But risk factors don't tell us everything. Having a risk factor, or even several risk factors, does not mean that you will get the disease. And some people who get the disease may not have any known risk factors. Even if a person with colorectal cancer has a risk factor, it is often very hard to know how much that risk factor may have contributed to the cancer.

Researchers have found several risk factors that may increase a person's chance of developing colorectal polyps or colorectal cancer.

Risk factors you cannot change

Age

While younger adults can develop colorectal cancer, the chances of developing colorectal cancer increase markedly after age 50. More than 90% of people diagnosed with colorectal cancer are older than 50.

Personal history of colorectal polyps or colorectal cancer

If you have a history of adenomatous polyps (adenomas), you are at increased risk of developing colorectal cancer. This is especially true if the polyps are large or if there are many of them.

If you have had colorectal cancer, even though it has been completely removed, you are more likely to develop new cancers in other areas of the colon and rectum. The chances of this happening are greater if you had your first colorectal cancer when you were younger than age 60.

Personal history of inflammatory bowel disease

Inflammatory bowel disease (IBD), which includes ulcerative colitis and Crohn's disease, is a condition in which the colon is inflamed over a long period of time. If you have IBD, your risk of developing colorectal cancer is increased, and you need to be screened for colorectal cancer on a more frequent basis (see the section, "Can colorectal cancer be found early?"). Often the first sign that cancer may be developing is called dysplasia. Dysplasia is a term that refers to abnormal cells that have the potential to progress to cancer.

Inflammatory bowel disease is different than irritable bowel syndrome (IBS), which does not carry an increased risk for colorectal cancer.

Family history of colorectal cancer

Most colorectal cancers occur in people without a family history of colorectal cancer. Still, up to 20% of people who develop colorectal cancer have other family members who have been affected by this disease.

People with a history of colorectal cancer or adenomatous polyps in one or more first-degree relatives (parents, siblings, children) are at increased risk. The risk is about doubled in those with a single affected first-degree relative, and is even higher in people with a stronger family history, such as:

• a history of colorectal cancer or adenomatous polyps in any first-degree relative (parent, sibling, or child) younger than age 60

• a history of colorectal cancer or adenomatous polyps in 2 or more first-degree relatives at any age

The reasons for the increased risk are not clear in all cases. Cancers can "run in the family" because of inherited genes, shared environmental factors, or some combination of these.

People diagnosed with adenomatous polyps or colorectal cancer should inform other family members. Those with a family history of colorectal cancer need to talk with their doctor about the possible need to begin screening before age 50.

Inherited syndromes

About 5% of people who develop colorectal cancer have an inherited genetic susceptibility to the disease. The 2 most common inherited syndromes linked with colorectal cancers are familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC).

Familial adenomatous polyposis (FAP): FAP is caused by changes (mutations) in the APC gene that a person inherits from his or her parents. About 1% of all colorectal cancers are due to FAP.

People with this disease typically develop hundreds or thousands of polyps in their colon and rectum, usually in their teens or early adulthood. Cancer usually develops in 1 or more of these polyps as early as age 20. By age 40, almost all people with this disorder will have developed cancer if preventive surgery (removing the colon) is not done.

FAP is sometimes associated with Gardner syndrome, a condition that involves benign (non-cancerous) tumors of the skin, soft connective tissue, and bones.

Hereditary non-polyposis colon cancer (HNPCC): HNPCC, also known as Lynch syndrome, is another clearly defined genetic syndrome. It accounts for about 3% to 4% of all colorectal cancers. HNPCC can be caused by inherited changes in a number of different genes that normally help repair DNA damage. (See the section, "Do we know what causes colorectal cancer?," for more details.)

This syndrome also develops when people are relatively young. People with HNPCC have polyps, but they only have a few, not hundreds as in FAP. The lifetime risk of colorectal cancer in people with this condition may be as high as 70% to 80%.

Women with this condition also have a very high risk of developing cancer of the endometrium (lining of the uterus). Other cancers linked with HNPCC include cancer of the ovary, stomach, small bowel, pancreas, kidney, ureters (tubes that carry urine from the kidneys to the bladder), and bile duct.

For more information on HNPCC, see the sections "Do we know what causes colorectal cancer?" and "Can colorectal cancer be prevented?"

Peutz-Jeghers syndrome: People with this rare inherited condition tend to have freckles around the mouth (and sometimes on the hands and feet) and large polyps in their digestive tracts. They are at greatly increased risk for colorectal cancer, as well as several other cancers, which usually appear at a younger than normal age.

Identifying families with these inherited syndromes is important because it allows doctors to recommend specific steps, such as screening and other preventive measures, at an early age.

Because several types of cancer can be linked with these syndromes, people should check their family medical history for polyps or any type of cancer. Those who develop polyps or cancer should inform other family members. People with a family history of colorectal polyps or cancer should consider genetic counseling to review their family medical tree and determine whether genetic testing may be right for them. If needed, this can help them to decide about getting screened and treated at an early age. More information on genetic counseling and testing can be found in the section "Can colorectal cancer be prevented?"

Racial and ethnic background

African Americans have the highest colorectal cancer incidence and mortality rates of all racial groups in the United States. The reason for this is not yet understood.

Jews of Eastern European descent (Ashkenazi Jews) have one of the highest colorectal cancer risks of any ethnic group in the world. Several gene mutations leading to an increased risk of colorectal cancer have been found in this group. The most common of these DNA changes, called the I1307K APC mutation, is present in about 6% of American Jews.

Lifestyle-related factors

Several lifestyle-related factors have been linked to colorectal cancer. In fact, the links between diet, weight, and exercise and colorectal cancer risk are some of the strongest for any type of cancer.

Certain types of diets

A diet that is high in red meats (beef, lamb, or liver) and processed meats (hot dogs, bologna, and luncheon meat) can increase colorectal cancer risk. Methods of cooking meats at very high temperatures (frying, broiling, or grilling) create chemicals that might increase cancer risk, although it's not clear how much this might contribute to an increase in colorectal cancer risk. Diets high in vegetables and fruits have been linked with a decreased risk of colorectal cancer. Whether other dietary components (fiber, certain types of fats, etc.) affect colorectal cancer risk is not clear.

Physical inactivity

If you are not physically active, you have a greater chance of developing colorectal cancer. Increasing activity may help reduce your risk.

Obesity

If you are very overweight, your risk of developing and dying from colorectal cancer is increased. While obesity raises the risk of colon cancer in both men and women, the link seems to be stronger in men.

Smoking

Long-term smokers are more likely than non-smokers to develop and die from colorectal cancer. While smoking is a well-known cause of lung cancer, some of the cancer-causing substances are swallowed and can cause digestive system cancers, such as colorectal cancer.

Heavy alcohol use

Colorectal cancer has been linked to the heavy use of alcohol. At least some of this may be due to the fact that heavy alcohol users tend to have low levels of folic acid in the body. Still, alcohol use should be limited to no more than 2 drinks a day for men and 1 drink a day for women.

Type 2 diabetes

People with type 2 (usually non-insulin dependent) diabetes have an increased risk of developing colorectal cancer. Both type 2 diabetes and colorectal cancer share some of the same risk factors (such as excess weight). But even after taking these into account, people with type 2 diabetes still have an increased risk. They also tend to have a less favorable prognosis (outlook) after diagnosis.

Factors with uncertain, controversial, or unproven effects on colorectal cancer

Night shift work

Results of one study suggested working a night shift at least 3 nights a month for at least 15 years may increase the risk of colorectal cancer in women. The study authors suggested this might be due to changes in levels of melatonin (a hormone that responds to changes in light) in the body. More research is needed to confirm or refute this finding.

Previous treatment for certain cancers

Some studies have found that men who survive testicular cancer seem to have a higher rate of colorectal cancer and some other cancers. This might be due to the treatments they have received.

Some early studies suggested that men who received radiation therapy to treat prostate cancer might have a higher risk of rectal cancer, as the rectum receives some radiation during treatment. However, other studies have not found such a link.

The American Cancer Society and several other medical organizations recommend earlier screening for people with increased colorectal cancer risk. These recommendations differ from those for people at average risk. For more information, speak with your doctor and refer to the table in the "Can colorectal cancer be found early?" section of this document.