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In the United States, more than 50,000 people die of colorectal cancer every year. Yet advanced cancers of the colon and rectum are highly preventable. That's because most of these cancers begin as an adenomatous polyp — a small clump of cells that forms on your colon lining. Although the great majority of colon polyps are harmless, some may turn cancerous over time.

As many as 20 percent of middle-age and older adults may have one or more colon polyps. Although anyone can develop colon polyps, you're at higher risk if you are 50 or older, overweight or a smoker, eat a high-fat, low-fiber diet, or have a personal or family history of colon polyps or colon cancer.

Sometimes colon polyps can cause signs and symptoms such as rectal bleeding, a change in bowel habits and abdominal pain. But most small colon polyps don't cause problems, which is why experts generally recommend regular screening. Colon polyps that are found in the early stages usually can be removed safely and completely.

Colon polyps range from smaller than a pea to golf ball sized. Small polyps, especially, aren't likely to cause problems, and you may not know you have one until your doctor finds it during an examination of your bowel. Sometimes, however, you may have signs and symptoms such as:

• Rectal bleeding. You might notice bright red blood on toilet paper after you've had a bowel movement. Although this may be a sign of colon polyps or colon cancer, rectal bleeding can indicate other conditions, such as hemorrhoids or minor tears (fissures) in your anus. Hemorrhoids don't usually bleed consistently over a period of weeks, however, so if your bleeding is prolonged, be sure to tell your doctor.
• Blood in your stool. Blood can show up as red streaks in your stool or make bowel movements appear black. Still, a change in color doesn't always indicate a problem — iron supplements and some anti-diarrhea medications can make stools black, and beets and red licorice can turn stools red.
• Constipation or diarrhea. Although a change in bowel habits that lasts longer than a week may indicate the presence of a large colon polyp, it can also result from a number of other conditions.
• Pain or obstruction. Sometimes a large colon polyp may partially obstruct your bowel, leading to crampy abdominal pain, nausea, vomiting and severe constipation.

Your digestive tract stretches from your mouth to your anus. As food travels along this 30-foot passageway, nutrients are broken down into a form that can be absorbed by your body and used to build cells and produce energy.

The last part of your digestive tract is a long muscular tube called the large intestine. The colon is the upper 4 to 6 feet of the large intestine; the rectum makes up the lower 8 to 10 inches. The colon's main function is to absorb water, salt and other minerals from digested food. Your rectum stores waste until it's eliminated from your body.

Why polyps form
The majority of polyps aren't cancerous (malignant), yet like most cancers, they result from abnormal cell growth. Healthy cells grow and divide in an orderly way — a process that's controlled by two broad groups of genes: proto-oncogenes and tumor suppressor genes. Mutations in any of these genes can cause cells to continue dividing even when new cells aren't needed. In the colon and rectum, this unregulated growth can cause polyps to form, and over a long period of time, some of these polyps may become malignant.

Polyps can develop anywhere in your large intestine. They can be small or large and flat (sessile) or mushroom shaped and attached to a stalk (pedunculated). Small and mushroom-shaped polyps are much less likely to become malignant than flat or large ones are. In general, the larger a polyp, the greater the likelihood of cancer.

There are three main types of colon polyps:

• Adenomatous. Once these polyps grow beyond the size of a pencil eraser — about 5 millimeters (mm), or 1/4 inch — there's a small but increasing chance that they'll become cancerous. This is especially true when their diameter exceeds 10 mm. For that reason, doctors normally take a tissue sample (biopsy) from polyps during a sigmoidoscopy and either biopsy or remove large polyps during a colonoscopy. Adenomas are divided into three sub-types — villous, tubular and tubulovillous. Villous adenomas tend to be larger than the other types and are the most likely to become malignant.
• Hyperplastic. These polyps occur most often in your left (descending) colon and rectum. Usually less than 5 mm in size, they're rarely malignant.
• Inflammatory. These polyps may follow a bout of ulcerative colitis. Although the polyps themselves are not a significant threat, having ulcerative colitis increases your overall risk of colon cancer.

A number of factors may contribute to the formation of colorectal polyps. They include:

• Age. The great majority of people with colorectal cancer are 50 or older. Your risk generally starts increasing around age 40, and the average age at which colorectal cancer is diagnosed is 62.
• Your sex. More men than women develop colon polyps and colon cancer.
• Ethnicity. In the United States, blacks have a higher rate of colorectal cancer than do other ethnic groups.
• Inflammatory intestinal conditions. Long-standing inflammatory diseases of the colon such as ulcerative colitis and Crohn's disease can increase your risk.
• Family history. You're more likely to develop colorectal cancer if you have a parent, sibling or child with the disease. If many family members have colon or rectal cancer, your risk is even greater. In some cases this connection isn't hereditary or genetic. Instead, cancers within the same family may result from shared exposure to an environmental carcinogen or from similar diet or lifestyle factors.
• Diet. Eating a high-fiber diet — one plentiful in fruits, vegetables and whole grains — can reduce your risk of colon polyps and colon cancer. Fiber seems protective against colorectal cancer because it provides bulk that moves your stool more quickly through your bowel. This means that cancer-causing substances (carcinogens) in the foods you eat aren't in contact with your bowel wall as long as they might be if you ate a low-fiber diet. Fruits and vegetables are also rich in antioxidants — substances that protect cells from damage caused by unstable molecules (free radicals) that may lead to cancer.
• Smoking and alcohol. Smoking significantly increases your risk of colon cancer — according to some estimates, people who smoke more than 20 cigarettes a day have 250 percent more polyps than nonsmokers do. Drinking alcohol in excess also makes it more likely that you'll develop colon polyps. If you both smoke and drink, your risk increases even more.
• A sedentary lifestyle. If you're inactive, you're more likely to develop colon cancer, although not rectal cancer. This may be because when you're inactive, waste stays in your colon longer.
• Obesity. Being significantly overweight — 30 pounds or more — has been linked to an increased risk of several types of cancer, including colorectal cancer. Extra fat appears to encourage the growth of cells in the colon and rectum and this is even more likely if the excess weight occurs in your waist rather than in your hips or thighs.

Inherited gene mutations
Another risk factor for colon polyps is genetic mutations. A small percentage of colorectal cancers result from identified gene mutations. These cancers are autosomal dominant, meaning you need to inherit only one defective gene from either of your parents. If one parent has the mutated gene, you have a 50 percent chance of inheriting the mutation. Although inheriting a defective gene greatly increases your risk, not everyone with a mutated gene develops cancer.

One genetic defect that plays a key role in colorectal cancer occurs in the adenomatous polyposis coli (APC) gene. When the APC gene is normal, it helps control cell growth. But if it's defective, cell growth accelerates, leading to the formation of multiple adenomatous polyps in your intestinal lining. Conditions related to APC gene defects include:

• Familial adenomatous polyposis (FAP). This is a rare, hereditary disorder that results from an APC gene defect. FAP causes you to develop hundreds, even thousands, of polyps in the lining of your colon and often in your upper intestine, beginning in your teenage years. If these go untreated, your risk of developing colon cancer by age 45 is nearly 100 percent. The encouraging news about FAP is that in some cases, genetic testing can help determine whether you're at risk of the disease.
• Gardner's syndrome. This syndrome is a variant of FAP. This condition causes polyps to develop throughout your colon and upper intestine. You may also develop noncancerous tumors in other parts of your body, including your skin (sebaceous cysts and lipomas), bone (osteomas) and abdomen (desmoids).
• Hereditary nonpolyposis colorectal cancer. This is the most common form of inherited colon cancer. It, too, results from a defect in the APC gene, but unlike people with FAP or Gardner's syndrome, people with hereditary nonpolyposis colorectal cancer tend to develop relatively few colon polyps. They do, however, often have tumors in other organs. Hereditary nonpolyposis colorectal cancer includes Lynch I and Lynch II syndromes. People with Lynch I syndrome usually develop a small number of polyps that quickly become malignant. Those with Lynch II syndrome tend to develop tumors in the breast, stomach, small intestine, urinary tract and ovaries as well as in the colon.

Some Jewish people of Eastern European descent have an inherited tendency to develop colorectal cancer, but the risk is much smaller than that caused by FAP, Gardner's syndrome or hereditary nonpolyposis colorectal cancer.

See your doctor if you notice blood in your stool, a change in your bowel habits that lasts longer than a week or abdominal pain. And if you're at high risk, don't wait until symptoms appear; talk to your doctor about the risks and benefits of regular screenings.

The American Cancer Society recommends colorectal screenings beginning at age 50 for most Americans and more frequent or earlier screening if you have other risk factors, such as a family history of the disease.

Nearly all colon cancers develop from polyps, but the polyps grow slowly, usually over a period of years. Screening tests play a key role in detecting polyps before they become cancerous. These tests can also help find colorectal cancer in its early stages, when you have a good chance of recovery. When early-stage cancers are found and removed during routine screening, the five-year survival rate may be as high as 90 percent.

Several screening methods exist — each with its own benefits and risks. Be sure to discuss these with your doctor:

• Digital rectal exam. In this office exam, your doctor uses a gloved finger to check the first few inches of your rectum for polyps. Although safe and relatively painless, the exam is limited to your lower rectum and can't detect problems with your upper rectum and colon. In addition, it's difficult for your doctor to feel small polyps.
• Fecal occult (hidden) blood test. This noninvasive test checks a sample of your stool for blood. It can be performed in your doctor's office, but you're usually given a kit that explains how to take the test at home. Be sure to follow the instructions carefully, because your diet and other factors can affect the results. You then return the test to a lab or your doctor's office to be checked. The problem is that most polyps don't bleed, nor do all cancers. This can result in a negative test result, even though you may have a polyp or cancer. On the other hand, if blood shows up in your stool, it may be the result of hemorrhoids or an intestinal condition other than cancer. For these reasons, many doctors recommend other screening methods instead of, or in addition to, fecal occult blood tests.
• Flexible sigmoidoscopy. In this test, your doctor uses a slender, lighted tube to examine your rectum and sigmoid — approximately the last 2 feet of your colon. Nearly half of all colon cancers are found in this area. Yet a sigmoidoscopy only looks at the last third of your colon, and doesn't detect polyps elsewhere in the large intestine. It's often combined with a barium enema to better visualize the entire colon, or your doctor may recommend performing a colonoscopy instead. A sigmoidoscopy can be somewhat uncomfortable, and though there's a slight risk of perforating the colon, the risks are less than they are for colonoscopy.
• Barium enema. This diagnostic test allows your doctor to evaluate your entire large intestine with an X-ray. Barium, a contrast dye, is placed into your bowel in an enema form. The barium fills and coats the lining of the bowel, creating a clear silhouette of your rectum, colon and sometimes a small portion of your small intestine. Air may also be added to provide better contrast on the X-ray. The test typically takes about 20 minutes and can be somewhat uncomfortable because the barium and air distend your bowel. There's also a slight risk of perforating the colon wall. Because barium enema has a high miss rate for colon polyps, it's not nearly as reliable as other screening tests.
• Colonoscopy. This procedure is the most sensitive test for colorectal polyps and colorectal cancer. It's better at detecting polyps than is a barium enema X-ray alone. Colonoscopy is similar to flexible sigmoidoscopy, but the instrument used — a colonoscope, which is a long, slender tube attached to a video camera and monitor — allows your doctor to view your entire colon and rectum. If any polyps are found during the exam, your doctor may remove them immediately or take tissue samples (biopsies) for analysis. A colonoscopy takes about a half-hour. You're likely to receive a mild sedative to make you more comfortable. The risks of diagnostic colonoscopy include hemorrhage and perforation of the colon wall; complications are more likely to occur when polyps are removed.
• Genetic testing. If you have a family history of colorectal cancer, you may be a candidate for genetic testing. This blood test may help determine if you're at increased risk of colon or rectal cancer, but it's not without drawbacks. The results can be ambiguous, and the presence of a defective gene doesn't necessarily mean you'll develop cancer. Knowing you have a genetic predisposition can alert you to the need for regular screening. Still, you'll also want to consider the psychological impact of what the test may reveal. Knowing you may develop cancer will affect not only your own life but also the lives of everyone close to you. Genetic testing for children is even more complex and problematic. These issues can be difficult to sort through, which is why it's best to discuss the ramifications of genetic testing with your doctor, a genetic counselor or both.
• Pill camera. Colonoscopy is effective at detecting polyps in the colon, but the colonoscope can't reach the small intestine. Until recently, a barium X-ray was the only way to screen the small intestine, but the test is often inaccurate. Now doctors have found that a tiny camera fitted inside a capsule that you swallow can identify polyps in the small intestine with a high degree of accuracy. But because small intestine polyps are rare, the test isn't routinely performed.
• New technologies. New technologies such as virtual colonoscopy (CT colonography) may make colon screening safer, more comfortable and less invasive. In virtual colonoscopy, you have a two-minute computerized tomography scan, a highly sensitive X-ray of your colon. Then, using computer imaging, your doctor rotates this X-ray in order to view every part of your colon and rectum without actually going inside. Before the scan, your large intestine is cleared of any stool, but researchers are looking into whether the scan can be done successfully without the usual bowel preparation. Although virtual colonoscopy potentially is a tremendous step forward, it may not be as accurate as regular colonoscopy, and it doesn't allow your doctor to remove polyps or take tissue samples.

Another new test checks a stool sample for DNA from abnormal cells. In preliminary studies, the test proved highly accurate, but results in the first large trial of the test were disappointing. In that trial, the DNA test found just half the colon and rectal cancers that colonoscopy did.

Although some types of colon polyps are far more likely to become malignant than others, a pathologist usually must examine a polyp under a microscope to determine whether it's potentially cancerous. For that reason, your doctor is likely to remove all polyps discovered during a bowel examination.

The great majority of polyps can be removed during colonoscopy or sigmoidoscopy by snaring them with a wire loop that simultaneously cuts the stalk of the polyp and cauterizes it to prevent bleeding. Some small polyps may be cauterized or burned with an electrical current. Risks of polyp removal (polypectomy) include bleeding and perforation of the colon.

Polyps that are too large to snare or that can't be reached safely are usually surgically removed — often using laparoscopic techniques. This means your surgeon performs the operation through several small incisions in your abdominal wall, using instruments with attached cameras that display your colon on a video monitor. Laparoscopic surgery may result in a faster and less painful recovery than does traditional surgery using a single large incision. Once the section of your colon that contains the polyp is removed, the polyp can't recur, but you have a moderate chance of developing new polyps in other areas of your colon in the future. For that reason, follow-up care is extremely important.

In cases of rare, inherited syndromes, such as FAP, your surgeon may perform an operation to remove your entire colon and rectum (total proctocolectomy). Then, in a procedure known as ileal pouch-anal anastomosis, a pouch is constructed from the end of your small intestine (ileum) that attaches directly to your anus. This allows you to expel waste normally, although you may have several watery bowel movements a day.

You can greatly reduce your risk of colon polyps and colorectal cancer by having regular screenings and by making certain changes in your diet and lifestyle. The following suggestions may help save your life:

• Pay attention to calcium. For four years, the Calcium Polyp Prevention Study followed nearly 1,000 adults who took either 1,200 milligrams of calcium or a placebo every day. At the end of the study, the participants who took the calcium had a 20 percent reduction in the risk of colorectal cancer as compared with those taking the placebo. Five years after the study, the calcium users showed a 41 percent reduction in the risk of polyps and a 35 percent reduction in colon cancers — almost double the numbers during the treatment phase. Calcium's protective effect didn't seem to last longer than five years, however. This study corroborates the results of several other studies linking calcium intake to a greatly reduced risk of colon cancer. Good food sources of calcium include skim or low-fat milk and other dairy products, broccoli, kale and canned salmon with the bones. Vitamin D, which aids in the absorption of calcium, also appears to help reduce the risk of colorectal cancer. You get vitamin D from foods such as vitamin-D fortified milk products, liver, egg yolks and fish. Sunlight also converts a chemical in your skin into a usable form of the vitamin. If you don't drink milk or you avoid the sun, you may want to consider taking both a vitamin D and a calcium supplement.
• Include plenty of fruits, vegetables and whole grains in your diet. These foods are high in fiber, which can cut your risk of developing colon polyps. Fruits and vegetables also contain antioxidants, which may help prevent cancer. The American Cancer Society recommends eating five or more servings of fruits and vegetables every day. Look for deep-green and dark yellow or orange fruits and vegetables such as Swiss chard, bok choy, spinach, cantaloupe, mango, acorn or butternut squash, and sweet potatoes, as well as vegetables from the cabbage family, including broccoli, brussels sprouts and cauliflower. Lycopene, a nutrient found in tomatoes and other red fruits and vegetables, such as strawberries and red bell peppers, may be a particularly powerful anti-cancer chemical.
• Limit fat. People who eat high-fat diets have a higher rate of colorectal cancer than do people who consume less dietary fat. Be especially careful to limit saturated fats from animal sources such as red meat. Other foods that contain saturated fat include whole milk, cheese, ice cream, and coconut and palm oils. Restrict your total fat intake to less than 35 percent of your daily calories, with no more than 8 percent to 10 percent coming from saturated fats.
• Get adequate amounts of folic acid and vitamin B-6. Research suggests that getting adequate amounts of the B vitamin folate or folic acid — the synthetic form of the vitamin — can help reduce your risk of colorectal cancer. Even people with a family history of the disease may be able to lower their risk by limiting alcohol consumption and taking 400 micrograms of folic acid daily. Folate is found in dark leafy greens such as spinach, and in pinto, kidney and navy beans, some nuts and seeds, and in fortified cereals. Most multiple vitamins also contain folic acid. Eating foods rich in folate can have added benefits for women. If you're pregnant, or think you may become pregnant, getting enough folic acid in your diet reduces the risk of certain birth defects. In addition, women with high levels of vitamin B-6 in their diets are less likely to develop colorectal cancer than are women with lower B-6 levels.
• Limit alcohol consumption. Consuming moderate to heavy amounts of alcohol — more than one drink a day for women and two for men — increases your risk of colon cancer. This is particularly true if you have a close relative, such as a parent, child or sibling, with the disease. A drink is considered to be a 4- to 5-ounce glass of wine, a 12-ounce can of beer, or a 1.5-ounce shot of 80-proof liquor. Curbing alcohol consumption can reduce your risk, even if colon cancer runs in your family.
• Stop smoking. Smoking can increase your risk of colon cancer and a wide range of other diseases. Talk to your doctor about ways to quit that might work for you.
• Stay physically active and maintain a healthy body weight. Controlling your weight alone can reduce your risk of colorectal cancer. And staying physically active may significantly cut your colon cancer risk. Exercise stimulates movement through your bowel and reduces the time your colon is exposed to harmful substances that may cause cancer. The American Cancer Society recommends at least 30 minutes of physical activity five or more days a week. Forty-five minutes of moderate or vigorous activity can lower your risk of breast and colon cancer even more. If you're overweight, lose weight until you are at a healthy level and maintain it.
• Talk to your doctor about aspirin. Women with a common gene mutation who take a regular aspirin two to seven times a week are less likely to develop colon polyps than are similar women who don't take aspirin. But aspirin seems to have no effect on polyp formation in women without the genetic mutation. What's more, aspirin can increase your risk of gastrointestinal bleeding, so check with your doctor before starting any aspirin regimen.
• Talk to your doctor about hormone therapy (HT). If you're a woman past menopause, hormone therapy (HT) may reduce your risk of colorectal cancer. Women who use HT have a somewhat lower risk of colorectal cancer than do women who don't use HT. But not all effects of HT are positive. Taking HT as a combination therapy — estrogen plus progestin — can increase your risk of breast cancer, dementia, heart disease, stroke and blood clots. Discuss your options with your doctor. Together you can decide what's best for you.
• If you're at high risk, consider your options. If you're at risk of FAP because of a family history of the disease, consider having genetic counseling. And if you've been diagnosed with FAP, start having regular colonoscopy tests in your early teens and discuss your options with your doctor. To prevent cancer from developing, most experts recommend having surgery to remove your entire colon when you're in your 20s. The risk for people with hereditary nonpolyposis colorectal cancer isn't quite as great as it is for those with FAP. Doctors recommend that people at risk of HNPCC begin having regular colonoscopies around age 20, but less often recommend removing the colon.