Colon cancer rates are rising among men and women under 50, the age at which guidelines recommend screenings start, a new analysis shows.

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 researchers reported.

"Colon cancer has traditionally been thought of as a disease of the elderly," said study lead author Dr. Samantha Hendren, an associate professor of surgery at the University of Michigan in Ann Arbor.

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"This study is really a wake-up call to the medical community that a relatively large number of colon cancers are occurring in people under 50," she added.

However, Hendren said it's too soon to say whether colon cancer screening guidelines should be altered to reflect that trend.

In the analysis, colon cancer among younger patients was often found at an advanced stage, meaning the disease has spread to lymph nodes or other organs. "Part of the reason for this is that these young patients are often diagnosed only after their cancers start to cause symptoms, such as anemia, bowel bleeding or a blockage in the colon," Hendren explained.

Doctors should be on the lookout for these warning signs of colon cancer, she added.

Not all bowel bleeding is caused by cancer, she said. "Bright red bleeding with a bowel movement is usually due to hemorrhoids or fissures, but dark blood or blood mixed with the stool is a warning sign," Hendren said.

People with a family history of colon cancer and others who are at higher risk should begin screening earlier than the age of 50, she said. "This is already recommended, but we don't think this is happening consistently, and this is something we need to optimize," she added.

The report was published online Jan. 25 in the journal Cancer.

Dr. Andrew Chan, an associate professor of medicine at Harvard Medical School in Boston, said the reasons for the increase in colon cancer among younger adults isn't known.

"It is unexplained," he said. "It's not simply a change in diagnoses, it's a very real increase. There may be an effect of our environment that could be contributing to the increase."

Smoking, obesity and physical inactivity are all risk factors for colon cancer, as is a poor diet, Chan said. "When we are faced with patients who have many of these elements, we should think more about the potential of them developing colon cancer at an earlier age," he said.

For the study, Hendren and her colleagues culled federal government data on nearly 260,000 patients diagnosed with colon cancer between 1998 and 2011.

Of these patients, nearly 15 percent were younger than 50. These patients were more likely to be diagnosed with advanced cancer and more likely to have surgery than older patients (72 percent versus 63 percent). Radiation therapy was also used more often in younger patients than in older patients (53 percent versus 48 percent), the researchers found.

Younger patients lived a little longer without a cancer recurrence, even though they tended to have more advanced cancer, Hendren said.

For patients under 50, about 68 percent survived five years, while about 67 percent of the patients 50 and older survived five years, she said. "It looks like patients' young age helps them in their cancer treatment and survival," she added.

These findings raise the question of whether screening for colon cancer should begin at an earlier age, Hendren said. "This would be a big and costly change, and I don't know whether it would help more people than it would hurt, so a lot of research would be required to understand this before any changes should be made," she said.

Chan noted that although the incidence of colon cancer is increasing among people under 50, the risk is still low. "I don't think the data at this point support expanding screening to younger age groups," he said.

Hendren said that "the cancer community needs to prepare for the increasing number of very young colorectal cancer survivors who will need long-term support to cope with the physical and psychological consequences of their disease and treatments."