The exact cause of bowel cancer is not known, but there are several factors that can increase a person’s risk of developing the disease

In general, colon cancer begins when healthy cells in the colon develop changes (mutations) in their DNA. A cell’s DNA contains a set of instructions that tell a cell what to do.

Healthy cells grow and divide in an orderly way to keep your body functioning normally. But when a cell’s DNA is damaged and becomes cancerous, cells continue to divide — even when new cells aren’t needed. As the cells accumulate, they form a tumor.

With time, the cancer cells can grow to invade and destroy normal tissue nearby. And cancerous cells can travel to other parts of the body to form deposits there (metastasis).

Risk factors

Factors that may increase your risk of colon cancer include:

• Older age. Colon cancer can be diagnosed at any age, but a majority of people with colon cancer are older than 50. The rates of colon cancer in people younger than 50 have been increasing, but doctors aren’t sure why.
• African-American race. African-Americans have a greater risk of colon cancer than do people of other races.
• A personal history of colorectal cancer or polyps. If you’ve already had colon cancer or noncancerous colon polyps, you have a greater risk of colon cancer in the future. Certain types of polyps called adenomas can develop into cancer over a long period of time. In fact, most bowel cancers develop from an adenoma. Your risk of developing cancer depends on how many polyps you have, and how large they are. 
• Inflammatory intestinal conditions. Chronic inflammatory diseases of the colon, such as ulcerative colitis and Crohn’s disease, can increase your risk of colon cancer.
• Inherited syndromes that increase colon cancer risk. Some gene mutations passed through generations of your family can increase your risk of colon cancer significantly. Only a small percentage of colon cancers are linked to inherited genes. The most common inherited syndromes that increase colon cancer risk are familial adenomatous polyposis (FAP) and Lynch syndrome, which is also known as hereditary nonpolyposis colorectal cancer (HNPCC).
• Family history of colon cancer. You’re more likely to develop colon cancer if you have a blood relative who has had the disease. If more than one family member has colon cancer or rectal cancer, your risk is even greater.
• Low-fiber, high-fat diet. Colon cancer and rectal cancer may be associated with a typical Western diet, which is low in fiber and high in fat and calories. Research in this area has had mixed results. Some studies have found an increased risk of colon cancer in people who eat diets high in red meat and processed meat.
• Medical conditions. People with certain medical conditions such as diabetes, gallstones and acromegaly may have an increased risk of colon cancer.
  • Diabetes: If you have diabetes, your pancreas doesn’t make enough of a hormone called insulin. People with diabetes may have an increased risk of bowel cancer. We don’t know why this is and scientists are looking into it.
  • Gallstones: The risk of rectal cancer is increased in people with gallstones compared to those without. Also, people with gallstones have double the risk of developing polyps in the large bowel (colon) compared to those without gallstones.
  • Acromegaly: People with acromegaly also have an increased risk of bowel cancer, although studies are still trying to find out why. Acromegaly is a condition where the pituitary gland produces too much growth hormone and causes an overgrowth of bones, especially in the face.
• A sedentary lifestyle. People who are inactive are more likely to develop colon cancer. Getting regular physical activity may reduce your risk of colon cancer.
• Obesity. People who are obese have an increased risk of colon cancer and an increased risk of dying of colon cancer when compared with people considered normal weight.
• Smoking. People who smoke may have an increased risk of colon cancer.
• Alcohol. Heavy use of alcohol increases your risk of colon cancer.
• Radiation therapy for cancer. Radiation therapy directed at the abdomen to treat previous cancers increases the risk of colon cancer.

It is important to note that having one or more of these risk factors does not necessarily mean that a person will develop bowel cancer. However, it is important for people who are at increased risk to be vigilant about getting screened for the disease and to make lifestyle changes that can help reduce their risk.