A strange divergence is happening in the diagnosis of colorectal cancer: Older people, who are most at risk from the disease, are seeing fewer cases while people under 50 are seeing more.

These diagnoses are often devastating, as younger people also tend to go longer between first experiencing symptoms and getting diagnosed, meaning most of their cancers are caught in a late stage that is more difficult to treat.

But what's driving this rise in young people? Doctors and researchers think some change that occurred after the 1960s is to blame, likely a shift in people's environments or lifestyles. Perhaps offering a clue as to the culprit, recent research also suggests that, whatever the factor, it primarily affects cancers of the rectum and the lowest part of the colon.

The rise of colorectal cancer in young adults

There were about 158,850 new cases of colorectal cancer diagnosed in the U.S. in 2026, according to the American Cancer Society (ACS). Over 86,000 cases — more than half — occurred in people over 65. But those numbers represent a decline of 2.5% a year in that age group since 2013.

Doctors chalk up the decline in colorectal cancer in people 65 and older up to the success of screening, said Dr. Aasma Shaukat, a gastroenterologist at New York University Langone Health. During routine colonoscopies, which start at 45 years old for people at typical risk, doctors remove precancerous polyps and thus prevent that tissue from becoming cancerous.

At the same time over-65s have seen lower cancer rates, people between 50 and 64 have seen a 0.4% increase per year and people between the ages of 20 and 49 have seen an even more alarming jump of 3% each year. The rise is seen in every racial and ethnic group.

What that translates to, according to research published in ACS's CA: A Cancer Journal for Clinicians, are 24,640 new cases a year for people 49 and younger and 47,600 new cases a year for people 50 to 64.

Most of these cases are what doctors call "left-side" cancers, occurring in the lower colon, which descends down the left side of the body, and the rectum. Between 1999 and 2023, rectal cancer mortality rose two to three times faster than colon cancer mortality across all demographics, according to research presented in May at Digestive Disease Week 2026, a major gastroenterology conference.

Possible environmental factors at play

Cancers arise due to genetic errors, some of which occur randomly when cells divide. And the cells in the gut divide constantly: The entire lining of the colon is shed and replaced every week, Shaukat said. The body has mechanisms to repair the myriad DNA mistakes that happen during this turnover, but when the rate of errors increases too quickly or when repair mechanisms fail, that's when most colorectal cancer occurs.

The environment in the gut can affect both this error rate and the efficacy of repair. Researchers have found that there's a complicated feedback loop between the gut microbiome, meaning the community of microorganisms in the colon, and health factors such as diet, exercise and insulin resistance.

Inflammation also factors into this feedback loop. While inflammation is a necessary immune response that repairs damaged tissue with new cell growth, when it becomes chronic, dividing cells become prone to mutation and overproliferation — in other words, cancer.

The culprits for increased chronic inflammation are many, Dr. Theodore Levin, a researcher and clinical lead for colon cancer screening at Kaiser Permanente Northern California, told Live Science. But "these are all thoughts or hypotheses," he cautioned.

One is the rise in sedentary lifestyles and obesity, which contributes to insulin resistance and, in turn, disrupts the microbial colonies that live inside the intestines. These microbes aid digestion and release compounds that can either promote or reduce inflammation.

Childhood antibiotic use rose after the 1960s, which can also perturb the gut microbiome. Additionally, microbiome-disturbing dietary changes since the 1960s include a rise in sugar-sweetened beverages, Levin said, as well as the introduction of a greater number of ultraprocessed foods, which typically lack the fiber content of fresh fruits and vegetables. Fiber, he said, promotes a diverse gut microbiome that tends to tamp down inflammation. (Fermented foods do the same thing, research suggests.)

Researchers are also concerned about the ubiquity of plastics, Levin added.

Some lab research suggests that colorectal cancer cells can take up microplastic particles and that the takeup of those particles increases the cells' propensity to migrate — at least in a lab dish. That could hint that microplastics make cancer more likely to spread through the body. Some animal studies also find that microplastics hamper immune function, which could theoretically help cancer cells to escape the body's defenses.

Plastic particles might also carry carcinogenic compounds deep into the gut, Levin said. Research is still ongoing, but some evidence links chronic exposure to perfluorooctanesulfonic acid (PFOS), a type of "forever chemical" found in nonstick cookware and firefighting foams, to colorectal cancer risk. Experiments using mouse intestinal tissue finds that PFOS reduces a gut enzyme that protects against cancer while spurring the production of proteins that encourage tumor growth.

But researchers have not yet found "one smoking gun" to explain the rise, cautioned Michael Sapienza, the Chief Executive Officer of the nonprofit Colorectal Cancer Alliance. For example, the results of observational studies of forever chemical exposure and colorectal cancer rates are mixed, with some even finding that more PFOS in the blood actually correlates with fewer colorectal cancer cases.

So while the hypotheses are many, the evidence remains murky. Older adults are exposed to all of these environmental and lifestyle changes, too, so researchers also need to understand why their cancer rates aren't rising along with those in younger people. It could be that the removal of precancerous lesions during colonoscopies are masking what would otherwise be a similar rise in the over-65s. Or perhaps it's early childhood exposure that matters most, a risk factor that the oldest adults escaped.

Raising awareness of the symptoms

Scientists aren't entirely sure why cancers of the lower colon and rectum are rising faster than the "right-sided" cancers of the upper intestines.

The lower and upper parts of the colon are very different, Levin said, even arising from different parts of the embryo in the very earliest stage of development. Cancers on the right side and left side often show different genetic mutations. Left-sided colorectal cancer frequently arise from what researchers call chromosomal instability, or mistakes in how chromosomes split between dividing cells during replication, when cells make a copy of their DNA before dividing. Right-sided cancers are more prone to arise from problems with DNA repair during replication.

These differing pathways could help researchers link environmental and lifestyle factors with the changes that lead to colorectal cancer. But that research will take time. For now, gastroenterologists are hoping to raise awareness about the symptoms of the disease and make sure other doctors think to check younger patients for cancer.

Early symptoms include rectal bleeding, unexplained weight loss, anemia, abdominal pain, blood in the stool, changes in bowel habits and unexplained nausea and vomiting. Unfortunately, many early symptoms of colorectal cancer match those of much less serious conditions. For instance, rectal bleeding can be caused by hemorrhoids, and changes in bowel habits could arise from anything from stress to an infection.

Jenna Scott, 40, of Atlanta was 29 and pregnant when her symptoms started. She didn't get a diagnosis until she was 31. By that time, the cancer was metastatic, having spread to other organs. Scott remains in treatment today.

"I don't know if it would have been as aggressive if they had caught it a year-and-a-half earlier," she told Live Science.

Scott's initial symptom was rectal bleeding, but her doctor brushed it off as hemorrhoids caused by pregnancy. Another classic colorectal cancer symptom, weight loss, seemed like typical postpartum shifts on the scale. Health systems and organizations like the Colorectal Cancer Alliance are working to raise awareness among doctors in all specialties so that they check these symptoms out thoroughly in young people rather than explaining them away.

Although routine colon cancer screenings don't start until 45, patients should be pushy with their doctors to get checked out earlier if they have any telltale symptoms, Sapiena said.

"It doesn't mean you have colorectal cancer," Sapiena said of these symptoms. "But it does mean you need to be your own advocate" and seek screening ASAP.