Previous studies picked up increases in young adults, but the new study provided new age-group details from almost 500,000 cases reported between 1974 and 2013. Colon cancer is the third most diagnosed cancer in men and women in the US, according to the American Cancer Society, and screenings like colonoscopies are part of the reason why so many cases can be diagnosed - often when the cancer is still treatable.
"Trends in young people are a bellwether for the future disease burden", said Siegel in a statement. Experts say colon and rectal cancer can be inherited and can also be influenced by the high-fat, low fiber diets that are common in the Western world.
Rectal cancer rates in adults in their 30s increased at the same pace - 3.2 percent each year - starting in 1980, the researchers found.
They analysed 490,305 cases among patients aged 20 and older across the USA who were diagnosed with invasive bowel cancer from 1974 through 2013.
While cases of colorectal cancer are declining in older adults, a dramatic surge in cases is occurring among Millennials and Generation Xers.
Recently though, studies have reported increasing colorectal cancer incidence in adults under age 50, for whom screening is not generally recommended.
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.
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The study did not uncover a reason for the change.
In the study, the researchers looked at data from the Surveillance, Epidemiology and End Results (SEER) Program, a government registry of cancer diagnoses. "We don't know how long it takes for the effects of obesity to act on cancer promotion", she added.
In addition, the authors suggest that the age to initiate screening people at average risk may need to be reconsidered.
With colorectal cancer rates increasing for people in their early 50s but decreasing for those in their late 50s, the study found a strikingly smaller gap in incidence within just that decade of life.
There has been significant interest to do more research in this area, with this group of patients that we wouldn't normally expect to be diagnosed with cancer at such a young age.
It's important for primary care doctors to be aware of this trend and act on symptoms of colon cancer even in their younger patients, she said. In adults 40 to 54, rates increased by 0.5% to 1% per year from the mid-1990s through 2013. In adults ages 40 to 54, rectal cancer rates increased by 2% per year from the 1990s to 2013.
Rates for adults older than 55 has been declining for about 40 years, researchers said.