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Why Has Rectal Cancer Quadrupled in Millennials?

The risk is still small, but that's a notable increase.
Image: Lawrence K. Ho / Getty

Cancers of the colon and rectum are on the rise in young people. Though the number of Americans over 55 diagnosed with the diseases has steadily dropped, Millennials now have double the risk of developing colon cancer and quadruple the risk of rectal cancer as someone born in 1950 at the same age, with rates as high as they were in 1890.

These were the findings in a new study, published Tuesday in the Journal of the National Cancer Institute. They looked at a well-known data set of nearly 500,000 patients 20 and older diagnosed with colorectal cancer between 1974 and 2013, and divvied them up by what year they were born and their age at diagnosis. They found that from the mid-80s to 2013, the incidence of colon cancer has been rising fastest for people between the ages of 20 and 29, by 2.4 percent per year (it also rose for people ages 30-39 in that same time frame, and, starting in the mid-90s, for people in their 40s, too). The increase was even more dramatic for rectal cancers, increasing by about 4 percent per year for adults in their 20s since since 1974.


It's important to note that, overall, the incidence of colorectal cancers has been declining in the US. H. Gilbert Welch, a professor of medicine at the Dartmouth Institute, told CNN that the numbers are actually much smaller than they appear—the increase translates to 1 or 2 more cases per 100,000 people per year, far fewer than the drop in older Americans, which has been about 100 cases per 100,000. Plus, Welch added, the uptick in diagnosis hasn't translated to more deaths of people in this age group.

The concern is that younger people might get diagnosed with later-stage, and more dangerous, disease because they aren't getting screened. And as a group, they carry that elevated risk forward as they age. One 2015 analysis predicted that one in 10 cases of colon cancer and one in four cases of rectal cancer are diagnosed in people under 50 by 2030; if that comes to pass, that might have troubling implications for when those young people get into their 50s, when those cancers are more common and deadly.

"It appears that under the surface, the underlying risk for this disease is actually increasing in the population," Rebecca Siegel, an epidemiologist at the American Cancer Society and the lead author of the paper, told NPR.

The findings don't point to any specific causes, but the researchers have a few ideas. "It is not surprising that the timing of the obesity epidemic parallels the rise in colorectal cancers because many behaviors thought to drive weight gain, such as unhealthy dietary patterns and sedentary lifestyles, independently increase colorectal cancer risk," the study authors write. Changes to the American diet, plus the fact that people have more of these risk factors earlier in life, might also make the body undergo genetic changes that can make cancer more likely in the future, they add.

But factors related to obesity probably aren't the only ones. It will take more research to figure out what is really going on, experts note. They urge younger people who notice possible symptoms, such as rectal bleeding or changes in bowel habits that last more than a few days, to talk to their doctors about getting screened.

The researchers stress the need for more research, so that public health officials can consider dropping the age at which people should start regularly screening for colorectal cancers.

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