Checking seemingly healthy people for cancer—what doctors call screening—seems like a simple process: Perform a test and either find cancer early and cure it or don’t find it and breathe easy.

Yet it’s actually not so simple. For many types of cancer, there is no test that can reliably detect the disease at a curable stage. The available tests aren’t always accurate. Testing may be invasive, risky, inconvenient, uncomfortable, or expensive. And some cancers (such as certain prostate tumors) are harmless, so detecting them can do more harm than good.

For colon cancer, we have several effective screening tests:

There is some uncertainty about just how much benefit these tests provide. Two new studies in yesterday’s New England Journal of Medicine aim to quantify the benefits.

In the first study, researchers tracked nearly 89,000 adults for 22 years. Some had colonoscopy or sigmoidoscopy. Others had no colon cancer tests. About 2% of the total group developed colon cancer, and 0.5% died of the disease.

Colonoscopy and sigmoidoscopy were linked with a lower risk of colon cancer. Those who had no problems found during a colonoscopy were about 56% less likely to develop colon cancer than those not screened. Those who had a polyp removed during sigmoidoscopy or colonoscopy or had no problems found during sigmoidoscopy were about 40% less likely to develop colon cancer than those not screened. Even more important, those screened by colonoscopy or sigmoidoscopy were less likely to have died of colon cancer.

In the second study, more than 46,000 adults were randomly divided into three groups. Two groups received stool testing either every year or every other year. A comparison group received “usual care,” and few people in this group had stool tests. The screening tests were done during two six-year periods. Up to 30 years later, about 2% of the total group had died of colon cancer.

Compared with the usual-care group, colon cancer deaths were 32% lower in the group that got yearly stool tests and 22% lower in the group that got the test every two years.

Taken together, these findings support the current recommendation that adults have a screening colonoscopy beginning at age 50. Sigmoidoscopy may be a worthwhile alternative, though its benefit was less. Doctors may recommend stool testing, too. But the ideal combination of tests has not been well studied.

People with average risk of colon cancer are advised to get a repeat colonoscopy every 10 years. These findings suggest that this may be more frequent than necessary.

People with risk factors for colon cancer may need to start earlier or have more frequent testing. Risk factors include a family history of inherited colorectal cancer syndromes, a first-degree relative (parent, sibling, or child) who developed colorectal cancer or polyps before age 60, a personal history of colorectal cancer or polyps, or a personal history of chronic inflammatory bowel disease, such as ulcerative colitis or Crohn’s disease.

There are a number of things you can do to prevent colon cancer.

Get screened. Talk to your doctor about when to begin testing, which test is best for you and how often to repeat it.

Exercise more. Studies suggest that regular exercise may reduce the risk of colon cancer.

Change your diet. A diet that is low in saturated fats may lower your cancer risk.

Don’t smoke. Cigarette smoking has been linked to many types of cancer, including colon cancer.

There is some evidence that taking a daily aspirin or folic acid may reduce the risk of colon cancer. However, this is not proven. Review these options with your doctor as well.

The biggest challenge for colon cancer screening is getting people to have the available tests. About 50,000 Americans die of colon cancer each year—many of these can be prevented with early screening. I am hopeful that this research and public health messages will help colon cancer screening become more widespread.

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