Garrow said other studies have found the same association, but he believes his study is the largest cross-sectional survey to evaluate the question of whether diabetics face a higher colorectal cancer risk.

His team based its findings on data involving more than 226,000 Americans, collected from 1997 to 2003 as part of the National Health Interview Survey.

A total of 5.9 percent of respondents had a history of diabetes. Even after compensating for other factors that affect risk -- age, gender, alcohol use, tobacco use and exercise -- the researchers found that individuals with diabetes were more likely than non-diabetics to develop colon cancer.

Exactly why that might be isn't certain, Garrow said, but he noted that elevated insulin levels in the blood of those with diabetes are thought to affect cells in the colon's mucosal lining.

"In the lab, these mucosal cells, when exposed to high levels of insulin, develop into cancer cells," he said. "There are insulin-like receptors on the colon's mucosal cells. The insulin seems to attach itself to the mucosal cells and cause changes that become cancer."
Most diabetics surveyed in the study had obesity-linked type 2 diabetes, in which the body becomes insensitive to insulin. A minority had the inherited type 1 form of diabetes, which results from the body's failure to produce insulin, the hormone that helps glucose enter cells of the body and supply them with energy.

An estimated 14.6 million people in the United States have been diagnosed with diabetes, according to the American Diabetes Association. More than 145,000 new cases of colorectal cancer are expected to be diagnosed in the United States this year, according to the American Cancer Society, and over 56,000 patients will die from the disease.

Garrow said the study highlights the fact that diabetics must be especially careful to follow colorectal cancer screening guidelines. The American Cancer Society now recommends that, beginning at age 50, men and women at average risk should be screened with tests such as the fecal occult blood test, invasive exams such as sigmoidoscopy and colonoscopy, and/or double barium enema.

Another expert, Dr. Kevin Adgent, said the research will inspire him to "look at my diabetic patients more closely."
Findings presented last Tuesday at the annual scientific meeting of the American College of Gastroenterology, in Honolulu.