Colon Cancer Is on the Rise in Young People—Is a Bacterial Toxin to Blame?

No one expects a young adult to get cancer. But in recent years, that sad scenario has been happening more and more frequently. Early-onset cancers—those affecting adults younger than 50—are being diagnosed increasingly often in countries around the world, including the United States.

At least 17 types of cancer are on the rise among younger Americans. And researchers have been especially concerned about an uptick in early-onset colorectal cancers, or those affecting the colon or rectum. The number of colorectal cancers diagnosed among U.S. adults younger than 55 increased by 1% to 2% each year from 2015 to 2019, according to the American Cancer Society. Compared to people born around 1950, those born around 1990 have double the risk of developing colon cancer and quadruple the risk of getting rectal cancer, indicates one sobering study. 

What’s driving that trend? Researchers don’t know for sure. But a study recently published in Nature points to one possibility: childhood exposure to a bacterial toxin called colibactin.

Colibactin is produced by bacteria commonly found in the digestive system, including certain types of E. coli. About 20% of healthy people—and even some 31% of babies—have colibactin-producing E. coli in their guts, previous studies estimate.

The toxin is known to damage DNA, and previous studies have suggested that colibactin-related mutations contribute to some cases of colorectal cancer. But Ludmil Alexandrov, PhD, a professor of cellular and molecular medicine at the University of California, San Diego and senior author of the new study, didn’t set out to research colibactin specifically. 

Originally, his team wanted to understand why people in different countries develop colorectal cancer at different rates. They genetically analyzed almost 1,000 colorectal cancer samples from people living in 11 countries to compare and contrast them.

“We did find some results which were country-specific,” Alexandrov said. “But actually, the thing that we found most exciting was this colibactin result.”

Alexandrov’s team found that DNA mutations associated with colibactin were 3.3 times more common in cancers diagnosed before age 40, compared to those diagnosed after age 70. They estimated that colibactin exposures likely happened early in study participants’ lives—probably before their 10th birthdays.

The results suggest that colibactin exposure could “put children on a trajectory for developing colorectal cancer 20 or 30 years earlier [than normal]—so instead of getting it at the age of 60 or 70, they’re getting it at the age of 30 or 40,” Alexandrov explained.

Analyzing such a large number of cancer samples, from people in so many countries, is “quite a challenging task,” so “it’s a great design in that regard,” said Shuji Ogino, MD, PhD, a pathologist at Brigham and Women’s Hospital in Boston who was not involved in the new research but has previously studied E. coli and colorectal cancer. 

But, Ogino said, “one study cannot address everything.”

Most crucially, the study could not definitively prove that colibactin caused people’s cancers; it just found an association between mutations related to the toxin and early-onset disease. “Our study provides very strong associative evidence,” Alexandrov said, but more research is necessary to prove causation.

The study also couldn’t nail down whether something has changed in our diets, environments, or lifestyles in recent decades to drive increasing exposure to colibactin, and thus the uptick in early-onset cancers. Nor did it touch on study participants’ other cancer risk factors, said Benoît Rousseau, MD, PhD, an assistant attending physician at Memorial Sloan Kettering Cancer Center in New York City who was not involved in the new research but studies gastrointestinal cancers.

“We don’t have a lot of information about their BMIs,” Rousseau said. “Are they obese patients, what is their exercise profile, what is their diet?” 

The study provides “strong, compelling, and convincing” data about a correlation between colibactin exposure and early-onset colorectal cancer, Rousseau told Health, but more research is required to confirm and expand upon the findings. 

Alexandrov’s team is already planning additional studies looking at how kids are exposed to colibactin, as well as whether tools like probiotics could protect against the toxin, he said. They are also working to develop a stool test that could detect signs of colibactin-related damage; if someone tested positive, they could then be monitored more aggressively for colorectal cancer. 

Although such a test isn’t available now, Alexandrov is optimistic that one could be developed within the next few years.

Even without a test on the market, there are some practical steps people can take now, Ogino said. His research suggests that eating lots of red and processed meats, sugar, and refined grains may increase the risk of developing colorectal cancer related to colibactin-producing bacteria. It’s smart to eat those things in moderation and instead opt for wholesome foods like vegetables, beans, and whole grains whenever possible, Ogino told Health. 

It’s important to follow a balanced diet from a young age, he added. “That means parents are critical,” Ogino said.

Limiting alcohol consumption, avoiding tobacco, exercising, and staying up-to-date on screenings are also important ways to reduce colorectal cancer risk, according to the Centers for Disease Control and Prevention. Routine screening tests, such as colonoscopies, are recommended for U.S. adults starting at age 45. 

Researchers are still trying to determine whether there are specific things people should do to prevent early-onset colorectal cancer, Alexandrov said. His study suggests it’s important for scientists to look not only for habits that adults can adopt but also for strategies that can minimize health risks in the earliest years of life.

“This study has one broader implication that’s lurking in it, and that’s the effect of childhood on later life,” Alexandrov told Health. “Something that may happen at the age of three or four can impact you at the age of 30 or 40 or even 50.”