Colorectal cancer begins when abnormal cells develop in the colon or rectum, parts of the digestive system that help process waste. While this disease affects hundreds of thousands of people each year, understanding its causes, symptoms, and risk factors can help with early detection and better outcomes.
What Is Colorectal Cancer?
Colorectal cancer is a disease where cells in the colon or rectum grow out of control. The colon, also known as the large intestine, is a long tube that helps carry digested food through your body. It is responsible for absorbing water and nutrients, and changing leftover waste into stool. The rectum is the final section before waste leaves the body through the anus. When cancer starts in the colon, it is called colon cancer. When it starts in the rectum, it is called rectal cancer. Because these two types share many similarities, they are often referred to together as colorectal cancer.[1][2]
Most colorectal cancers begin as small clumps of cells called polyps, which are growths that form on the inner lining of the colon or rectum. Not all polyps are cancerous, but some types can turn into cancer over time. This process usually takes many years—often around 10 years or more—which is why regular screening is so important. Finding and removing polyps before they become cancerous can actually prevent colorectal cancer from developing.[3][6]
Your colon wall is made up of several layers, including mucous membrane, tissue, and muscle. Cancer starts in the mucosa, which is the innermost lining. This layer contains cells that make and release mucus and other fluids. If these cells change or mutate, they may form a polyp. Over time, if left undetected, the cancer can work its way through the tissue and muscle layers, and may spread to other parts of the body through the lymph nodes or blood vessels.[6]
Epidemiology: Who Gets Colorectal Cancer?
Colorectal cancer is the third most common cancer diagnosed in both men and women in the United States, not counting skin cancers. It is also the second most common cause of cancer-related deaths when men and women are combined. Worldwide, colorectal cancer is the third most frequently diagnosed cancer and the second leading cause of cancer deaths.[4][6][9]
According to estimates, in 2025 there will be over 106,000 new cases of colon cancer and more than 46,000 new cases of rectal cancer in the United States. Colorectal cancer causes approximately 50,000 deaths each year in the country. The lifetime risk of developing colorectal cancer is about one in 23 for men and one in 26 for women.[6][7]
Men are slightly more likely to develop colorectal cancer than women. The disease also affects Black people at higher rates compared to other racial or ethnic groups. Black individuals have both an increased risk of developing colorectal cancer and an increased risk of dying from it.[6][8]
Colorectal cancer typically affects older adults, and age is one of the main risk factors. Most cases are diagnosed in people age 50 and older. However, over the past 15 years, there has been a concerning trend: the number of people between ages 20 and 49 being diagnosed with colorectal cancer has been increasing by about 1.5% each year. Medical researchers are still trying to understand why younger adults are developing this disease more frequently. Because of this trend, 30% of all colorectal cancer diagnoses now occur in people under age 55. Each day, 56 cases are diagnosed in patients ages 46 and younger.[3][6][7]
Causes of Colorectal Cancer
Colorectal cancer happens when there are changes in the genetic material, or DNA, of cells in the colon or rectum. These changes are also called mutations or variants. Often, the exact reason why these genetic changes occur is not known. The mutations happen during a person’s lifetime rather than being inherited from parents.[8]
Some genetic changes that raise the risk of colorectal cancer, however, are inherited. This means a person is born with them. About 5% of people with colorectal cancer have inherited genetic conditions such as Lynch syndrome (also called hereditary non-polyposis colorectal cancer or HNPCC), Familial Adenomatous Polyposis (FAP), or MUTYH-Associated Polyposis (MAP). These inherited conditions greatly increase the chances of developing the disease.[7][8]
Besides genetics, other factors including lifestyle choices and environmental exposures can affect the risk of developing colorectal cancer. The disease is considered multifactorial, meaning it results from a combination of genetic, behavioral, and environmental influences working together over time.[4]
Risk Factors: What Increases Your Chances?
A risk factor is anything that increases your chance of getting a disease. Some risk factors for colorectal cancer can be changed, such as diet or smoking habits. Others, like age or family history, cannot be changed.[12]
One of the strongest risk factors is having a family history of colorectal cancer. If a parent, sibling, or child has had colorectal cancer, your risk is two to three times higher than someone without that family history. If the relative was diagnosed at a young age, or if more than one family member has had the disease, your risk increases even more—up to three to six times higher than the general population. People with this family history may need to start screening earlier than age 45.[7][8][12]
Having a personal history of colorectal cancer or certain types of polyps also raises your risk. Adenomas are polyps that look abnormal under a microscope or are larger than one centimeter. These are considered high-risk because they can turn into cancer over time. People who have had adenomas in the past need more frequent monitoring.[8][12]
Certain health conditions increase risk as well. Having chronic ulcerative colitis or Crohn disease for eight years or longer raises the chance of developing colorectal cancer. Both of these are inflammatory bowel diseases that cause long-term irritation and damage to the colon lining.[8][12]
Lifestyle choices play a significant role in colorectal cancer risk. Smoking cigarettes is linked to an increased chance of developing the disease. Heavy alcohol use—defined as having three or more alcoholic drinks per day—also raises risk. Being overweight or obese is another important risk factor. People who do not get regular physical activity are at higher risk compared to those who exercise regularly.[8][12]
Diet can influence your risk as well. Eating a lot of red meat (such as beef, pork, or lamb) and processed meats (such as hot dogs, bacon, or deli meats) has been shown to increase the risk of colorectal cancer. On the other hand, eating a diet rich in fruits, vegetables, whole grains, and fiber may help reduce risk.[23][27]
Symptoms: What to Watch For
Many people with colorectal cancer do not have any symptoms at first, especially in the early stages. This is why screening is so important—it can find cancer before symptoms appear, when treatment is most effective. When symptoms do occur, they can vary depending on the size of the cancer and where it is located in the colon or rectum.[3][5]
One of the most common symptoms is blood in or on the stool. The blood may be bright red, or the stool may appear dark, tarry, or brick-red in color. It is important to remember that blood in the stool does not always mean cancer. Other conditions like hemorrhoids, anal tears, or even certain foods like beets can cause changes in stool appearance. However, it is always best to talk to a healthcare provider if you notice blood.[5][6]
Changes in bowel habits are another warning sign. These changes might include persistent diarrhea or constipation, or feeling as though the bowel does not empty completely after using the bathroom. Some people notice that their stools become narrower than usual. If these changes last for more than a few days, it is worth discussing with a doctor.[5][8]
Abdominal pain or discomfort is also a common symptom. This might include frequent gas pains, bloating, fullness, or cramping. While many things can cause belly pain, pain with no known cause that does not go away or is severe should be checked by a healthcare provider.[6]
Some people experience unexplained weight loss or persistent fatigue. These symptoms occur because the cancer may be using up the body’s energy or causing internal bleeding that leads to a low level of red blood cells, a condition called anemia.[6][8]
It is worth noting that many of these symptoms can be caused by conditions other than colorectal cancer, such as infections, irritable bowel syndrome, or hemorrhoids. However, because colorectal cancer often has no symptoms early on, anyone experiencing these changes should talk to a doctor rather than assuming it is something less serious.[5]
Prevention: Steps You Can Take
The most effective way to reduce your risk of colorectal cancer is to get screened regularly, starting at age 45. Screening tests can find polyps before they turn into cancer, and they can also detect cancer at an early stage when it is easier to treat. Almost all colorectal cancers begin as precancerous polyps, which may be present for years without causing symptoms. Screening allows doctors to find and remove these polyps before they become dangerous.[1][23]
There are several types of screening tests available, including colonoscopy, which allows a doctor to examine the entire colon using a flexible tube with a camera. Other options include stool-based tests that check for blood or abnormal DNA. Your doctor can help you decide which test is right for you and how often you should be screened.[1][8]
An estimated one in three adults in the United States are not being screened as recommended. This is a missed opportunity for prevention, since screening can actually stop cancer from developing in the first place. If you think you may be at increased risk due to family history or other factors, talk to your doctor about starting screening earlier than age 45.[7]
Beyond screening, there are lifestyle changes you can make to help lower your risk. Increasing physical activity is one of the most important steps. Regular exercise has been linked to a reduced risk of colorectal cancer. Aim to stay active most days of the week.[22][23]
Maintaining a healthy weight is also important. Being overweight or obese increases colorectal cancer risk, so eating healthfully and exercising regularly to stay at a healthy body weight can make a difference.[22][23]
Diet plays a role in prevention as well. Eating a diet low in animal fats and high in fruits, vegetables, and whole grains may help reduce risk. Foods rich in fiber, such as whole wheat bread, brown rice, beans, and legumes like lentils and peas, are especially beneficial. Some studies suggest that dairy products may also have a protective effect against colorectal cancer.[23][25][27]
Limiting your intake of red and processed meats is another important step. High consumption of these foods has been shown to increase colorectal cancer risk. Avoiding or limiting alcohol is also recommended, as heavy alcohol use may raise your chances of developing the disease. If you smoke, quitting is one of the best things you can do for your overall health, including reducing your cancer risk.[22][23]
Pathophysiology: What Happens in the Body
Understanding how colorectal cancer develops can help explain why early detection is so important. The process begins when normal cells in the lining of the colon or rectum undergo genetic changes. These changes cause the cells to grow and divide in an uncontrolled way, forming a polyp. While many polyps remain harmless, some accumulate additional mutations over time and transform into cancer.[4]
The colon wall has several distinct layers. The innermost layer, called the mucosa, is where cancer begins. This layer is made up of cells that produce mucus to help move waste through the digestive system. When these cells mutate, they can form a polyp that sticks out from the colon wall. Not all polyps are the same. Some types, especially adenomas, have a higher chance of becoming cancerous.[6]
As cancer develops, it starts to invade deeper into the colon wall. It moves from the mucosa through the underlying tissue and muscle layers. This is a gradual process that usually takes many years, which is why colorectal cancer is more common in older adults. The slow progression also explains why screening is so effective—it provides many opportunities to catch the disease before it advances.[6]
If colorectal cancer is not detected and treated, it can spread beyond the colon or rectum. Cancer cells can enter the lymph nodes, which are small structures that are part of the body’s immune system. From there, cancer can travel through the lymphatic system to other parts of the body. Cancer can also spread through the bloodstream, reaching distant organs such as the liver, lungs, or other sites. When cancer spreads to other parts of the body, it is called metastasis.[4][6]
The blood supply to the colon comes mainly from arteries, and veins carry blood away from the colon. The extensive network of blood vessels in the colon is one reason why cancer can spread if it grows through the colon wall and reaches these vessels. Understanding the anatomy and function of the colon helps explain why surgery to remove the affected section, along with nearby lymph nodes, is often part of treatment.[4]
Several factors influence how aggressive a colorectal cancer is and how quickly it grows. Some cancers have specific genetic mutations or molecular features that make them more likely to spread or more resistant to certain treatments. Testing the cancer tissue for these features, known as biomarker testing, can help doctors choose the most effective treatment plan.[10]
