"These are people in the most productive periods of their lives, and they are often diagnosed much later, so there is a substantial risk of mortality".
"Our finding that colorectal cancer risk among millennials has escalated back to the level of those born in the late 1800s is very sobering", Siegel said.
"It is not surprising that the timing of the obesity epidemic parallels the rise in colorectal cancer because many behaviours thought to drive weight gain, such as unhealthy dietary patterns and sedentary lifestyles independently increase colorectal cancer risk", the authors wrote. In adults ages 40 to 54, rectal cancer rates increased by 2% per year from the 1990s to 2013. The findings come as rates of these cancers have continued to decline in adults ages 55 and up, according to the study.
"Trends in young age groups are a bellwether for future disease burden", she said. In contrast, rectal cancer rates in adults age 55 and older have generally been declining for at least 40 years, well before widespread screening.
"If they are particularly concerned about their individual risk of colorectal cancer, for example, if they have a family history of the disease, they should talk to their doctor about whether to start screening earlier", he said.
The trend was even worse for rectal cancers, showing that younger adults were about four times as likely to be diagnosed with those cancers versus older adults, the team found.
The results showed that after decreasing since 1974, colon cancer incidence rates increased by 1 to 2 percent per year from the mid-1980s through 2013 in adults ages 20 to 39.
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Ms Siegel added: "Educational campaigns are needed to alert clinicians and the general public about this increase to help reduce delays in diagnosis - which are so prevalent in young people - but also to encourage healthier eating and more active lifestyles to try to reverse this trend".
Rates of colon and rectal cancer in millennials are rising sharply in the U.S., according to new research, prompting concern that poor diets and sedentary lifestyles are contributing to a resurgence of the deadly disease.
Many young patients have no obvious risks, Weber said, so "we suspect there may be additional factors at play". The data included all cases of colon and rectal cancer that occurred from 1974 to 2013 in nine regions of the United States in adults ages 20 and older.
The researchers found that in the late 1970s and early 1980s, the rates of colon cancer among adults ages 50 and up increased, and the rates among adults under age 50 decreased.
"But it appears that under the surface, the underlying risk for colorectal cancer is rising, and it is rising pretty quickly among young adults".
"If you're young or even if you're in your early 50s, if you have symptoms consistent with colorectal cancer like blood in the stool, abdominal cramps, changes in bowel patterns, go to your doctor and insist that they follow up", she said.
Rates for adults older than 55 has been declining for about 40 years, researchers said.