New diagnoses of colorectal cancers, which have been on the decline for years among those over 50, are increasing "exponentially" among those between the ages of 20 and 49, new research has found.

By the year 2030, the rate of new diagnoses among the under-50 set can be expected to nearly double, even as that rate declines by a third among those over 50, who generally get routine screening for the disease, according to an epidemiological study by researchers at the MD Anderson Cancer Center in Houston.

The jump in youthful cancer risk isn’t unique to cancers of the colon or rectum: New diagnoses of breast cancer, too, are rising among younger women. The reasons for the jump in either cancer aren’t entirely clear.

But when it comes to colorectal cancer, the researchers have their suspicions: increasing obesity, sedentary lifestyles and the widespread embrace of a Western diet that’s high in processed foods and red meat and low in fruits and vegetables all may be contributing to the rate of new diagnoses in young people, they wrote.

Given the rising incidence of such cancers in young people, physicians seeing younger patients with potential symptoms of colorectal cancer should not dismiss their suspicions, the authors wrote. And in all patients, they should emphasize the importance of good nutrition and regular exercise as a means of colorectal cancer prevention, they added.

The study was published Wednesday in the journal JAMA Surgery. The research team was led by Dr. Christina E. Bailey, a surgical oncology fellow at MD Anderson.

Colorectal cancer is still a disease of aging. Close to 90 percent of the 136,830 new cases diagnosed in 2014 in the United States are expected to be in those over 50. And between 2007 and 2011, the median age of diagnosis was 68.

But new diagnoses have grown fastest among adults between 20 and 34, a group in which the disease was virtually unheard of until fairly recently. Between 1975 and 2010, new diagnoses in this group grew by roughly 2 percent per year, the new research found. And the uptick in new diagnoses was at all stages of the cancer, from local to advanced.

Among those 35 to 49, a pool that accounts for about 7 percent of colorectal cancer diagnoses over the study’s 35 years, rates of new diagnoses increased by a more modest 0.4 percent.

Spun forward, those numbers spell trouble for a disease that cancer specialists thought they had brought under control with increased screening. Most colorectal cancers start from slow-growing polyps, which can be detected early by fecal occult testing, sigmoidocopy or colonoscopy and removed before they cause trouble. Largely because such screening is widespread among those over 50, rates of new colorectal cancers in this group fell during the study’s 35-year span by roughly 1 percent yearly.

But among patients younger than 50, such screening is recommended only for those with a family history of colon cancer or polyps. Without any changes in those recommendations — or to risk factors such as poor diet and exercise patterns that may be contributing to the uptick — the MD Anderson researchers concluded that by 2030, the rate of new colon cancer diagnoses among patients 20 to 34 years old can be expected to increase 90 percent for colon cancers and 124 perccent for rectal cancers.

"This is an important moment in cancer prevention," said Dr. George J. Chang, a cancer surgeon at MD Anderson and the senior author of the study. If no changes are made in public education and prevention efforts, he added, the potential effect of colorectal cancer could be very real in patients under 50.

"This is the moment to reverse this alarming trend," Chang said in a statement released with the study.