Today, I'm going to go over the subtle symptoms and signs of colon cancer that you should never ignore. Colon cancer is one of those devastating cancers where, on one hand, it's one of the most preventable cancers if caught early. Still, on the other hand, it's unfortunately one of the more deadly cancers, becoming incredibly hard to treat if found at a later, more advanced stage.
This matters because rates of colon cancer are on the rise, especially among young people. The number of colorectal cancer cases in people under 55 has doubled over the past few decades, and colon cancer is becoming so prevalent in younger people that, according to the National Cancer Institute, it's now the number one cause of cancer death in people between the ages of 20 and 49.
To put this in perspective, the 5-year survival rate for localized colon cancer is 91%, but it drops all the way down to 13% once it spreads outside the colon and metastasizes to other organs. Recognizing early symptoms could save lives, especially for younger people who historically haven't had to worry much about this cancer.
What Is Colon Cancer?
Before diving into the signs, let's quickly break down what colon cancer, or colorectal cancer, actually is. Colon cancer begins in the large intestine, or colon, and more often than not, it starts as a benign, non-cancerous polyp. Some of these polyps can turn cancerous over time. The good news is that if caught early, they can be easily removed, which is exactly what happens during a routine screening colonoscopy. Colon cancer is one of the few cancers where screening and treatment can happen at the same time.
1. Changes in Bowel Habits
The first sign to watch for is any change in bowel habits, whether that's new constipation, frequent loose stools, or diarrhea. Having diarrhea or constipation for a week or two can be fairly common and usually isn't a major concern. Still, if that change persists for longer than a few weeks, it could indicate a more serious problem.
This happens because the colon is responsible for absorbing water and forming stool, so any disruption to its function due to a tumor or mass can present as chronic diarrhea or constipation. Diarrhea occurs because the growth irritates the bowel lining, increasing water secretion and speeding up transit time. Constipation, on the other hand, typically shows up when a tumor creates a physical blockage that slows or prevents stool passage. Alternating patterns of diarrhea and constipation can actually signal that a tumor is intermittently obstructing the colon.
2. Unexplained Weight Loss
The next sign to discuss is unexplained weight loss. Colon cancer can lead to weight loss in two ways: first, by physically interfering with proper nutrient absorption in the intestine, and second, because many cancers, including colon cancer, redirect the body's nutrients to support their own growth. They do this by releasing substances called cytokines that hijack the body's normal signaling processes to fuel their own proliferation. If you find yourself losing weight unintentionally, without a clear reason, it's worth getting evaluated, not just for colon cancer, but for other cancers as well.
3. Blood in the Stool
Another subtle warning sign of colon cancer is blood in the stool, medically known as hematochezia. This is actually one of the hallmark signs of colon cancer. That said, the most common cause of blood in the stool isn't colon cancer, but hemorrhoids. Another fairly common cause is a condition called diverticulosis, where small pouches form in the walls of the intestine and can occasionally bleed or become infected.
That said, anytime you notice blood in your stool, in the toilet, or on toilet paper after wiping, you shouldn't assume it's simply hemorrhoids. It's important to get checked out to rule out something more serious.
4. Dark or Black Stools
Another sign to watch for is dark or black stools. This dark color is also caused by bleeding, but it typically indicates the source is in the upper digestive system, such as the stomach or small intestine. As blood travels through the GI tract, it interacts with digestive chemicals, which change its color from red to black. That said, this dark blood can also originate from the colon, specifically the ascending colon on the right side of the abdomen. So while black stools typically point to an issue higher up in the digestive tract, in some cases, they can indicate a bleeding source in the colon that needs to be evaluated for cancer.
5. Iron Deficiency Anemia
This brings us to another sign that should always be evaluated for colon cancer: iron deficiency anemia. This happens because colon cancers may not always cause obvious, visible bleeding. Instead, they can cause chronic, low-grade blood loss from a bleeding tumor, which gradually depletes the body's iron reserves. Since iron is needed to build and replenish hemoglobin in red blood cells, this constant turnover eventually leads to iron deficiency and anemia.
Colon cancer isn't the most common cause of iron deficiency anemia, more often it's due to inadequate iron intake, heavy menstrual periods, or kidney disease. But if you're diagnosed with low iron and anemia, especially if you're a man, colon cancer should definitely be ruled out as a cause.
6. A Feeling of Incomplete Bowel Movements (Tenesmus)
Another symptom that could point to colon cancer is a feeling of fullness after a bowel movement, or a sense of incomplete evacuation, medically known as tenesmus. This happens because, as a cancer grows, especially in the lower colon or rectum, it can press on the bowel wall or mimic the sensation of stool remaining in the rectum. This symptom is often mistaken for a common issue like irritable bowel syndrome, but it can also be an early sign of cancer, especially if it persists for several weeks.
7. Excessive Gas or Bloating
Another symptom worth evaluating if it persists for more than a few weeks is excessive gas or bloating. This can happen through several mechanisms. If a tumor grows large enough, it can create a physical blockage in the colon that traps gas, leading to bloating and abdominal discomfort.
Tumors can also disrupt normal digestion and fermentation of food in the colon, which throws off the balance of gut bacteria, and any disruption to the gut microbiome can lead to more gas and bloating. In some cases, partial blockage from a tumor slows the passage of waste, giving bacteria more time to ferment undigested food, which further increases gas production.
If you have persistent bloating without any recent dietary changes, or if bloating and gas persist for more than a few weeks, that's a red flag worth having evaluated.
The Most Common Early Sign: No Symptoms at All
Now let's talk about the most common "early sign" of colon cancer: having no symptoms at all. Colon cancer often has to be fairly advanced before it starts producing noticeable symptoms, which is exactly why aggressive screening is so important. The best time to find colon cancer is when you have no symptoms whatsoever, and the earlier it's found, the higher your chances of survival.
When and How to Get Screened
In the US, the recommendation is to start colon cancer screening at age 45 for people at average risk. Screening can be done either with a colonoscopy or with a more non-invasive at-home stool test.
Colonoscopy involves inserting a long, flexible tube with a camera into the rectum, allowing your GI doctor to examine the entire colon for polyps or abnormal tissue.
If any concerning polyps are found, they can typically be removed right away and sent for biopsy. Colonoscopy is considered the gold standard for screening and diagnosing colon cancer. Still, it does have some downsides: it requires bowel preparation the day before the procedure, which isn't the most pleasant experience, and there's a small risk of complications like bleeding or perforation during the procedure, as well as a small risk from the sedation typically used in the US.
Stool-based tests are a less invasive alternative. These include the fecal occult blood test and the fecal immunochemical test (FIT), which check a stool sample for blood that could indicate possible colon cancer. Some stool tests can also check for DNA changes associated with colon cancer or large polyps. The advantage of these tests is that they're non-invasive and don't require a procedure or sedation. Still, they're less effective at detecting polyps or cancer precursors than a colonoscopy, and they need to be repeated every 1 to 3 years, depending on the specific test.
There are pros and cons to each option, but the most important thing is getting screened, regardless of which test you choose.
When to Start Screening Earlier
Age 45 is the latest recommended starting point for average-risk individuals, but in some cases, screening should start much earlier. If you have inflammatory bowel disease, like Crohn's disease or ulcerative colitis, you'll need to start screening earlier. If you have a family member with a history of colon cancer or high-risk polyps, the recommendation is to start screening at age 40, or 10 years before the age at which your relative was diagnosed, whichever comes first. For example, if a relative were diagnosed at age 35, your screening would need to start 10 years earlier, at age 25.
There's some additional nuance depending on other risk factors and how closely related you are to the affected family member. Still, your doctor can help guide you toward the best test and timing for your specific situation.
Conclusion
Colon cancer is uniquely both highly preventable and, if caught late, highly dangerous, which makes awareness of its subtle warning signs, changes in bowel habits, unexplained weight loss, blood in the stool, dark stools, iron deficiency anemia, tenesmus, and persistent bloating, genuinely lifesaving. Since colon cancer often produces no symptoms until it's fairly advanced, screening remains the single most powerful tool for catching it early, whether through colonoscopy or a stool-based test. Starting screening at age 45, or earlier if you have relevant risk factors like inflammatory bowel disease or a family history, gives you the best chance of catching this cancer while it's still highly treatable.
FAQ
What are the early warning signs of colon cancer?
Early signs can include changes in bowel habits (new constipation or diarrhea), unexplained weight loss, blood in the stool, dark or black stools, iron deficiency anemia, a feeling of incomplete bowel movements (tenesmus), and persistent gas or bloating.
Does colon cancer always cause noticeable symptoms?
No, and this is actually one of the most important things to understand. Colon cancer often causes no symptoms until it's fairly advanced, which is why regular screening, rather than waiting for symptoms, is so important.
At what age should colon cancer screening start?
For people at average risk, screening should start at age 45. Those with inflammatory bowel disease or a family history of colon cancer or high-risk polyps may need to start earlier, sometimes as young as their 20s or 30s, depending on individual risk factors.
What's the difference between a colonoscopy and a stool-based test?
Colonoscopy is considered the gold standard, allowing direct visualization and removal of polyps during the same procedure, but it requires bowel prep and carries small procedural risks. Stool-based tests (like FIT or fecal occult blood tests) are non-invasive and easier to do at home, but are less sensitive at detecting polyps or early cancers and need to be repeated more frequently.
Is blood in the stool always a sign of colon cancer?
No. The most common causes of blood in the stool are hemorrhoids or diverticulosis, not cancer. However, any blood in the stool should still be evaluated by a doctor to rule out more serious causes.
Why are colon cancer rates rising in young people?
The transcript notes that colorectal cancer cases in people under 55 have doubled over recent decades, making it now the leading cause of cancer death in people aged 20 to 49, though the full explanation for this trend involves ongoing research into diet, lifestyle, and other risk factors.