No one knows what underlying lifestyle, environmental or genetic factors may be driving the rise in cases.

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A sobering new study has found that younger Americans aren’t just getting colon-cancer diagnoses earlier. They are dying of colorectal cancer at slightly higher rates than in previous decades, and no one knows why.

“This is real,” said Rebecca Siegel, an epidemiologist with the American Cancer Society and the lead author of the study, published as a research letter in The Journal of the American Medical Associated (JAMA). “It’s a small increase, and it is a trend that emerged only in the past decade, but I don’t think it’s a blip. The burden of disease is shifting to younger people.”

The study found that even though the risk of dying from colon and rectal cancers has been declining in the population overall, death rates among adults ages 20 to 54 had increased slightly, to 4.3 deaths per 100,000 people in 2014, up from 3.9 per 100,000 in 2004.

“This is not merely a phenomenon of picking up more small cancers,” said Dr. Thomas Weber, who was not involved in the study but is a member of the steering committee of the National Colorectal Cancer Roundtable. “There is something else going on that’s truly important.”

No one knows what underlying lifestyle, environmental or genetic factors may be driving the rise in cases.

While rates of cancers tied to human papillomavirus, or HPV, have been rising in recent years, that virus causes cancers mainly of the cervix, back of the throat and anus, and scientists do not believe sexual behaviors or HPV are driving the increase in colon or rectal cancer (anal and rectal cancers are distinct).

Obesity, a diet high in red or processed meats and lack of physical activity are among the factors tied to increased risk, but new research is looking at other possible causes. One recent study found, for example, that prolonged use of antibiotics during adulthood was associated with a greater risk of developing precancerous polyps, possibly because antibiotics can alter the makeup of the gut microbiome.

Scientists are also exploring whether the colorectal cancers emerging in younger adults are different from those seen in older people — and whether they can be detected and treated with the same tools. There is some evidence that young people are more likely to have precancerous polyps that are harder to see and remove during a colonoscopy because of their location in the colon or because they are flat rather than tubular, according to Dr. Otis Brawley, chief medical officer for the American Cancer Society.

The findings add to the urgency to find reliable ways to detect colorectal cancer early in young people. Most medical groups have for years recommended people start routine screening at age 50 unless they have specific risk factors, such as a family history of the disease or chronic conditions like inflammatory bowel disease that raise the risk.

Warning signs of colorectal cancer include rectal bleeding, bloody stools, unexplained weight loss, fatigue and digestive complaints, or persistent changes in bathroom behavior.

Anemia in men is also a warning sign and should be explored further, and while many doctors typically attribute anemia in a premenopausal woman to menstruation, experts say that if a woman is experiencing any other symptoms, doctors should assess her for colon cancer.