The study found after decreasing since 1974, colon cancer incidence rates increased by 1% to 2% per year from the mid-1980s in adults aged 20 to 39.
For starters, he said, "Although relative rates are rising in younger people, the absolute risk is still low in the younger population".
"Some of the behaviors that are thought to drive the obesity epidemic, like a more sedentary lifestyle and unhealthy dietary patterns, are also independent risk factors for colorectal cancer", Siegel told KCBS.
Both colon and rectal cancer incidence rates in 50 to 54 year olds were half those of 55 to 59 year olds in the early 1990s.
The authors of the study said more research was needed to explain why colorectal cancer rates were rising in younger people, but noted a correlation with higher rates of obesity in the US.
About 135,000 Americans are diagnosed annually with the cancers (and 50,000 die each year, reports USA Today), but those 55 and younger now make up an appreciable number of diagnoses: 29% of rectal cancer and 17% of colon cancer cases.
Recommendations by the American Cancer Society and other expert groups generally have called for colonoscopies or other tests starting at age 50 for a person with average risk but earlier screening for individuals with a family history of cancer.More news: Mavericks get Noel from 76ers for Bogut, Anderson
The study, which appears in the Journal of the National Cancer Institute, found that colon cancer incidence rates increased by one to two per cent a year from the mid-1980s through 2013 in adults aged 20 to 39.
For rectal cancer, the changes in the rates were more pronounced, the study said.
The study, which included scientists at the NCI, didn't determine the reason for the shift.
Rates of colon and rectal cancer in millennials are rising sharply in the United States, according to new research, prompting concern that poor diets and sedentary lifestyles are contributing to a resurgence of the deadly disease. But among younger adults, rectal cancer rates have been increasing even faster than colon cancer rates. There were 490,305 cases. They conducted a retrospective study of all patients 20 years and older diagnosed with invasive CRC from 1974 through 2013 in the nine oldest Surveillance, Epidemiology, and End Results (SEER) program registries.
Many young patients have no obvious risks, Weber said, so "we suspect there may be additional factors at play".
In the meantime, as they say, an ounce of prevention is worth a pound of cure - so building healthy habits while you're still young isn't just helpful to stave off disease now, but guard yourself against longterm risk for a wide variety of health conditions, including cancer. In adults 40 to 54, rates increased by 0.5% to 1% per year from the mid-1990s through 2013. Rectal cancer rates in adults 55 and older, however, have been dropping for at least 40 years, Siegel said.