Colon cancer is officially the deadliest cancer for people under 50. Experts are shocked at how quickly this happened.

• Colon cancer is now the leading cause of cancer death in people under 50.
• Experts are surprised at how quickly it replaced lung cancer and breast cancer.
• The rapid rise is under investigation, but diet, environment, and genetics may all play a role.

Colon cancer is officially the deadliest cancer for people under 50 in the US — a shift that happened more rapidly than cancer researchers expected.

New data shows that in 2023, colon cancer eclipsed breast cancer, which had been the leading cause of cancer death in young adults for over a decade. Before that, lung cancer was the big killer.

American Cancer Society epidemiologist Rebecca Siegel, senior author of the new report published in JAMA on January 22, told Business Insider she was "very surprised by the speed of this shift."

Until the mid-1990s, colon cancer was less deadly in people under 50 than brain cancer, leukemia, lung cancer, and breast cancer. It was only in 2021 when the US Preventative Services Task Force lowered the colon cancer screening age recommendation from 50 to 45, realizing that the risks had changed.

The absolute number of colon cancer deaths in working-age people under 50 is still relatively small, with the American Cancer Society estimates suggesting around 3,750 people under 50 died from colon cancer in 2023. But unlike those four other leading causes of cancer death, where Siegel said there's "great news" to report, the trend lines with young colon cancer are going in the wrong direction.

Colon cancer death rates keep going up in people under 50, despite aggressive treatment

Something unique appears to have happened to the cohort of people born after 1950, and it's putting every subsequent generation at a greater risk of developing colon cancer early and fast.

Colon cancer has been the leading cause of cancer death in men under 50 since about 2014, when it beat out lung cancer, but until 2023, breast cancer remained the leading cause of cancer death in people under 50 overall.

Experts had been projecting that colon cancer would likely become the leading cause of cancer death for everyone under 50 years old at some point before 2030, but the pace of the increase surprised even the most seasoned cancer experts.

To go from fifth to first place over the course of about two decades "is not typical in cancer research," Siegel said. It's remarkable it all happened within the lifespan of her young adult daughter, she said.

The reasons for such a rapid shift are complex. While the rate of aggressive colon cancer cases has been going up, doctors have been getting better at treating and preventing other deadly cancers. Plus, lung cancer deaths plummeted, as people quit smoking en masse. Breast cancer is still the leading cause of cancer death in women under 50, when men are left out of the equation, but the number of women dying of breast cancer every year has also gone down steadily since the mid-1990s.

Experts aren't exactly sure why colon cancer death rates are soaring, but point to a likely mix of complex factors influencing our environment, our genetics, and early life exposures during a baby's first months in and out of the womb.

A healthy diet doesn't appear to be enough to stop colon cancer, but regular colonoscopies can prevent it

Prevention isn't as simple as the old standard advice to eat a healthy diet and get enough exercise, though those factors are still part of the equation. The air we breathe, the foods we consume, and the movement we do all play a role, but there seems to also be an element of luck involved. For example, one twin may get colon cancer, while another shows no signs.

"In the end, it's still a chance event," oncologist Tim Cannon previously told Business Insider.

Cannon has treated several young, fit colon cancer patients who've died from aggressive, late-stage forms of the disease.

The steepest rise in young colon cancer patients has been among people just starting adulthood, young patients in their 20s and 30s.

"They get more treatment — and yet they're not living longer," Dr. Kimmie Ng, director of Dana-Farber's Young-Onset Colorectal Cancer Center, previously told Business Insider.

Experts have a few key pieces of advice. Everyone should start getting colonoscopies at age 45. They aren't just screening tools, they are also intestinal housecleaning measures, designed to remove precancerous polyps before cancer develops. Eat plenty of fiber, especially from foods like leafy greens, other vegetables, and whole grains. And if you see blood in the toilet, or have persistent stomach pain for weeks and weeks that will not go away, get it checked out.