Harvard Health Letter

How to lower your risk for colon cancer

The search for answers circles back to some familiar recommendations.

There are no guarantees, of course, but most of us can lower our chances of getting colon cancer in several ways: don't eat too much red meat, don't get heavy, exercise, and for heaven's sake, don't smoke.

Starting at about age 50, regular screening — colonoscopy is increasingly favored — improves the chances of avoiding colon cancer and also of surviving even if you do get it. Screening works because colon cancer is usually a slow-growing disease that starts with adenomatous polyps, small, protuberant growths inside the colon that may contain precancerous cells. Screening tests are designed to find these polyps so doctors can remove them before they become cancerous.

Reasons we procrastinate

But the fact of the matter is that most of us tend to put off screening tests, perhaps especially so the ones for colon cancer. The fecal occult blood test involves gathering stool samples. Sigmoidoscopy and colonoscopy require cleaned-out colons. The preparation, which involves a heavy-duty laxative and then a lot of time in the bathroom, can be something of an ordeal. Many people say the preparation for the procedure is more unpleasant than the procedure itself.

So-called virtual colonoscopy might be more appealing. This test lets the doctor view the colon from the outside with a CT scanner instead of from the inside with an endoscope, so it's less invasive and can't cause an accidental perforation of the colon, a one-in-a-thousand occurrence with regular colonoscopy. But for now, it still requires a cleaned-out colon, and it remains virtual only if polyps aren't found. If they are, you will need a regular colonoscopy to have them removed. Besides, virtual colonoscopy is still considered experimental and may not be covered by insurance.

As for the other admonitions — cutting back on red meat may not be so difficult. But the gap between word and deed looms especially large when it comes to weight control and exercise.

So we do need easier, more reliable ways of preventing colon cancer, which is the third most common cancer in the United States (about 154,000 new cases a year) and the second most lethal (52,000 deaths annually).

It's usually not inherited

As with many of the most common cancers, some colon cancer is clearly caused by inherited genetic mutations that are passed down from generation to generation. But that's a relatively rare event. Fewer than 5% of cases are caused by the two main genetic disorders that have been identified so far, familial adenomatous polyposis and hereditary nonpolyposis colorectal cancer.

Another 25% of patients fall into a gray area. The disease runs in their families, so there seems to be some inherited genetic component, but the pattern of who is affected and who isn't suggests other causative factors as well.

About 70% of colon cancer cannot be explained by family history or an obvious inherited factor. Doctors sometimes refer to these as "sporadic" cases although they are the most common. In most instances, therefore, colon cancer has more to do with what we eat, how much we exercise, how much we weigh, and a number of other factors.

Aspirin is too risky

The daily, low-dose (81- to 325-milligram) aspirin is one possibility. Many people already take aspirin to reduce their risk of having a second heart attack or stroke — or of having their first one if they fall into a high-risk group (a not-very-exclusive club that includes many men over 40 and postmenopausal women). Aspirin also seems to have anticancer properties. In lab and animal experiments, it has prevented the rapid cell division that's associated with cancer. And epidemiologic studies show that people who take aspirin are about half as likely to get colon cancer as those who don't, although the protective effect seems to emerge only after 10 years or more of regular intake.

When aspirin has been put to the test in randomized trials, the outcomes have been mixed — and confusing. One trial that enrolled people who had previously had a polyp found that taking a 325-mg aspirin daily lowered the risk of getting a polyp again. But another trial that compared different daily dosages (81 mg vs. 325 mg) found the smaller amount modestly protective but not the larger one.

In 2007, the U.S. Preventive Services Task Force recommended against taking aspirin — or any other nonsteroidal anti-inflammatory drug (Aleve, Motrin, others) — for colon cancer prevention if your colon cancer risk is average. The well-known risks of these drugs (intestinal bleeding, kidney problems, and "bleeding" strokes) outweigh the possible colon cancer protection, the experts decided. Their recommendation didn't address people in a higher risk category, such as those who have had polyps discovered during a screening test.

It's a closer call, but even then use of aspirin isn't routinely recommended. Part of the thinking is that people at high risk should be screened more often anyway, and that those additional tests will find polyps at an early, less dangerous stage.

Still, there's room for exercising some judgment. If a person at high risk for colon cancer is at low risk for complications from aspirin — and would benefit from taking it for other reasons (presumably cardiovascular) — then it's reasonable to at least consider aspirin for reducing colon cancer risk.

Seven tips about what will and won't lower your risk

1. Cut back on the red and processed meat. The studies don't line up perfectly, but many point to increased colon cancer risk among people who eat a lot of red (beef, lamb, pork) and processed meat (bacon, ham, hot dogs, sausage, and so on). The risk from processed meat might be greater than it is from red meat. Why red meat might cause colon cancer is unclear. Some studies suggest that the heme iron it contains promotes cell division and cancerous growth. Chicken doesn't seem to increase colon cancer risk, and one study found it might even reduce the risk.

2. Stay active. Whether it is on or off the job, physical activity seems to protect against colon cancer (but curiously, not against rectal cancer). Some studies suggest that you'll get even more protection if you exercise vigorously. But Danish researchers reported in 2006 that activity level wasn't as important as the number of different activities people participate in.

3. Stay slender. Harvard studies identified a connection between obesity and colon cancer in 1995. Some nuances have emerged since. Obese men of all ages are at risk, but for women, premenopausal more than postmenopausal obesity is the danger. Being overweight (a BMI of 25 to 29.9) increases your risk, but only half as much as being obese (a BMI of 30 or higher). Visceral fat, which accumulates in the belly and clings to the abdominal organs, may pose more of a risk than fat under the skin that widens hips, thighs, and buttocks.

4. Adequate calcium intake is good enough for most. High milk and calcium consumption have been linked to lower colon cancer risk in epidemiologic studies. And one important trial showed that in people who'd already had a polyp removed, taking 3,000 mg of calcium carbonate daily (1,200 mg of elemental calcium) reduced the risk of recurrence by 20%. But when daily calcium (1,000 mg) and vitamin D (400 IU) supplements were tested in the landmark Women's Health Initiative study, they had no effect on colon cancer rates. Some research suggests that calcium is protective but that daily intake over 1,000 mg probably doesn't add much. Other data suggest that we've paid too much attention to calcium and that vitamin D is the real risk reducer. Men are expressly advised by the American Cancer Society not to get too much calcium (1,500 mg or more daily) because large amounts may increase prostate cancer risk. The bottom line: People who have had polyps should talk to their doctors about taking a calcium supplement. High levels of calcium intake have been shown to reduce the risk of getting more polyps. But others can get the calcium they need by eating a balanced diet that includes nonfat dairy products and fruits and vegetables.

5. Nothing special about fiber. Loading up on fiber was once thought to be our best defense against colon cancer. No more. In clinical trials, fiber hasn't worked to reduce recurrent polyps, and the epidemiological evidence that it protects against first-time polyps is uneven. (Diets high in fiber do seem to reduce the risk of heart disease and diabetes, however, so don't forget about it entirely). Whole grains are a great source of dietary fiber, but have other ingredients as well. Results from a National Institutes of Health–AARP study published in 2007 showed that whole-grain consumption, but not fiber specifically, was associated with a modest reduction in colon cancer risk.

6. Hormone therapy isn't worth the other risks. If colon cancer were the only health concern, many women might consider taking hormones to prevent it. In the Women's Health Initiative study, postmenopausal women who took an estrogen-progestin combination lowered their risk for colorectal cancer by 44%. But hormone therapy has other risks associated with it (including breast cancer and heart disease, depending on when it's taken relative to menopause), so it's not recommended for colon cancer prevention.

7. Get screened. The number of Americans dying each year from colon cancer has been declining. Treatment has improved, so people with the disease are living longer. But screening, so cancers are caught earlier, has played a role. The American Cancer Society recommends that people at average risk get their first screening test at age 50. Partly because it involves inspection of the entire colon, the colonoscopy has become increasingly popular. How often screening tests need to be done depends, of course, on whether anything suspicious is found. A low-risk patient (no or insignificant polyps) needn't get another colonoscopy for 10 years. A high-risk patient may be scheduled for a test every three years — and even more often than that, depending on family history and other factors.

Folic acid falters

Another contender for colon cancer prevention-in-a-pill has been folic acid, the synthetic form of folate, which is found naturally in leafy green vegetables and other foods. One of the B vitamins, folic acid wears several hats, all of them important. It's crucial to the development of red blood cells and therefore the prevention of anemia. In the United States and several other countries, flour and grains are fortified with folic acid because it prevents spina bifida and other birth defects affecting the spinal cord. Along with other B vitamins, folic acid has been tested as a way of reducing the risk of heart attack and stroke; the results are mixed. It has even been tested as a treatment for depression.

On top of all these other credentials, folic acid looks pretty good as an anticancer agent. It's crucial to the creation of DNA, the molecule that genes are made of, so if folic acid is in short supply, aberrations in DNA that could give rise to cancer-causing mutations may develop. Evidence from animal studies hints at cancer protection from folic acid, and epidemiologists have found a connection between low folic acid intake and colon cancer, particularly in people who drink alcohol (the same is true of breast cancer).

With all this good news about folic acid rolling in, the investigators conducting the trial that compared aspirin dosages decided to add folic acid to their study. But their report in the June 6, 2007, issue of the Journal of the American Medical Association (JAMA) was not of the good-news variety. The study volunteers who took folic acid pills (1,000 micrograms daily) were more, not less, likely to get recurrent polyps than the volunteers who took a placebo pill. Furthermore, large polyps were more common in the folic acid group and, generally speaking, large polyps are more likely to develop into cancer than smaller ones.

The researchers also reported that the folic acid group had more of other sorts of cancer, especially prostate cancer (24 cases in the folic acid group vs. nine in the placebo group).

Soon after these results appeared in JAMA, Tufts University researchers reported in a cancer epidemiology journal that colon cancer rates in the United States and Canada started to go up after folic acid supplementation began in the two countries in the late 1990s. Although the increase could very well be the result of more people getting screened for colon cancer, the Tufts researchers argued that the surge in our folic acid intake thanks to supplementation might also be to blame.

So what's going on? Does folic acid join the growing list of vitamins and minerals (vitamin A, vitamin E, perhaps selenium) that go from being healthful to harmful when taken in large amounts in pill form? No one knows for sure. Indeed, the Tufts researchers went out of their way to emphasize that their findings were food for thought, and far from conclusive. Still, they speculated that perhaps the surplus folic acid in our diets overwhelms systems in the intestine that metabolize the vitamin. Others have theorized that folic acid in certain amounts may prevent colon cancers from forming, but in large amounts may "feed" small, microscopic cancers if they're already there.