Colorectal cancer is now the leading cause of cancer death in Americans under 50, and doctors still cannot fully explain why. A new study published in Nature Medicine does not answer why cases are rising, but it offers a concrete clue to why younger patients' tumors may be harder to treat.

By reading gene activity in individual cells from 189 colorectal tumors, researchers found that cancers in younger patients contained a higher share of connective tissue cells relative to cancer cells. Those surrounding cells may help shield tumors from the immune system and from chemotherapy.

For younger adults, the practical message is less about this one study and more about awareness. Routine screening starts at 45, and symptoms in younger people are often dismissed. Knowing the warning signs remains one of the most useful steps anyone can take.

Inside 189 Tumors, Cell by Cell

The research team, including scientists at Harvard Medical School, Brigham and Women's Hospital, the Dana-Farber Cancer Institute and the Broad Institute, used a method called single-nucleus RNA sequencing. It reads which genes are active in individual cells and works on frozen, banked tumor tissue.

The tumors came from patients ranging from age 29 to over 90, including 54 people diagnosed before 50 and 135 diagnosed later, Live Science reported. That range let the researchers look for gradual changes across age rather than comparing only two groups.

"The clinical data suggests that the disease in the younger patients is more aggressive overall," said co-author Ana Anderson, a professor of neurology at Harvard Medical School and a core faculty member at the Gene Lay Institute of Immunology and Inflammation at Brigham and Women's Hospital. "The patients, being very healthy otherwise, do not do well with treatment."

Connective Tissue as a Possible Shield

The central finding involves the tumor's surroundings — not the cancer cells alone. In younger patients, tumors held a higher proportion of connective tissue cells than malignant cells. In older patients, the pattern reversed.

That difference may matter in two ways. Connective tissue cells can release compounds that suppress the immune system, which could help tumors avoid immune attack. Dense connective tissue can also act as a physical barrier, potentially keeping chemotherapy drugs from reaching cancer cells. Either effect could help explain why younger patients, who are often otherwise healthy, may respond poorly to standard treatment.

The researchers also found that cancer cells in younger patients showed more gene activity tied to cell-to-cell communication. Similar signals have been linked to cell migration, aggressive tumors and metastasis in other cancers. The tumors also contained many long noncoding RNAs, genetic molecules that do not make proteins and whose role here is unknown.

"It's a new avenue for research into this disease that we previously had no idea about," Anderson said.

Clues, Not Causes

The study's limits are important. It describes differences in tumor makeup but does not prove those differences cause worse outcomes, and it does not explain why colorectal cancer is becoming more common in younger people. The sample, while large for this type of analysis, came from banked tissue and may not represent every population. The function of the long noncoding RNAs is still being tested in laboratory experiments.

The findings could still guide treatment research. If connective tissue helps protect tumors in younger patients, drugs that target that tissue or the signals it sends might one day be combined with chemotherapy or immunotherapy. That idea would need to be tested in clinical trials before it could help patients.

The rise itself is well documented. Colorectal cancer rates in people under 50 have climbed since the mid-1990s even as they fall in older adults, who benefit from routine screening. Cases are rising in children and teens as well, though the disease remains rare in young people. In CDC data, rates among 10- to 14-year-olds rose from about 1 to 6 per million between 1999 and 2020, Live Science reported. Researchers are studying possible contributors including the gut microbiome, diet, antibiotics, and inactivity, but no single cause has been confirmed.

Warning Signs and Screening Age

The U.S. Preventive Services Task Force and the American Cancer Society recommend that adults at average risk begin colorectal cancer screening at 45. People with a parent, sibling or child who had colorectal cancer or advanced polyps, or with inflammatory bowel disease, may need to start earlier and should ask a doctor.

Symptoms deserve attention at any age. According to the American College of Surgeons, warning signs include persistent changes in bowel habits, recurring rectal bleeding, dark or narrow stools, unexplained weight loss, and fatigue. Rectal bleeding was the strongest predictor of a diagnosis in the research the group highlighted. Ongoing abdominal pain and iron-deficiency anemia are also red flags.

These symptoms often have other, less serious causes — such as hemorrhoids. That is exactly why they are easy to dismiss. Anyone with symptoms that last more than a few weeks or keep coming back should see a health care provider and mention family history. Screening tests range from colonoscopy to at-home stool tests, and a doctor can help choose the right option.

The bottom line: a cell-level atlas of 189 tumors suggests younger patients' colorectal cancers may be surrounded by more protective connective tissue, which could help explain resistance to treatment. It is a lead for new therapies, not an explanation for rising cases, and paying attention to symptoms remains essential.

What Readers Want to Know

What did the study find? Tumors from younger colorectal cancer patients had more connective tissue cells relative to cancer cells, which may help shield them from the immune system and chemotherapy.

Does this explain why colorectal cancer is rising in young adults? No. It offers a possible reason younger patients' tumors resist treatment, not a cause for rising cases.

How many tumors were studied? 189 tumors from patients aged 29 to over 90, including 54 diagnosed before age 50.

When should colorectal cancer screening start? At age 45 for people at average risk, or earlier for those with a strong family history or certain conditions.

What symptoms should younger adults watch for? Rectal bleeding, persistent bowel changes, dark or narrow stools, unexplained weight loss, fatigue, ongoing abdominal pain, and anemia.

What should I do if I have symptoms? See a health care provider if symptoms last more than a few weeks or keep returning, and share any family history of colorectal cancer.