Colorectal cancer is rising again after 20 years, and scientists don't know why

Colorectal cancer is already the second leading cause of cancer deaths in the United States and the leading cause among adults younger than 50. Although rising cases in younger people have attracted widespread attention, the latest findings show that the problem extends into older generations. Researchers warn that a continued increase could place additional pressure on the healthcare system as more patients require cancer treatment and long-term care after diagnosis.

Colorectal Cancer Rates Reverse Decades of Decline

To investigate the changing pattern, Caitlin Murphy, PhD, MPH, Professor of Pediatrics and Public Health Sciences at the University of Chicago, and her colleagues examined the latest colorectal cancer incidence data from the National Cancer Institute. Their analysis tracked changes in diagnoses from 2000 through 2023.

The results revealed a significant reversal. In 2000, the age-adjusted colorectal cancer incidence rate was 55.2 cases per 100,000 people. By 2022, that figure had dropped to 36.7. However, the rate increased to 38.3 in 2023, marking the first overall rise after more than two decades of falling rates.

"This is the first time that we have seen an increase in the overall rate since in more than 20 years," Murphy said. She emphasized that the long-standing downward trend had been a major feature of colorectal cancer statistics for decades. The new analysis indicates that rates are now increasing in every age group through ages 65 to 69.

Murphy, an epidemiologist who specializes in colorectal cancer patterns, had anticipated that the increase initially observed in younger adults would eventually become apparent among older people. She attributes this expected progression to a phenomenon called the "birth cohort effect."

This effect describes how people born during the same general period can share risk patterns that become more apparent as they grow older. In this case, individuals born in the 1960s and subsequent decades appear to face an elevated risk of colorectal cancer as they enter middle age and later life.

"I think of a birth cohort effect as the interaction of age and time," Murphy explained. "As people born around the same time move through time and age together, we're going to see more colorectal cancers being diagnosed."

A Sharp Increase Among Adults Ages 45 to 49

One of the most striking findings involved people between 45 and 49 years old. From 2019 through 2023, colorectal cancer incidence in this group increased by 12.76% annually, the largest rise recorded among the age groups examined.

Researchers believe much of this particular increase can be explained by changes in cancer screening recommendations. National guidelines lowered the recommended starting age for routine colorectal cancer screening from 50 to 45, allowing more people to be tested and more previously undetected cancers to be identified.

"That means that screening is doing as we intend it to do -- it's catching cancer earlier," Murphy said.

But increased screening cannot account for the broader pattern. Colorectal cancer rates are also rising among adults ages 65 to 69, while the decline has stalled among those ages 70 to 74. Both populations have historically experienced substantial benefits from screening, and approximately 80% of people in these age groups are current with recommended testing.

These findings are particularly concerning because they suggest that additional, unidentified risk factors could be counteracting some of the progress achieved through decades of cancer screening.

The research team also investigated whether disruptions during the COVID-19 pandemic might have distorted the results. Cancer screenings and diagnoses were affected during the pandemic, potentially changing when cases were detected. However, the upward trend remained even after the researchers removed data from 2020 and 2021. Screening activity had also largely recovered to its earlier levels by January 2021, making pandemic-related disruptions an unlikely explanation for the increase recorded in 2023.

"These results motivate us to understand reasons for this rise, so risk factors can be identified and possibly reduced," said Sonia Kupfer, MD, director of the Gastrointestinal Cancer Risk and Prevention Clinic at UChicago Medicine.

Why Colorectal Cancer Is Rising Across Generations

The growing number of colorectal cancer diagnoses among younger Americans has become a major health concern. American Cancer Society data show that incidence among adults ages 20 to 49 increased by approximately 3% per year between 2013 and 2022.

This increase in colorectal cancer among people younger than 50, often referred to as early-onset colorectal cancer, has drawn considerable public and scientific attention. The latest study suggests that understanding why younger adults are developing the disease more frequently could also help explain what is happening in older populations.

"There are many unique challenges when you're diagnosed with cancer at a young age, so it's important that we shine a spotlight on that problem," Murphy said. "But in terms of understanding the root cause, I think our study shows that we can consider that whatever might be causing colorectal cancer in younger people is now also causing it to increase in older people as well."

The underlying causes remain uncertain, but researchers are investigating whether exposure to chemicals in the environment could play a role.

Murphy and her colleagues are working with Xin Hu, PhD, a toxicologist at Emory University, to examine this possibility. Their approach involves analyzing small blood samples for evidence of thousands of environmental chemical exposures simultaneously. By identifying which substances are associated with potential risks, the team hopes to narrow the list of chemicals that deserve more detailed investigation.

The researchers then plan to collaborate with Kupfer to explore how these chemicals affect colon tissue. To do so, they will use organoids, laboratory-grown models that reproduce important features of the colon. These miniature tissue systems allow scientists to study biological responses in a controlled environment and could help uncover how certain exposures contribute to colorectal cancer.

Why Colorectal Cancer Screening Remains Critical

With cancer rates increasing across a broad range of ages, the researchers emphasize the need to keep tracking national cancer statistics as new registry information becomes available each year.

Murphy cautions that the consequences could become substantial if the current pattern continues. As large numbers of Americans born in more recent decades grow older, rising incidence rates could translate into many additional cancer diagnoses. That would increase demand for medical treatment and services supporting people who survive colorectal cancer.

While scientists work to understand the factors behind the increase, screening remains one of the most effective ways to find colorectal cancer early and reduce the risk of dying from the disease.

Current national recommendations advise adults at average risk to begin regular colorectal cancer screening at age 45 and continue through age 75. For people younger than 45 or older than 75, screening decisions should be tailored to individual circumstances, including family medical history, symptoms, and other health conditions.

Colonoscopy continues to be considered the gold standard for colorectal cancer screening, but patients also have additional testing options. These include newer blood-based screening tests and stool tests that can be completed at home.

"There are several ways to do screening that should be discussed with a clinician, so people know what the different screening tests are intended for and how often these need to be repeated," Kupfer added.

She also stressed that people younger than 45 should not ignore possible warning signs simply because they have not reached the age recommended for routine screening. Those without known genetic or family history risk factors should still seek medical attention if they notice blood in their stool, persistent changes in bowel habits, or unexplained weight loss.

Study Funding and Future Cancer Research

The research, titled "Colorectal cancer among U.S. adults, 2000-2023," received financial support from the National Institutes of Health and the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research Ablon Scholars Program.

Along with Murphy, the study's authors include Folasade P. May of the University of California Los Angeles, Xin Hu of Emory University in Atlanta, and Samir Gupta of the University of California San Diego.

The University of Chicago's medical and biological sciences programs are also continuing to expand their work in cancer research and patient care. In April 2027, UChicago Medicine is scheduled to open the AbbVie Foundation Cancer Pavilion, which will be Chicago's first freestanding cancer pavilion. The facility is intended to bring together advanced diagnostic services, new cancer treatments, research that translates laboratory discoveries into clinical care, and comprehensive support for patients and the surrounding community.