‘Strongest predictor of colorectal cancer’ under 50 revealed

Ignore this red flag, and it could come back to bite you in the rear.

A new study has identified the “strongest predictor of colorectal cancer” in adults under 50 — increasing the odds of a diagnosis by a staggering 850%.

“This research lends support to the question of who does or doesn’t warrant a colonoscopy,” Dr. Sandra Kavalukas, a colorectal surgeon at the University of Louisville School of Medicine and lead author of the study, said in a statement.

Generally, Americans without a family history of the disease are advised to start getting screened at age 45. But experts warn that this guidance leaves many younger adults — who are experiencing the fastest rise in colorectal cancer rates — dangerously exposed.

“Many of the early-onset colorectal cancers that I see have no family history,” said Kavalukas, who hopes this research will help doctors identify which young symptomatic patients could benefit from earlier colonoscopies.

In the study, researchers combed through the records of 443 patients under 50 who got colonoscopies between 2021 and 2023 at University of Louisville Health.

Nearly half of them were diagnosed with early-onset colorectal cancer, with rectal bleeding standing out as the biggest warning sign.

“If they’re 35 and they come in with rectal pain, they probably don’t need a colonoscopy,” Kavalukas said. “But if they come in with a bleeding complaint, they are 8.5 times more likely to have a colorectal cancer.” 

Notably, 70% of the young cancer patients had no family history of the disease, and just 13% carried the genetic markers often linked to hereditary cancers.

While family history did increase risk, researchers found it was associated with only a two-fold rise in the odds of diagnosis.

The study also revealed that former smokers faced nearly twice the risk of early-onset colorectal cancer compared to non-smokers.

And when it came to getting a colonoscopy, symptoms were the big driver: a whopping 88% of cancer patients had the procedure because of issues like rectal bleeding — compared to just over half of those without cancer.

The findings were presented Friday at the American College of Surgeons Clinical Congress 2025.

The study comes amid a growing health crisis: colorectal cancer is on the rise in young Americans, with 1 in 5 diagnoses now in people under 55, per the Cancer Research Institute.

This troubling trend has been building since the ’90s — even as cases among older adults have declined.

The exact reasons behind the rise in early-onset colorectal cancer remain unclear, but research suggests modern lifestyle factors — such as sedentary habits, obesity and heavy consumption of processed foods — are contributing to the problem.

Oncologists also suspect that environmental pollutants in the air, soil and water may be stoking the flames.

Alarmingly, younger patients are often diagnosed at advanced stages because their symptoms are overlooked or mistaken for less serious health concerns.

As a result, colorectal cancer has become the leading cause of cancer death among American men under 50 and the second deadliest for women in the same age group.

In 2023 alone, 3,750 young adults died from the disease, while nearly 20,000 were diagnosed, according to the American Cancer Society.

“If you have a person below the screening age with rectal bleeding, you should seriously consider a colonoscopy,” Kavalukas stressed to doctors. 

While blood and stool tests can help screen for colorectal cancer, colonoscopies remain the only way to prevent it — allowing doctors to find and remove precancerous polyps before they turn deadly.