What Causes Colorectal Cancer?

This content has been reviewed and approved by

Howard Burris, MD
Director, Drug Development
Sarah Cannon Cancer Center
Tennessee Oncology

The exact cause of most cases of colorectal cancer is unknown. Colorectal cancer develops when the body rapidly produces unnecessary cells. These out-of-control cells can form bumps on the inside of the large intestine called polyps. Polyps can turn into cancer if they are not removed.

Risk Factors for Colorectal Cancer

A risk factor is something that increases a person’s chance of getting a disease. About three-quarters of all cases of colorectal cancer develop in people who have no known risk factors, except for age.

Research has shown that certain risk factors appear to increase a person’s chance of getting colon and rectum cancer:

• Age - Most people who have colorectal cancer are over 50. However, people can get this disease at any age.
• Diet - Colorectal cancer is often associated with a diet high in fat (especially animal fat) and calories and low in fiber. Heavy alcohol consumption can also increase a person’s risk of colorectal cancer.
• Ethnic background - Ashkenazi Jews (those with ancestors from Eastern Europe) have a higher risk of colorectal cancer than other ethnic groups.
• Family history - People with a sibling, parent, or child who developed colorectal cancer or polyps before age 60 have an increased risk of colorectal cancer.
• Inherited disease - A small percentage of people with colorectal cancer have inherited an increased risk of getting this disease. Most of these people have hereditary nonpolyposis colorectal cancer (HNPCC), also known as Lynch syndrome. About 80 percent of people with the HNPCC genetic condition develop colorectal cancer at some point during their lifetime. An even smaller percentage of people with colorectal cancer have inherited familial adenomatous polyposis (FAP). People with FAP tend to develop hundreds of adenomatous polyps, or noncancerous tumors, in the colon or rectum. Many people with FAP get colorectal cancer at a young age.
• Personal history of cancer - People who have had colorectal cancer, even if the cancer has been removed, or colorectal polyps (growths that stick out from the lining of the colon or rectum) are more likely to develop new colorectal cancers. People who have had ovarian, uterine, or breast cancer have a slightly higher risk of colorectal cancer.
• Personal history of diabetes - People with diabetes are more likely to develop colorectal cancer.
• Personal history of inflammatory bowel disease - When the colon is inflamed, or irritated, for a long time, this is known as inflammatory bowel disease. People who have had inflammatory bowel disease, such as ulcerative colitis (ulcers and irritation in the colon and rectum) or Crohn’s disease (ongoing inflammation of the stomach and intestine), have a higher risk of colorectal cancer.
• Physical activity and obesity - People who are not physically active and have large amounts of body fat are more likely to develop colorectal cancer.
• Race - African Americans have a slightly higher chance of getting colorectal cancer.
• Smoking - Some studies have shown that people who smoke cigarettes have a higher chance of getting colorectal cancer. The risk of colorectal cancer increases with the number of cigarettes smoked daily and the number of years of smoking. However, colorectal cancer risk decreases each year after a person stops smoking.