Colorectal cancer rates are rising sharply in people under 40, and doctors are baffled and worried

The rates of colorectal cancer have been steadily dropping for people born before 1950, but a sharp rise in colon and especially rectal cancer in people in their 20s and 30s has doctors worried and flummoxed. On Tuesday, researchers with the American Cancer Society published a study in the Journal of the National Cancer Institute estimating that Americans under 50 will be hit with 13,500 new cases of colon and rectal cancers this year, a growing percentage of the some 95,000 colon cancer and 40,000 rectal cancer diagnoses among all ages.

"People born in 1990, like my son, have double the risk of colon cancer and quadruple the risk of rectal cancer" as someone born in 1950 at the same age, epidemiologist Rebecca Siegel, the lead author of the study, tells The New York Times. Worse, "they carry the risk forward with them as they age." The analysis is the largest and most detailed to date of colorectal cancer incidence, and it found a 1-2.4 percent increase in colon cancer rates among people 20 to 39 every year since the mid-1980s, versus a 0.5-1.3 percent annual increase in adults 40 to 54 and a decline among people 55 and older. The rates for rectal cancer are worse, rising by 3.2 percent a year for Americans in their 20s from 1974 to 2013.

Colorectal cancer is hard to diagnose from external factors — the symptoms, when they are present, include things like prolonged diarrhea or constipation, abdominal pain, bloody stools, or other digestive ailments. Colonoscopies aren't encouraged (or generally covered by health insurance) until age 50, and less invasive or cheaper tests are still not on par. Doctors have some theories about why colorectal cancer cases are rising sharply in younger people — risk factors including obesity, sedentary lifestyle, heavy alcohol use, and certain chronic illnesses that are on the rise — but "the honest truth is nobody knows 100 percent why there is an increase," said Dr. Mohamed E. Salem at Lombardi Comprehensive Cancer Center at Georgetown University, who says 60 percent of his patients are younger than him, and he's 42. You can read more about the worrisome mystery at The New York Times, or learn more in the CBS News report below. Peter Weber

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