What you can do

Get screened. It's a good idea to consider having regular screening tests starting at age 40 or earlier if you are at increased risk due to genetic markers, family history, symptoms, or certain other conditions that raise your risk, such as ulcerative colitis. Medical guidelines recommend that everyone else should start being screened at age 50. With a colonoscopy, polyps can be detected and removed before they have the chance to become cancerous. Screening can also pick up colon cancers early, when treatment is most effective. There are many different screening options; see the "Early detection" section below for more details.

Consider taking baby aspirin. Ask your doctor whether you should take a baby aspirin (81 mg) every day. A study published in 2003 followed 1,100 patients who had had precancerous polyps or colon cancers removed. They found that those who took a baby aspirin daily were 20 percent less likely to develop additional polyps and 40 percent less likely to suffer a recurrence of their cancer. (Note, however, that even baby aspirin may cause gastrointestinal bleeding.)

Maintain a healthy weight. Studies suggest that obesity — particularly if the excess weight is centered on your belly — may increase your risk for polyps and colon cancer by 50 to 80 percent. Experts suspect that may be because obesity can lead to higher levels of insulin in your blood, which in turn may promote abnormal cell growth in the colon.

Eat a healthy diet. The typical Western diet — heavy on red meat and saturated fats, light on fruits, vegetables and fiber — has long been associated with a heightened risk of colon cancer. There's no lone culprit here, nor any single food that can prevent the illness. But an overall healthy diet may lower cancer risk.

• Limit your consumption of red meat to no more than twice a week. Studies have consistently found that people who eat a lot of beef, pork or lamb have higher rates of colon cancer.
• Get plenty of fiber. A number of studies have found that people with a fiber-heavy diet have lower rates of colon cancer and pre-cancerous polyps. Fiber's power may be due to its ability to bind with carcinogens (cancer-causing agents) and help move waste through the digestive system more quickly. Though it's not clear what the optimal dose might be, experts recommend a bowl of bran cereal for breakfast or a daily tablespoon of a psyllium supplement such as Metamucil.
• Limit your alcohol intake. Colon cancer has been linked with heavy drinking. One theory is that alcohol lowers the levels of folic acid, a nutrient that is essential to DNA synthesis. The American Cancer Society recommends men have no more than an average of two drinks a day and women average no more than one a day. (A drink is defined as one beer, one glass of wine or one shot of alcohol.)
• Eat apples. Research suggests that substances in apples called oligomeric procyanidins may help prevent colon cancer.

Eat your omega-3's. Consider incorporating omega-3 fatty acids into your diet. Two servings a week of fatty fish, such as salmon, trout, herring, sardines, tuna and mackerel, are a good source of these fats, which may suppress the growth of tumor cells. (You will need to be aware of the mercury content of some of these fish, especially if you are pregnant or might get pregnant. Check our chart for help selecting fish high in omega-3s but low in mercury.) If you don't eat much fish, ask your doctor whether you should take a dietary fish oil supplement of about 1,000 or 1,200 mg. The supplements may cause excessive bleeding in some people.

Get regular exercise. Studies from around the world have consistently shown that physical activity can decrease the risk of colon cancer by as much as 50 percent. Exercise may help the bowel work well and also reduces levels of blood sugars and insulin, both of which can promote tumor growth. The more exercise, the stronger the effect, but even a moderate level of activity — such as three hours of brisk walking a week — can substantially lower your risk.

Consume calcium. Make sure you get adequate amounts of calcium, either through diet or a supplement. (The recommended intake for people aged 19-50 is 1,000 mg per day; for those over 50, 1,200 mg.) Very low levels are associated with a greater risk of pre-cancerous polyps and colon cancer.

Don't smoke. Smoking increases your risk of developing colon cancer and cancer-causing polyps. Smokers are 30 to 40 percent more likely than non-smokers to die from colon cancer. Women, in particular, should take note: One study found that smoking raises a woman's chances of colon cancer even more than it does a man's.

Get enough vitamin D. There's emerging evidence that this vitamin may help protect against many forms of cancer, including colon cancer. In people who'd already had at least one colon polyp, taking 800 IU of vitamin D per day increased colon cells' production of a protein that helps kill damaged cells, one study found. Some researchers think that the current Vitamin D guidelines (200 IU per day for people aged 19-50; 400 IU for ages 51-70; 600 IU for ages 71 and older) are inadequate, and would prefer a recommendation of 1,700 IU per day. Talk your physician about the right amount for you.

• Your test does not mean you definitely will or will not get colon cancer — it's not a diagnosis, just an indicator of your risk.
• If you are under 50, your doctor is unlikely to order a colonoscopy unless you also have symptoms, a family history, or increased risk based on a genetic test. The colonoscopy is an invasive test that can cause complications.
• A more likely recommended course of action might be annual fecal occult blood tests or other non-invasive screening tests, a healthy lifestyle and regular visits to your doctor.
• If you have Crohn's disease or your Navigenics test indicates that you are at increased risk for Crohn's disease as well as colon cancer, it's important to be alert to potential Crohn's symptoms and treat them promptly. Treating inflammation of the bowel can reduce colon cancer risk.

Colon cancer often causes no symptoms for years. But you should see your doctor if you have any of the following signs:

• A change in bowel habits that lasts for more than a few days. That can include diarrhea, constipation or stools that are narrower than usual.
• A feeling that you need to have a bowel movement that is not relieved by doing so.
• Blood (bright red or dark red) in the stool.
• Frequent gas pains, bloating, fullness or cramps.
• Jaundice (yellowing of skin and eyes).
• Weakness and fatigue.
• Vomiting.
• Unexplained weight loss.

Experts are not in total agreement over which screening tests should be done, at what age they should start or how often they should be performed. But all agree that the most thorough and accurate test is a colonoscopy.

• Colonoscopy: Although some doctors perform the procedure without anesthesia, it's most likely that you will be mildly sedated for it. The doctor inserts a slender, flexible tube equipped with a light and tiny camera into your rectum and threads it through the length of the colon to examine the inside walls of the intestine. If any polyps or other abnormalities are found, they can be removed and biopsied. Because colonoscopies are costly and can cause complications, some experts recommend them as a first-line screening test only for people who have an increased risk of colon cancer because of their medical or family history. How often the procedure should be done depends on a person's individual risk. The American Cancer Society recommends that people at average risk for colon cancer get a colonoscopy every 10 years. Those at higher risk probably will need more frequent screenings, on the order of every three to five years. (You might want to schedule your colonoscopy first thing in the morning: One study found that the first colonoscopies of the day tended to find more polyps.)
• Fecal occult blood test: You use a special kit provided by your physician or pharmacy to check your stool for hidden blood. If the test is positive, a follow-up colonoscopy is recommended. The test is easy and inexpensive, but also has a high rate of both false negatives and false positives.
• Immunochemical fecal occult blood test: A newer kind of stool blood test, this works like the fecal occult blood test but is not affected by foods or vitamins, and so is less apt to produce a false positive result. If the test does suggest there's blood in your stool, your doctor will probably follow up with a colonoscopy or other appropriate diagnostic test.
• Stool DNA test: Instead of looking for blood in the stool, this test looks for abnormal DNA from cancer or polyp cells. It may produce false negatives as well as false positives. It is more expensive than some of the other tests, and like other stool screening tests requires a colonoscopy to confirm positive results.
• Flexible sigmoidoscopy: Your doctor inserts a slender, flexible tube with a light and tiny camera into your rectum to examine the lower third of the colon. The test is not able to detect abnormalities in the upper colon, meaning it could miss as many as half of all colon cancers, and if polyps or other suspicious lesions are found on sigmoidoscopy, you'll need a full colonoscopy. Also, unlike a colonoscopy, a sigmoidoscopy is done without sedation. This may appeal to some people, but can be moderately uncomfortable. Because of the limitations of stool occult blood tests and of sigmoidoscopy compared to colonoscopy, the American Cancer Society recommends getting a stool test every year and a sigmoidoscopy every five years.
• Double contrast barium enema: You will be given an enema that contains a white dye called barium and then a radiologist will take X-rays of your colon after it has been filled with air, looking for suspicious growths. If any are found, your doctor will probably follow up with a colonoscopy. Though simpler and less expensive than a colonoscopy, double contrast barium enema is also less accurate.
• Virtual colonoscopy: This relatively new test works like a barium X-ray but uses an imaging technology similar to a CT scan. A federal study concluded that it is effective at detecting large cancers and precancerous growths in people who are at average risk for colon cancer. It can be uncomfortable, though, because unlike a colonoscopy, it is generally done without anesthesia. It is starting to gain acceptance, but does have some drawbacks.

• Has anyone in your family ever had colon cancer, and at what age?
• Have you noticed any changes in your bowel movements, especially constipation, narrowing of the stools or blood in the stool?
• Are you having regular abdominal pain or losing weight without trying?
• Could anything else be causing these symptoms, such as stress, anxiety or depression; dietary changes or excesses; medication or herbs; or unusual travel where you might have picked up a parasite?
• Are you taking aspirin or any other anti-inflammatory drugs?
• Are you worried enough to want testing?

• Take a careful history and do a physical.
• Check several stool samples for blood.
• Order blood tests for anemia, among other things.
• Refer you to a specialist, if needed. The gastroenterologist may elect to do a colonoscopy or other tests.