Bloomberg — Rates of colorectal cancer are increasing among adults younger than 50 with the biggest rise, more than five percent, recorded among those aged 20 to 29, according to a study.

As routine screening for those age 50 and older became more common in the mid-1980s, rates of colon cancer began to drop at a quickening pace, dropping 2.8 percent a year in men by 2005, said the report, published in the journal Cancer Epidemiology Biomarkers and Prevention. Younger Americans, who make up nine percent of cases, aren't routinely screened for the disease, the second most common cause of cancer death in US adults.

While the research didn't identify the cause of the rising rates, potential reasons may include more obesity in this age group, diets heavy in red and processed meats and the recent rise in rates of type 2 diabetes, said Elizabeth Grant, the study author and vice president of surveillance and health policy research at the American Cancer Society.

"The fact that cancer is affecting younger people is worrisome," Grant said in a telephone interview. "We want to educate health-care practitioners and the public that this increase is happening, and it may be related to obesity and unfavourable dietary patterns."

The malignancy climbed 1.5 percent a year in men aged 20 to 49 and 1.6 percent a year in women aged 20 to 49 from 1992 through 2005, according to the cancer society report. The surge in malignancy rate was seen largely on the left side of the colon and rectum, sites where red meat consumption is a risk factor, the report said.

The study should raise vigilance about symptoms reported in younger adults, including rectal bleeding, abdominal pain, changes in bowel habits and anemia with blood in the stool, the researchers wrote. Early cancer detected by screening usually doesn't have symptoms, so by the time doctors detected cases in the younger adults, the cancers were invasive, the report said.

The report didn't have a national count of cases in each age group. The percentages in the study represent a projection of rising national rates based on a sampling of cases diagnosed in people aged 20 to 49 from 1992 through 2005, rather than from the total number of cases, the authors said. The data were drawn from federal cancer registries.

Colorectal malignancy is expected to strike 146,970 and cause 49,920 deaths overall in 2009, the society said. About 91 percent of the cases are in people 50 and older. This is the first study to take a comprehensive look at this cancer's increase in younger adults. Rates and case counts before 1998 weren't included in the study, said spokesman David Sampson.

While tobacco and alcohol are potential risk factors for colorectal cancer in general, the researchers said they are unlikely to explain this rise because alcohol intake has edged down since 1981, and tobacco use takes at least 30 years to lead to colon cancer, the study said.

"I was alarmed reading this. I had no idea this was happening," said Peter Gann, a physician and cancer epidemiologist at the University of Illinois at Chicago College of Medicine in a telephone interview. "The big news has been the decline in incidence. Of course, this is a population that is not screened so it wouldn't be affected by screening."

Gann, who wasn't involved in the research, questioned whether there may also be a link with increases in anal cancers driven by human papillomavirus (HPV), a sexually transmitted infection that raises cancer risk.

The American Cancer Society, based in Atlanta, has recommended since the 1980s that adults age 50 and older get routine screening for colorectal cancer.

"For people younger than 50 we don't recommend routine screening at this point," said Ward in an interview, "because the rate isn't high enough that the benefit exceeds the risks." Testing of people under 50 is advised if they have genetic risk factors, a family history of the tumors or inflammatory bowel disease that raise their odds of malignancy, she said.