Rates of colorectal cancer, which overall have been declining for decades in the United States, are instead rising sharply among young and middle-aged adults, according to a new study that startled researchers and is sparking questions about whether screening should start earlier than age 50.
The study, published recently in the Journal of the National Cancer Institute, found that between the mid-1980s and 2013, colon cancer rates increased about 1 to 2 percent per year for people in their 20s and 30s. Rates for middle-aged adults also rose, but at a slower pace.
Rectal cancer rates climbed even faster in recent decades, at about 3 percent per year for people in their 20s and 30s and 2 percent annually for those ages 40 to 54. As a result, three in 10 new cases of rectal cancer now are diagnosed in patients younger than 55 — double the proportion in 1990. By contrast, rectal cancer rates in adults ages 55 and older have dropped for four decades.
American Cancer Society researcher Rebecca Siegel, who led the study, said that earlier work had signaled a growing incidence of colorectal cancer among the groups known as Gen X and millennials. But the magnitude of the increase identified “was just very shocking,” she said, as was the finding that the underlying risk of colorectal cancer appears to be rising.
The study, which included scientists at the NCI, didn’t determine the reason for the shift. But Siegel suggested one explanation might be a complex interaction involving the same factors that have contributed to the obesity epidemic — changes in diet, a sedentary lifestyle, excess weight and low fiber consumption.
Colorectal cancer refers to malignancies in the colon or rectum, which are parts of the large intestine. Most cancers there start as polyps, or growths, on an inner wall. Most polyps are benign, but over time some can develop into cancer.
The American Cancer Society estimates that more than 95,000 new cases of colon cancer and almost 40,000 new cases of rectal cancer will be diagnosed in 2017. About 50,000 people are expected to die of colorectal cancer in the United States this year.
The latest research involved a retrospective look at more than 490,000 people ages 20 and older who received diagnoses of invasive colorectal cancer between 1974 and 2013, with a specific focus on years of birth and five-year age groups.
By comparing different generations at similar ages, it found that people born around 1990 have double the risk of colorectal cancer and quadruple the risk of rectal cancer of people born around 1950.