Although overall colon cancer rates are declining, the rates among Americans under 50 have jumped more than 11 percent in the past decade, a new study finds.

Over the same decade, the number of cancers in those 50 and older fell by nearly 3 percent, the study found.

"Our findings suggest that health care providers should be more vigilant about detecting symptoms in younger patients and also should consider lowering the threshold for colonoscopy screening," lead researcher Dr. Elie Sutton said during a media briefing. Sutton is a research fellow at Mount Sinai West Hospital in New York City.

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"We really don't know why colon cancer is increasing in younger patients," he said. "We can speculate that it's due to increases in inflammatory bowel disease or a change in diet, but really there is no clear consensus on that."

The researchers also found that colon cancer among those under 50 was often diagnosed when the cancer was already advanced, Sutton said.

About five years ago, a study found a similar trend toward young onset, Sutton said. "Between the time of the previous research and our study, we still have not adequately addressed the risk of colorectal cancer in people under the age of 50. It's critical that we reverse this trend so that we are able to reduce, and hopefully eliminate, it in all populations, regardless of age," he said.

Colon and rectal (colorectal) cancer is the third most common cancer, according to the U.S. National Cancer Institute (NCI). The NCI estimates there will be more than 134,000 new cases in 2016. Colorectal cancer is the second leading cause of cancer deaths, second only to lung cancer, the NCI reports.

For the study, Sutton and colleagues reviewed data on more than 1 million colorectal cancer cases listed in the National Cancer Database from 2004 to 2013.

While the study found that the number of these cancer cases is rising about 1 percent a year in people under 50, the majority of cases still occur after 50, Sutton said.

These findings mirror those of a study from the Jan. 25 online issue of the journal Cancer. That study found that one in seven colon cancer patients is under 50. Younger patients are more likely to have advanced stage cancer, but they live slightly longer without a cancer recurrence because they are treated aggressively, the University of Michigan researchers said.

The findings from these studies made both research teams wonder if colon cancer screening, which usually starts at 50, might need to begin earlier.

"This is a really important issue because rates of colon cancer are increasing in young adults," said Rebecca Siegel, the strategic director of surveillance information services at the American Cancer Society.

"People are guessing it has to do with obesity and changing patterns in diet, but we need more research in this area -- no one knows why this is happening," she said.

New screening guidelines that take into account these findings are in the works now, Siegel said.

But even under the current guidelines some people under 50 should have a colonoscopy. These include people with a family history of colon cancer, and parents and siblings who have had benign tumors in the colon, called adenomas or polyps, she said.

The results of the study were scheduled to be presented Tuesday at Digestive Disease Week in San Diego. Findings presented at meetings are typically viewed as preliminary until they've been published in a peer-reviewed journal.