A single colon screening performed in midlife—one procedure, one afternoon—may reduce the risk of dying from colorectal cancer for at least two decades afterward, according to one of the longest colorectal screening trials ever conducted.

The research from Norway, which followed nearly 100,000 adults for 23 years, found that men who underwent a one-time flexible sigmoidoscopy, a less invasive exam than a standard colonoscopy, were 28 percent less likely to develop colorectal cancer and 37 percent less likely to die from the disease than men who were not screened.

Women also experienced fewer cancer diagnoses, though the study did not find a significant reduction in deaths.

The findings, published in the Annals of Internal Medicine, add to growing evidence that a single flexible sigmoidoscopy exam in midlife may have lasting effects on colorectal cancer risk.

“The findings are consistent with other trials of flexible sigmoidoscopy,” Stephen W. Duffy, an emeritus professor of cancer screening at Queen Mary University of London and a researcher on a major UK sigmoidoscopy trial, who was not involved in the new study, told The Epoch Times in an email. He noted that the UK trial also found significant long-term reductions in colorectal cancer incidence and mortality after sigmoidoscopy screening.

Colorectal cancer is the second leading cause of cancer deaths in the United States. Because the disease often develops without noticeable symptoms in its early stages, many cases are diagnosed only after the cancer has advanced—making screening and early detection especially important.

Stronger Long-Term Benefits in Men

The findings come from the NORCCAP trial, a large population-based screening study conducted in Norway that invited adults ages 50 to 64 to take part between 1999 and 2001.

About 20,000 adults were assigned to receive a one-time flexible sigmoidoscopy, while more than 78,000 were in a comparison group that received usual care and no screening through the study.

Compared to a full colonoscopy, flexible sigmoidoscopy uses a shorter,  thinner, and more maneuverable scope. Because it can only view the lower portion of the colon, it can typically be performed in about 20 minutes without full sedation and with a simpler bowel preparation. When doctors see suspicious growths during the exam, patients are referred for a more comprehensive colonoscopy, where precancerous polyps and early cancers can be removed.

Among men, the cumulative risk of developing colorectal cancer over 23 years was 4.3 percent in the screened group, compared with 6 percent among those not screened. Deaths from the disease also fell, from 2.2 percent to 1.4 percent.

The benefit was concentrated in cancers of the lower, or distal, colon—the area the sigmoidoscope can reach—and persisted throughout the follow-up period.

Men in the trial were more likely to have lower-colon polyps that led to follow-up colonoscopies, increasing the chances that precancerous growths were found and removed before they could turn into cancer.

“Since a large number of colorectal lesions occur in the lower colon, prevention of cancer in this part of the colon has a substantial effect on colorectal cancer incidence and mortality overall,” Duffy said.

Researchers also observed two distinct declines in deaths among men after screening. The first occurred within two to five years of screening, reflecting cancers detected earlier, when treatment is more effective.

A second decline emerged about nine to 12 years after the initial screening. Researchers say this later drop likely reflects cancers that never developed because precancerous polyps had been removed during follow-up colonoscopies.

In other words, the test changed men’s long-term risk not by curing anything on its own, but by triggering colonoscopies where dangerous growths could be removed.

Why the Benefits Were Smaller for Women

Women who underwent sigmoidoscopy screening saw an 11 percent reduction in colorectal cancer incidence, but no clear reduction in deaths from the disease. Their 23-year cancer risk fell from 4.7 percent in the comparison group to 4.2 percent in the screening group.

The difference between men and women reflects where tumors tend to develop, gastroenterologist Dr. Aasma Shaukat, a global leader in colorectal cancer prevention and professor at NYU Grossman School of Medicine, who was not involved in the study, told The Epoch Times in an email.

“Women have more proximal colon cancers,” Shaukat said. Flexible sigmoidoscopy examines only the lower colon and does not reach the upper, or proximal, colon, where women are more likely to develop tumors.

Researchers suspect that a mix of biological factors—such as differences in hormones, genes, and tumor biology—helps explain this pattern, but the exact reasons are still being studied.

Women also tend to have a lower baseline risk of colorectal cancer than men at the same age, she added, with risk in women lagging by roughly a decade. “A 60-year-old woman’s risk is more similar to that of a 50-year-old man,” Shaukat said. “When baseline risk is lower, the absolute benefit of screening is also smaller.”

What the Findings Mean for Screening

Doctors say a person’s risk of colorectal cancer plays an important role in deciding when to begin screening and which test to choose.

Risk increases with age, and people with a family history of colorectal cancer, inflammatory bowel disease, or a personal history of colon polyps may need earlier or more frequent screening.

Current U.S. guidelines recommend that adults at average risk begin colorectal cancer screening around age 45, with several options available depending on individual risk, access, and preference.

Lifestyle factors—including smoking, heavy alcohol use, obesity, physical inactivity, and diets high in red or processed meats—have also been linked to higher risk.

Colonoscopy, typically performed every 10 years when no polyps are found, remains the most commonly used screening test in the United States because it examines the entire colon and allows doctors to remove polyps during the same procedure. People at higher risk—for example, those with previous polyps or strong family history—are often advised to have colonoscopies more frequently than every 10 years.

Stool-based tests, such as fecal immunochemical test or FIT, are noninvasive and are usually repeated every one to three years. Flexible sigmoidoscopy is used less often in the United States, but may be performed every five years when chosen.

Studies have shown that all three approaches can reduce colorectal cancer risk and deaths, though they differ in what they detect. “There are no head-to-head trials comparing them directly,” Shaukat said. “But across studies, all three modalities are effective in reducing colorectal cancer incidence and mortality.”

The Takeaway

In the United States, colonoscopy remains the preferred screening approach.

While flexible sigmoidoscopy typically involves less sedation, both procedures typically require similar bowel preparation, making colonoscopy the more comprehensive option, Shaukat said.

The new findings do not change current U.S. recommendations, but they provide evidence that even a single screening exam in midlife can have lasting effects—particularly for men.

The broader message of the study is straightforward: Colorectal cancer is often preventable, and the benefits of detecting and removing precancerous growths can last for decades.