Colorectal adenocarcinoma is the most common form of cancer affecting the colon and rectum. When doctors talk about colon cancer, they are usually referring to this type, which starts in the inner lining of the large intestine and can grow through layers of tissue over time.
Understanding Colorectal Adenocarcinoma
Colorectal adenocarcinoma is a type of cancer that begins in the large intestine, which includes both the colon and the rectum. These two organs work together as the final part of the digestive system. The colon, which makes up most of the large intestine and stretches about five feet long, absorbs water and nutrients while storing waste. The rectum connects to the anal canal and helps move waste out of the body.[5]
The term adenocarcinoma refers to cancer that starts in glands that produce mucus and other fluids. In the case of colorectal adenocarcinoma, the cancer develops in cells lining the inside of the colon or rectum. This is not just one rare type among many—it represents over 95% of all colorectal cancers, making it by far the most common form of this disease.[17]
What makes this cancer particularly important to understand is that it often develops slowly over many years. The cancer typically begins as small clumps of cells called polyps, which are growths that form on the inner lining of the colon. While most polyps are not cancerous, certain types can transform into cancer over time, usually taking about 10 to 15 years for this change to occur.[1][4]
Who Gets Colorectal Adenocarcinoma
Colorectal cancer stands as the third most commonly diagnosed cancer worldwide, accounting for approximately 10% of all cancer cases. In 2020 alone, more than 1.9 million new cases were identified globally, along with over 930,000 deaths from the disease.[9]
The disease affects different groups of people in varying ways. Age plays a major role, with most cases occurring in people aged 50 and older. However, a concerning trend has emerged over the past 15 years: the number of people between ages 20 and 49 diagnosed with colon cancer has been rising by about 1.5% each year. Medical researchers are still working to understand why younger adults are increasingly affected.[4]
Gender differences exist, though they are not dramatic. Males are slightly more likely to develop colorectal cancer than females according to the U.S. Centers for Disease Control and Prevention.[4]
Race and ethnicity also influence who gets this disease. In the United States, Black individuals face higher rates of colorectal cancer compared to people of other ethnic groups or races. African Americans have the highest risk among all racial groups in the country.[1][4]
Geographic patterns reveal striking differences across the world. The highest rates of new cases appear in Europe, Australia, and New Zealand, while Eastern Europe experiences the highest death rates from this cancer. By 2040, experts predict the global burden will increase dramatically, with an estimated 3.2 million new cases per year—a 63% increase from 2020 levels—and 1.6 million deaths annually, representing a 73% increase.[9]
What Causes This Cancer
The exact cause of colorectal adenocarcinoma remains unclear, but scientists understand that changes in the DNA of cells drive the development of cancer. Your DNA makes up your genes, which determine traits like your appearance and how your body functions. This genetic material is passed down from your parents.[1]
For colon cancer to form, many different genes need to undergo mutations, which are changes in the DNA sequence. These mutations can happen in two main ways. First, some mutations are inherited, passing from one generation to the next through family genes. However, inherited mutations account for only a small number of colorectal cancer cases. The second and much more common type involves acquired mutations, which are not related to family genes but instead occur during a person’s lifetime. These acquired mutations cause most cases of colorectal adenocarcinoma.[1]
The process begins when cells in the innermost lining of the colon—called the mucosa—mutate or change. These cells normally produce and release mucus and other fluids. When mutations occur, the cells may form a polyp. Over time, additional genetic changes can cause some of these polyps to become cancerous. Left undetected or untreated, the cancer works its way through layers of tissue and muscle in the colon wall, and may eventually spread to other parts of the body through lymph nodes or blood vessels.[4]
Risk Factors That Increase Your Chances
A risk factor is anything that increases your chance of developing a disease. For colorectal adenocarcinoma, numerous factors can raise your risk, though having risk factors does not mean you will definitely get cancer. Many people with risk factors never develop the disease, while others with no known risk factors do.[8]
Family history plays a significant role. Having a first-degree relative—meaning a parent, sibling, or child—with a history of colon or rectal cancer increases your risk. Personal medical history matters too. If you have previously had colon, rectal, or ovarian cancer, your risk goes up. The same applies if you have a history of high-risk adenomas, which are colorectal polyps that measure 1 centimeter or larger, or that contain cells that look abnormal under a microscope.[8]
Certain inherited genetic conditions substantially increase risk. Changes in specific genes can lead to familial adenomatous polyposis, or FAP, and Lynch syndrome, also called hereditary nonpolyposis colorectal cancer. These genetic conditions are responsible for about 5,000 new colorectal cancer cases each year in the United States.[8][17]
Long-standing inflammatory bowel conditions raise risk as well. Having chronic ulcerative colitis or Crohn disease for eight years or more increases the likelihood of developing colorectal cancer.[8]
Lifestyle factors contribute significantly to risk. Smoking cigarettes, drinking three or more alcoholic drinks per day, and having obesity all increase your chances of developing this cancer. A diet high in fat and low in fiber, eating too much red meat and processed meats, and not getting enough exercise are additional risk factors.[1][8]
People with type 2 diabetes face higher risk as well. Among demographic factors, women who are 50 years or older represent the group at highest risk, though the disease affects people of all genders and ages.[1]
Recognizing the Symptoms
One of the challenges with colorectal adenocarcinoma is that many people experience no symptoms at all in the early stages. When the cancer is small and most treatable, it often causes no noticeable problems. This is why screening tests are so important—they can find cancer before symptoms appear.[3][7]
When symptoms do occur, they depend on the cancer’s size and location in the large intestine. It is also important to know that these symptoms can be caused by many other, less serious conditions. However, you should always talk to a healthcare provider if you notice any of these warning signs.[3]
Blood in or on your stool is a common symptom that should never be ignored. You might notice blood in the toilet after you have a bowel movement, see it on toilet paper after wiping, or observe that your stool looks dark or bright red. While blood in stool can result from other issues like hemorrhoids or anal tears, it always warrants a conversation with your doctor.[4][7]
Changes in bowel habits that persist deserve attention. This includes ongoing constipation, diarrhea that does not go away, or feeling like you still need to have a bowel movement even after you have just gone to the bathroom. You might also notice that your stools become narrower than usual.[4][7]
Abdominal pain without a clear cause, especially if it does not go away or becomes severe, should prompt a doctor’s visit. Many conditions can cause belly pain, but unusual or frequent abdominal discomfort should always be evaluated by a healthcare provider.[4]
A bloated feeling in your stomach that lasts more than a week or gets worse needs medical attention. Similarly, a full feeling in your bowels even after a bowel movement can signal a problem.[1][4]
Unexplained weight loss and persistent tiredness or weakness are additional symptoms that may appear. These general symptoms can occur with many health conditions, but when combined with other signs or when they persist without explanation, they should be discussed with your doctor.[1]
Prevention Strategies
Several lifestyle changes can help reduce your risk of developing colorectal adenocarcinoma. While no prevention method is foolproof, these steps can lower your chances significantly.
If you smoke tobacco, quitting represents one of the most important steps you can take. Smoking may increase colorectal cancer risk, and stopping this habit benefits your overall health in numerous ways beyond cancer prevention.[23]
Limiting alcohol consumption matters too. Heavy alcohol use may increase the risk of developing colorectal cancer, so doctors advise drinking only in moderation if you choose to drink at all.[1][23]
Your diet plays a crucial role in prevention. Eating a healthy diet rich in fruits, vegetables, and whole grains while keeping red meat and processed meat intake low may help reduce your risk. A diet high in fat and low in fiber has been linked to increased colorectal cancer risk, so making opposite choices—more fiber, less fat—can be protective.[1][23]
Physical activity provides powerful protection. An active lifestyle has been linked to reduced risk of colorectal cancer. You do not need to become an athlete—simply getting regular exercise appropriate for your fitness level can make a difference. Talk with your doctor about creating an exercise program that works for you.[23]
Maintaining a healthy body weight reduces risk. Being overweight or having obesity can increase your chances of developing colorectal cancer. Eating nutritious foods and exercising regularly work together to help you achieve and maintain a healthy weight.[1][23]
Perhaps the single most important prevention strategy is getting screened regularly. Colorectal cancer is unique because recommended screening tests can not only detect cancer early but can actually prevent cancer from developing in the first place. Colonoscopy and other screening methods can identify polyps before they become cancerous, and these growths can be removed during the procedure. According to the U.S. Multi-Society Task Force on Colorectal Cancer Screening, most men and women at average risk should start screening at age 45. For people with certain risk factors, screening may need to start earlier.[1][6]
How the Disease Develops in Your Body
Understanding how colorectal adenocarcinoma changes normal body functions helps explain why symptoms occur and how the disease progresses. The large intestine includes the colon and rectum, with the colon being divided into right and left sections. The right colon—consisting of the cecum, ascending colon, and right two-thirds of the transverse colon—mainly absorbs water and some nutrients. The left colon—including the left one-third of the transverse colon, descending colon, and sigmoid colon—primarily stores and prepares waste for excretion.[5]
The colon wall consists of multiple layers: an innermost mucous membrane lining, layers of tissue, and muscle on the outside. Blood vessels and lymphatic channels run throughout these layers, allowing the colon to function but also providing potential pathways for cancer to spread.[4]
Colorectal adenocarcinoma begins in the mucosa, the innermost lining where cells produce mucus and other fluids. When these cells undergo genetic mutations, they may form a polyp—a growth that protrudes from the colon lining. Not all polyps become cancer, but certain types have that potential.[4]
The transformation from polyp to cancer typically takes 10 to 15 years. During this time, additional genetic changes accumulate in the polyp’s cells. Eventually, if enough mutations occur, the cells become cancerous and start to invade deeper layers of the colon wall. They move through the mucosa into underlying tissue and muscle layers.[1][4]
As cancer grows, it can cause physical effects that produce symptoms. A tumor might partially block the passage of stool through the colon, leading to changes in bowel habits like constipation or narrow stools. Cancer can bleed, which results in blood appearing in the stool. Large tumors may cause pain by pressing on nearby tissues or by stretching the colon wall.[7]
In advanced stages, cancer cells can break away from the original tumor and travel to other parts of the body through the lymphatic system or bloodstream. This process, called metastasis, allows cancer to form new tumors in distant organs such as the liver, lungs, or other sites. Once cancer has spread, it becomes much more difficult to treat and can affect the function of multiple organ systems.[4]
The body’s normal digestive processes can be disrupted at various points. The colon’s ability to absorb water and electrolytes may be impaired, leading to diarrhea. Tumor growth can cause partial or complete blockage of the intestine, preventing normal waste movement and causing severe constipation, pain, and bloating. Blood loss from the tumor, even if not visible, can lead to anemia—a low red blood cell count that causes fatigue and weakness.[1]