(CNN) Adults in the United States are dying from colon and rectal cancers at an increasing rate about age 50, when they should just be beginning screenings, according to a new study from the American Cancer Society.

Since routine screening is generally not recommended for most adults under 50, the cancers found in younger adults are often in advanced stages and more deadly, said Dr. James Church, a colorectal surgeon at the Cleveland Clinic in Ohio.

Church, who was not involved in the new study, said he has seen this trend in death rates up close. Last year, on separate occasions, Church saw two 36-year-olds with stage IV colon cancer, he said.

In both of those patients, who had no relation to each other, the cancer spread to their livers, making it so he couldn't operate. Both died, he said.

"They both had young families, both little girls, and they lost their father in one case and their mother in the other, forever, because of this nasty disease when it's advanced," Church said.

"It makes a big impact on me, and it makes me keenly interested in trying to solve this issue," he said. "Everybody in colorectal surgical circles is seeing increased incidence of colon cancer in the young, defined as younger than 50."

According to the previous study, adults born in 1990 could have twice the risk of colon cancer and four times the risk of rectal cancer at the same age had they been born in 1950.

The reason for the rise in both incidence and death rates remains unclear.

"We've known that there's this increasing trend in people under 50 for incidence, but a lot of people were saying, 'Hey, this is good news. This means people are getting more colonoscopies, and cancer's being detected earlier,' " said Rebecca Siegel, an epidemiologist at the American Cancer Society and lead author of the new study.

Now, "what (the new study) indicates is that the increase in incidence is a true increase in disease occurrence and not an artifact of more colonoscopy use," she said. "If it was just colonoscopy use, you wouldn't expect to see an effect on death rates, or even you might see a decline in death rates."

Colorectal cancer, which includes both colon and rectal cancers, is the third-leading cause of cancer-related deaths in women in the United States and the second leading cause in men, and this year, it's expected to result in about 50,260 deaths, according to the American Cancer Society

Globally, colorectal cancer is the third most common cancer, according to the World Cancer Research Fund International

A 'surprising' racial divide

The new study included data on colon and rectal cancer diagnoses and death reports for adults ages 20 to 54 in the United States from 1970 to 2014.

The mortality data came from the National Cancer Institute's Surveillance, Epidemiology and End Results Program , as reported by the National Center for Health Statistics, which tracks cause-specific mortality rates.

After analyzing the data, the researchers found that colon and rectal cancer mortality rates among 20- to 54-year-olds declined overall from 1970 to 2004 but then increased by 1% annually from 2004 to 2014. In 2014, the total colorectal mortality rate in that age group was 4.3 people per 100,000.

Additionally, "when we looked at the trend by race, the increase in death rates is confined to whites, and in blacks, we see a slight decline over the entire 45-year study period in death rates," Siegel said.

"That's very surprising, because whites and blacks have similar patterns in the major risk factors for colorectal cancer, like obesity," she said. "A lot of people want to look to the natural culprit, obesity, but that probably isn't what's completely driving this increase in colorectal cancer."

It turns out that what's driving the increase in both colorectal cancer incidence and death rates remains a mystery, Siegel said.

"It's important to mention that still the risk for colorectal cancer is low in people under 55. We don't want to be alarmists. The risk is low," Siegel said.

"In whites, the increase over the past decade was a 14% increase in the rate. So it's not enormous, but it's concerning, because the trend has changed direction," she said. "It was declining, and now it's increasing."

Some key factors that can impact mortality rates include the stage of cancer at the time of diagnosis, treatments received by patients and the molecular subtype of the cancer, said Dr. Nancy You, a colorectal cancer surgeon and associate professor of surgical oncology at the MD Anderson Cancer Center in Houston.

Still, while such large population-level studies are excellent for showing large-scale trends, they often don't allow for deciphering the causes for those trends, said You, who was not involved in the new study.

For instance, the data analysis in the study didn't reveal the proportion of advanced versus early-stage colorectal cancers over time or whether the proportion of patients who received stage-specific treatments changed over time, You said.

"The study also did not compare the cancer-specific mortality rates of young versus older adults with colorectal cancer in the same time," You said.

"Finally, young age-of-onset rectal cancer appears to differ from young age-of-onset colon cancer, because the rise in incidence rates of rectal cancer is much more dramatic," she said. "The mortality rate difference based on colon versus rectum would also be interesting for a future analysis."

You added that, between 2004 and 2014, there were many advances in surgical and chemotherapy treatments for colorectal cancer.

"So the findings reported here raise the concern that 'why are such treatment advances failing to deliver their promise of improving survival among young adults?' " she said.

All in all, the study "tells us that we need to get messages out for people when they turn 50, they need to call and schedule their colorectal cancer screening, because increasing death rates for people who should be screened is very concerning," said Siegel, the study's lead author.

The Affordable Care Act required coverage of colorectal cancer screening tests , but patients still should check with their health insurance providers to determine coverage for colorectal cancer screening, which can range in cost.

Screenings can be performed using a fecal blood test, a stool DNA test, a sigmoidoscopy, a virtual colonoscopy or the standard colonoscopy, according to the National Cancer Institute , which also notes that other tests to screen for colorectal cancer are not generally recommended.

Jim Risk, a 51-year-old patient at the Cleveland Clinic who was not involved in the new study, agreed that the study's findings are a reminder for more adults to get screened and to pay attention to potential colorectal cancer symptoms , which include diarrhea, blood in the stool, cramping or bloating.

Risk was 40 when the Cleveland Clinic's Church diagnosed him with stage I rectal cancer.

'Had I ignored it ... I probably wouldn't be alive today'

About 10 years ago, Risk experienced rectal bleeding, and at first, he figured it was a result of a lingering hemorrhoid, he said.

Yet after he casually mentioned the bleeding to his primary care physician in an annual physical exam, Risk's doctor recommended that he get a colonoscopy to make sure the bleeding was nothing more serious.

So Risk visited the Cleveland Clinic's main campus, where "they pulled basically a golf-ball-sized polyp out of me," he said.

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Colon polyps are growths of tissue on the lining of the colon and rectum, and some polyps can become cancerous . Risk's polyp was tested for cancer, and a week later, Church called him with the test results.

"He was sort of stumbling around with his words, and that's when I knew I had a problem," said Risk, who is now healthy after surgical treatment for the cancer.

"I was unbelievably blessed. I caught it very early, and had I ignored it another year, I probably wouldn't be alive today," he said. "You have to be a good steward of your own body, and when you feel that there's something going on, you're probably better off getting it checked out as early as possible."

Even in the new study, the researchers wrote that escalating mortality rates in young and middle-aged adults "highlight the need for earlier (colorectal cancer) detection" through age-appropriate screening and timely followups for symptoms.

Risk said, "I think I've had 11 colonoscopies in my life, and the prep work is horrible, but just go in and just do it and get it over with."

To reduce your risk of colon and rectal cancers , Siegel recommended maintaining a healthy body weight, staying physically active, avoiding drinking alcohol excessively and avoiding smoking.

Meanwhile, regular screening tests may find colorectal cancer early, when treatment is likely to be more effective.

Yet whether there should be changes in screening recommendations remains to be debated, Church said. Some colorectal cancer screening tests can come with risks or false positive results.

For instance, the risk in colonoscopies includes a possible reaction to sedatives or anesthesia, bleeding, perforation of the colon, pain in the abdomen or even an extremely rare risk of fatal complications, according to the National Institutes of Health

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"The big question that we're struggling with is, as a society of doctors, should we screen people earlier? Is it worth it? And part of the big issue is, we don't know why there is this increased incidence of colon cancer in young people," Church said.

"We were taught in medical school that colon cancer is more common as you get older, and we didn't expect the rates going up," he said. "What we can do right now is pay attention to symptoms and pay attention to risk factors like family history, and by we, I mean largely primary health docs and patients."