Deaths from rectal cancer are rising rapidly among younger adults, an alarming trend that is confounding scientists trying to understand why millennials are so hard-hit.

“The rate of rectal cancer seems to be increasing more than two to three times compared to colon cancer,” said Mythili Menon Pathiyil, lead author of a new study and a gastroenterology fellow at SUNY Upstate Medical University in Syracuse, New York.

If the trend continues, rectal cancer deaths will exceed the number of colon cancer deaths — already the nation’s No. 1 cause of cancer death in people under age 50 — by 2035.

According to the American Cancer Society, 158,850 new colorectal cancers will be diagnosed in 2026. About 55,230 patients will die from the disease, with nearly a third of those deaths in people under age 65. Colon cancer and rectal cancer are similar but form in different parts of the digestive tract.

The new research, which hasn’t yet been published in a peer-reviewed journal, is scheduled to be presented at a national meeting of gastroenterologists in May.

The findings, however, strengthen an American Cancer Society study released in March showing that a rise in rectal cancer rates is driving increases in colorectal cancer diagnoses in people younger than age 65. Colorectal cancer rates have been increasing 3% each year for adults under age 50 since the late 1990s and scientists are scrambling to understand why.

“This is a medical crisis,” Dr. Ben Schlechter, a gastrointestinal medical oncologist at Dana-Farber Cancer Institute in Boston, said. “This is not something that should be ignored.” Schlechter was not involved with the new study.

Pathiyil’s research used Centers for Disease Control and Prevention death records from 1999 through 2023, specifically for people ages 20 to 44. Deaths from early-onset colorectal cancer overall rose throughout that time period, with the rate of rectal cancer deaths accelerating the most: up to three times faster.

Hispanic adults saw the steepest rise in rectal cancer deaths, according to the study, and had the fastest-growing mortality rates of any demographic group.

The researchers didn’t identify why the tumors were increasing or whether they are becoming more aggressive.

“If we could ‘Magic School Bus’ our way into the tumor, we could see what’s going on inside those cells,” said Andreana Holowatyj, an assistant professor of hematology and oncology at Vanderbilt University Medical Center. “This stresses the need to understand some of those core characteristics so that we can not only reverse these trends, but also improve treatment.” Holowatyj was not involved with the new research.

Standard treatments for rectal cancer include not only surgery, but radiation and chemotherapy to the pelvic region. All might affect a patient’s bladder, bowel and sexual functioning.

Why are rectal cancer rates rising among millennials?

There is no single answer yet. Scientists are working to find common links among younger adults who develop rectal cancer as they reach their 30s and 40s.

It seems the bulk of such cancers leading the surge in cases today are not hereditary. An exposure to increase the risk likely occurred in the 1960s or 1970s, Schlechter said.

“It’s not like there’s a new gene in the population,” he said. “This is something modern.”

It’s possible that portions of the colon’s microbiome were disrupted at some stage in life, possibly by consuming a lot of sugar-sweetened beverages as children, Schlechter said.

At the same time, many younger patients with rectal cancer have “zero risk factors,” he said. “That’s really troubling.”

Rectal cancer symptoms that shouldn’t be ignored

Colon cancer symptoms can be vague, including fatigue, abdominal pain, bloating and unexplained weight loss. Rectal cancers are often accompanied by bright red blood on toilet paper or in the toilet bowl and a real urgency to use the bathroom even after a bowel movement.

Any noticeable differences in bowel function should be checked out, Schlechter said. “If things are different, you should really see a doctor and ask explicitly, ‘Could this be colon cancer? I hear it’s happening in young people.’”

The U.S. Preventive Services Task Force recommends colon cancer screening for the general population starting at age 45. Most often, that’s a colonoscopy. That recommendation should go out the window for people younger than 45, experts say, if a person has a family history of the disease.

That leaves younger adults without a family history but who are experiencing symptoms without a clear path to a diagnosis. Pathiyil said there can be a seven-month gap in the time when people develop symptoms and when they’re finally diagnosed.

“That can make a difference in outcomes,” she said. “Colorectal cancer is curable when detected early. But 75% of individuals less than the age of 50 are diagnosed in an advanced stage. That in itself could explain why we are seeing rising mortality.”