

Call 911 or seek emergency care if you develop the following:

  • Heavy rectal bleeding
  • Severe abdominal pain and cramping
  • Rigid abdominal muscles
  • Fever and chills
  • Vomiting
  • Cool, clammy skin
  • Rapid heartbeat

These could be signs of peritonitis, a potentially life-threatening condition associated with intestinal perforation.

Conditions That Mimic Colon Cancer

Changes in stool or bowel movement may be suggestive of colon cancer—particularly if you have risk factors for the disease—but there are numerous other explanations for these symptoms. These conditions are far more common but can mimic many of the signs and symptoms of colon cancer. These include:

Despite the similarities, certain signs and symptoms can differentiate the conditions and point the doctor in the direction of colon cancer.

  Colon Cancer Hemor-rhoids DD IBS IBD
Abdominal pain X   X X X
Cramping related to defecation X   X X X
Constipation X X X X  
Diarrhea X   X X X
Gas and bloating X   X X X
Narrow stools X        
Tenesmus X     X X
Rectal itching   X     X
Dyschezia X X   X X
Rectal bleeding X X     X
Blood in stool X   X   X
Mucus in stool   X   X  
Fatigue X       X
Nausea X   X    
Loss of appetite X   X    
Weight loss X       X

A Word From Verywell

Because colon cancer is often asymptomatic, it is important to understand your risk factors for the disease—including older age, a family history of colon cancer, and inflammatory bowel disease—and to adhere to the screening recommendations by the American College of Gastroenterology (ACG).

The ACG's 2021 guidelines recommend screening for all adults starting at age 45 up until 75. Depending on the type of test used, screening may take place every one to three years (for stool-based tests) or every five to 10 years (for endoscopic tests).

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