Deaths from colorectal cancer have increased in adults younger than 55 since the mid-2000s, but the higher mortality is being seen largely among whites, not blacks, medical investigators report in research published Wednesday.

The findings, which appear in the Journal of the American Medical Association, are part of a continuing body of research by the American Cancer Society, which earlier this year uncovered a disturbing trend: a growing incidence of colorectal cancer in younger age groups.

Now, medical scientists are discovering not only are younger people increasingly at risk, deaths appear to be concentrated among younger whites.

“The higher incidence and mortality are expected in men and women over the age of 50,” said Dr. Robert C. Amajoyi, a colorectal cancer surgeon at South Nassau Communities Hospital’s Gertrude & Louis Feil Cancer Center in Valley Stream. He was not involved in the cancer society’s study.

“In the past year alone, my team and I have operated on five young patients who are under the age of 40. The youngest was only 26,” he said.

“Right now it is difficult to know what is causing this increase, but while we try to figure it out, we should not take any symptoms in young people lightly,” Amajoyi said.

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The American Cancer Society’s research was led by epidemiologist Rebecca Siegel, who also headed the study published in February. That investigation found millennials and members of Generation X were experiencing a higher-than-expected incidence of the cancer. Siegel and her colleagues underscore that the increased incidence and death rates among young people come nowhere near those of older people. The trend, nevertheless, is disturbing, the scientists said.

“Although the [overall] risk of colorectal cancer remains low for young and middle-aged adults, rising mortality strongly suggests that the increase in incidence is not only earlier detection of prevalent cancer, but a true and perplexing escalation in disease occurrence,” Siegel said in a statement Tuesday.

The American Cancer Society estimates about 13,500 new cases of colorectal cancer will be diagnosed in people under 50 nationwide this year. But more than 135,000 cases of the disease will be diagnosed in all age groups, the majority over age 50, according to the society’s annual statistical projections.

In the new study, Siegel and colleagues found colorectal cancer mortality rates had been declining in young people from 1970 to 2004, from 6.3 deaths per 100,000 to 3.9. Then, there was an abrupt 1 percent annual increase. That increase was confined to white colorectal cancer patients for whom mortality rates also increased by 1.4 percent annually, the study’s data show.

Doctors have long noted a greater prevalence of the disease in African-Americans, and in 2010 the United States Preventive Services Task Force recommended an earlier routine screening age for blacks compared with the rest of the population. Colonoscopy screening is recommended at age 45 for blacks, compared with age 50 for the rest of the population at average risk, and with no family history of the disease.

Dr. Michele C. Reed, a family practice physician in Garden City, said younger black patients are heeding the task force’s recommendation. “I am seeing a wave of younger [black] people taking advantage of what they need to do,” Reed said of screening.

“I think it wouldn’t be a bad idea to bring down the screening age to 45 for everybody,” Reed said.