(CNN) Cases of colon and rectal cancer are on the rise in young adults -- and the median age of patients diagnosed is getting lower.
Colorectal cancer is the third deadliest cancer among men and women in the United States -- after lung and prostate in men and lung and breast in women.
Actor Chadwick Boseman died of colon cancer Friday at age 43 after battling the disease for years. Colorectal cancer cases among those under 50 have been increasing since the 1990s, according to a March report by the American Cancer Society.
Half of all new diagnoses are in people under age 66, the report said.
The American Cancer Society recommends regular screening for colon and rectal cancer from age 45. Other health organizations still recommend routine screening starting at age 50.
In 2017 -- the most recent data available -- 52,547 people died of colorectal cancer nationwide, according to the Centers for Disease Control and Prevention.
More younger people have been diagnosed over the years. The American Cancer Society report found that the median age for people diagnosed with colorectal cancer was 72 in 1989. It stayed that way until the early 2000s and then fell to 66 by 2016.
The rate at which people are diagnosed with colorectal cancer in the United States is dropping among those 65 and older but rising in younger adults.
Scientists knew colorectal cancer cases were going up in younger age groups. "But we were surprised by how fast it is happening," said Rebecca Siegel, study co-author and scientific director of surveillance research at the American Cancer Society in Atlanta.
"This report is very important because it not only provides a snapshot of the current colorectal cancer burden, but also a window to the future," Siegel said.
If the increases in younger adults continue, doctors will face unique challenges such as the need for the preservation of fertility and sexual function, as well as the risk of long-term treatment effects because of their extended life expectancy she added.
The report included data on colorectal cancer cases and deaths from the National Cancer Institute and the US Centers for Disease Control and Prevention, among others.
Based on an analysis of the data, researchers found colorectal cancer cases among people younger than 50 have been increasing since the mid-1990s.
From 2012 to 2016, incidence rates among that age group rose by 2.2% annually and included tumors found in both the colon and the rectum, the study found.
Among adults ages 50 to 64, researchers found that colorectal cancer incidence declined during the 2000s, then reversed course and rose by 1% annually between 2011 and 2016. Among adults ages 65 and older, the study found that a rapid decline in colorectal cancer incidence persisted during the 2000s and that rates fell by 3.3% annually from 2011 to 2016.
"Much of the decline in incidence in older aged adults is because of increased screening, but the cause for rising incidence in younger age groups is still unknown," Siegel said.
"The obesity epidemic is probably contributing, but doesn't seem to be the sole cause," she said. "Diet has a large influence on colorectal cancer risk and there is a lot of research going on looking at how different things we consume, including drugs such as antibiotics, influence gut health, specifically their role in determining the microorganisms that make up our microbiome."
The review also revealed trends in colorectal cancer deaths and determined future projections for the disease.
The analysis found from 2008-2017, colorectal cancer death rates dropped by 3% annually in adults 65 and older and by 0.6% annually in adults ages 50 to 64 -- but the rates jumped by 1.3% a year in adults younger than 50.
The report also found that "striking disparities" by race and geography persist, with mortality rates among Alaska Natives almost three times higher than those in whites and about double those in blacks.
As for incidence, during 2012 through 2016, rates ranged from 30 cases per 100,000 people among Asian/Pacific Islanders and 38.6 in Whites to 45.7 in Blacks and 89 in Alaska Natives, according to the data.
There were limitations in the research and scientists were unable to determine exactly why certain declines or increases in colorectal cancer occurred among various age groups.
Yet researchers were able to make some projections in future trends, predicting an estimated 53,200 colorectal cancer deaths in 2020 with an estimated 7%, or 3,640 deaths, in adults younger than 50.
The analysis also projected 147,950 newly diagnosed cases of colorectal cancer in the US this year with about 12%, or 17,930 cases, diagnosed in adults younger than 50.
The analysis may build more consensus for screening before age 50, Siegel said.