Colorectal cancer is rising in adults under 50 and is now their leading cancer killer; warning signs and risk factors younger people should know

For decades, colorectal cancer was largely viewed as a disease of later life. But that picture is changing in a way cancer researchers can no longer ignore. In the United States, colorectal cancer is now the leading cause of cancer death among people younger than 50, according to American Cancer Society researchers. At the same time, the disease is becoming less common among older adults, creating a striking divide by age.

The numbers are particularly concerning among people in their 20s, 30s and 40s. Between 2013 and 2022, colorectal cancer incidence increased by about 3% a year among adults aged 20 to 49, while rates among people 65 and older continued to fall. And researchers still cannot point to one clear reason.

A cancer once associated with ageing is changing its age profile

Colorectal cancer refers to cancers that begin in the colon or rectum. It can develop from abnormal growths called polyps, some of which can become cancerous over time.

The overall picture in the US has actually improved: screening and better prevention have helped drive colorectal cancer rates down among older adults. Yet those gains have not been shared equally across generations.

The American Cancer Society estimates that 45% of new colorectal cancer diagnoses now occur in people younger than 65, compared with 27% in 1995. Among people under 50, roughly three-quarters of cancers are diagnosed at an advanced stage, including about 27% that have already spread to distant parts of the body.

That does not mean colorectal cancer is suddenly common among young adults. It remains far more frequent with increasing age. But the direction of the trend is what has alarmed researchers.

Why is this happening? There is no single answer

Stanford Medicine gastrointestinal oncologist Shruti Patel told the university's publication that researchers do not yet have a single culprit behind the increase.

Known risk factors include obesity, physical inactivity, smoking, alcohol consumption and diets high in red and processed meat. Diets containing more vegetables, fruit and fibre and regular physical activity are associated with lower risk. But these factors do not fully explain the rapid change in younger generations.

Genes are another piece of the puzzle, particularly inherited conditions such as Lynch syndrome and familial adenomatous polyposis. Yet most younger patients do not have a known hereditary explanation for their cancer.

Scientists are therefore investigating whether changes in the environment and lifestyle over recent generations could be contributing. Researchers are studying everything from metabolic health and diet to the gut microbiome and other environmental exposures.

For now, however, several proposed explanations remain under investigation rather than established causes.

The warning signs can look deceptively ordinary

One reason early-onset colorectal cancer can be difficult to catch is that its symptoms can resemble everyday digestive problems.

A large systematic review and meta-analysis published in JAMA Network Open examined 81 studies involving more than 24.9 million people under 50. Among younger patients who developed colorectal cancer, the most common warning signs included blood in the stool, abdominal pain and changes in bowel habits.

The researchers found that delays were common: the median time from symptoms appearing to diagnosis was about four months, while the mean was roughly six months.

That matters because younger adults may not immediately associate these symptoms with colorectal cancer, while clinicians also need to consider early-onset disease when such warning signs appear.

The US National Cancer Institute similarly notes that younger adults may overlook warning signs because colorectal cancer is not what they expect at their age.

Screening starts at 45, but age isn't the whole story

For people at average risk in the US, routine colorectal cancer screening now begins at 45, rather than 50. The change was made because of the rise in younger-onset disease.

But there is an important limitation: half of colorectal cancer diagnoses occurring before 50 are in people younger than 45, according to the American Cancer Society. That means screening alone cannot explain or solve the problem.

People with a strong family history or certain inherited conditions may need screening earlier and should discuss their individual risk with a clinician. And anyone experiencing concerning symptoms should not simply wait until reaching the routine screening age.

The bigger mystery is still unsolved

The rise in younger adults is happening alongside major improvements in colorectal cancer prevention and treatment among older people.

That contrast has left researchers with an unusually important question: what changed for the generations born more recently?

Scientists suspect that multiple factors may be interacting rather than one new cause suddenly appearing. But until research identifies which exposures matter, and how they might interact with genetics, metabolism, the gut microbiome and other biological processes, the reason for the generational shift will remain unresolved.

For now, the most practical message is simpler than the mystery itself: colorectal cancer is no longer a disease that can be dismissed as something that happens only much later in life. Persistent changes in bowel habits, blood in the stool, unexplained abdominal pain, anemia, fatigue or unexplained weight loss deserve medical attention, regardless of age.