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Why is colorectal cancer increasing among young people?

Colon and rectal cancers share many similarities and are typically lumped into one category, called colorectal cancer. Studies, however, show that the increase in diagnoses is mainly driven by a rise in rectal cancers and cancers found in the left, or distal, side of the colon, near the rectum. “That maybe provides an important clue for understanding what might be going on,” said Caitlin Murphy, an associate professor and cancer researcher at UTHealth Houston.

Colorectal cancers in younger people also tend to be more aggressive, and they are often found at a more advanced stage, Dr. Murphy said. But most people affected by early-onset colorectal cancer are too young to be recommended for routine cancer screenings, which have helped decrease rates in adults over 50. In 2021, the U.S. Preventive Services Task Force reduced the recommended age for starting colorectal cancer screening by just five years — from 50 to 45.

A vast majority of colorectal cancer diagnoses are still made in people 50 and older. The American Cancer Society predicted last year that roughly 153,000 new diagnoses would be made in the U.S. in 2023, of which 19,550 would be in people younger than 50. But millennials born around 1990 now have twice the risk of colon cancer compared with people born around the 1950s, while millennials’ risk for rectal cancer is about four times higher than that of older age groups, according to a study published in the Journal of the National Cancer Institute. That means diagnoses are likely to “continue going up as these higher-risk generations age,” Dr. Murphy said.

When cancer is found at a younger-than-usual age, doctors usually suspect that genetic mutations may be to blame. And some molecular studies suggest that tumors in early-onset colorectal cancers do have different mutations driving the cancer compared with tumors in older adults. Another piece of evidence that there is a genetic component: It is clear that having a first-degree relative who had colorectal cancer — or even a precancerous polyp — can increase your risk, Dr. Cecchini said. But genetic changes do not explain the full picture, he said.

Some research has linked lifestyle and dietary changes to increased rates of colorectal cancer in both young people and older adults. Recent generations have consumed more red meat, ultraprocessed foods and sugary beverages, and have been known to binge drink more frequently; between 1992 and 1998, cigarette smoking also increased before declining again, while physical activity has continuously declined for decades. All of these factors — along with the rise in obesity rates since the 1980s — are associated with cancer risk. But once again, none of them fully account for the increase in early-onset colorectal cancer.

“For a lot of these risk factors, like smoking, you have to be exposed for long periods of time before the cancer develops,” said Dr. Andrea Cercek, a co-director of the Center for Young Onset Colorectal and Gastrointestinal Cancers at Memorial Sloan Kettering Cancer Center. And many patients in their 20s and 30s do not even fit in these risk groups, she said. “Many of our patients are athletes,” she said. “Many of them were never heavy, not even in childhood.”

Experts are beginning to investigate if there are other environmental drivers of early-onset cancer. For instance, some small studies have hinted at the idea that people who develop colorectal cancer at an early age have an imbalance of “good” and “bad” bacteria in their gut. Researchers are not only looking at antibiotic use, which can alter the gut microbiome, but also nonsteroidal anti-inflammatory drugs that are used as painkillers, proton pump inhibitors that are used to counter stomach acid issues and several psychiatric medications that may be absorbed through the intestinal lining and have increased in use in recent decades, Dr. Cercek said.

Some experts believe exposure to toxic chemicals in the environment may also be to blame. “There’s patterns of environmental exposures by geography, by race, by sex, by all the things that we know colorectal cancer rates also differ by,” Dr. Murphy said.

For instance, for many years, the rates of colorectal cancer diagnoses were highest among non-Hispanic Black people, but research shows that these cancers increased more among non-Hispanic white people in the 1990s and early 2000s, Dr. Murphy said. Now, both groups have fairly similar rates of cancer. “Does this mean that white people are now being exposed to something that Black people have been exposed to for many, many years? We just don’t know yet,” Dr. Murphy said.

There are also geographic disparities in the increase in cancer, with experts seeing more cases emerge in cities and towns along the Mississippi River, in Southeastern states and in Appalachia, which may be explained by occupational exposures to trace elements like arsenic, chromium, and nickel, which are often used in coal production, chemical plants and other industries in those regions. So-called forever chemicals like per- and polyfluoroalkyl substances, better known as PFAS, have been linked to other cancers and could also be driving some of the increase in early-onset colorectal cancer.

“I don’t think there’s going to be one smoking gun that explains everything,” Dr. Murphy said. “It’s a whole bunch of things.”