(HealthDay)—While rates of colon cancer have declined among people 50 and older, they’re on the rise for younger Americans. Now, new research suggests widening waistlines may be one reason why.

In the study, women aged 20 to 49 who were overweight or obese had up to twice the risk for colon cancer before age 50, compared with normal-weight women.

“Our findings really highlight the importance of maintaining a healthy weight, beginning in early adulthood, for the prevention of early onset colorectal cancer,” said study co-author Yin Cao. She’s an assistant professor of surgery at Washington University in St. Louis.

Even though obesity has been floated as a possible reason for rising colon cancer rates among the young, “we were surprised by the strength of the link,” Cao said in a university news release.

The study wasn’t designed to prove cause and effect, only an association. But one colon cancer expert wasn’t surprised by the finding.

Dr. Jeffrey Aronoff, a colorectal surgeon at Lenox Hill Hospital in New York City, noted that obesity has long been a risk factor for colon cancer in people over 50. “I do believe that a healthy lifestyle, which includes diet, exercise,” may help curb even younger people’s odds for the disease, he said.

In the new study, Cao and her colleagues collected data on more than 85,000 U.S. women ages 25 to 44 who took part in a large, ongoing study.

Women who were heavy as teens and gained weight in early adulthood had an increased risk of colon cancer before age 50, the researchers found.

In fact, they estimated that about 22 percent of early onset colon cancers could have been prevented if those who were diagnosed had maintained a healthy weight. Across the whole American population, that could represent thousands of cases of early onset colon cancer that might be prevented.

The risk of early onset colon cancer for overweight and obese women was the same regardless of whether or not the woman had a family history of the disease.

Cao and her team members cautioned that the study cannot prove that increased weight causes early onset colon cancer, only that the two are associated. It is possible that weight is just a marker for other risk factors, such as diabetes or metabolic issues like high blood pressure or higher cholesterol, which have also been on the rise.

And the researchers stress that despite the rise in colon cancer among people under 50, it remains relatively rare, at about 8 cases per 100,000 people. Still, because screening for colon cancer usually starts at 50, those who develop it younger are often diagnosed when the disease is in its late stages and more difficult to treat.

That’s why the American Cancer Society recently lowered its recommended age at which most people should have a first screening colonoscopy. The new guidelines advise that screening begins at 45, not 50 as in the previous guidelines.

Colon cancer expert Dr. Sherif Andrawes directs endoscopy at Staten Island University Hospital in New York City. He said the study “is very important and confirms a recent observation among clinicians and experts in the field.”

And Andrawes said there’s another reason to urge Americans to get screened for colon cancer earlier.

“A bigger concern is those younger patients with cancer present symptomatic at diagnosis—which may reflect aggressive disease and an advanced stage at onset of discovery, which leads to overall worse outcomes in a younger individual,” he said.

And what about the risk for young obese men? According to Cao’s team, one limitation of the study is that it included mostly white women, so more research is needed to see if these associations hold for men and other populations.

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