FRIDAY, Oct. 1, 2004 (HealthDayNews) -- People with type 2 diabetes who require long-term insulin therapy may be at greater risk of colorectal cancer, a small study suggests.

The study is believed to be the first to specifically examine whether taking insulin promotes cancer of the colon or rectum among people with type 2 diabetes. Previous studies have suggested a link between type 2 diabetes and increased colorectal cancer risk, the authors said.

"Basically, this study showed that type 2 diabetes patients who had received chronic insulin have a significantly higher risk of having colorectal cancer compared to type 2 patients who had not received insulin," said lead author Dr. Yu-Xiao Yang, an instructor of medicine in the division of gastroenterology at the University of Pennsylvania School of Medicine.

And the longer patients received insulin, the greater their risk of colorectal cancer. People who received at least three years of insulin therapy had three times the risk of developing that cancer compared with patients who received no insulin. The risk for people on more than five years of insulin therapy was four times higher than the no-insulin group, Yang said.

"There seems to be a duration-response effect," he said.

However, Dr. Gene Barrett, professor of medicine at the University of Virginia and immediate past president of the American Diabetes Association, said the study offers no conclusive evidence of a link between insulin dependence and colorectal cancer.

In the study, the cancer risk for people who've been on insulin for more than five years is based on just four patients, Barrett pointed out. "So this is a very small study that's going to be highly influenced by statistical fluctuation," he cautioned.

Additional studies are needed to confirm the findings, Yang acknowledged. "If this association is confirmed, I think it has huge public health implications," he said.

The study appears in the October issue of the journal Gastroenterology.

Colorectal cancer is the third leading cause of cancer-related death in the United States, according to the American Cancer Society (ACS). An estimated 146,940 new cases will be diagnosed this year, and 56,730 people will die from it, the ACS said.

People with diabetes have a 30 percent to 40 percent increased chance of developing colorectal cancer and tend to have a higher death rate after being diagnosed, the ACS reports.

Most of the 18 million Americans who suffer from diabetes have type 2, a condition in which the body fails to use insulin properly. As the prevalence of diabetes in the Unites States increases, due primarily to the obesity epidemic, the number of patients who will eventually require insulin therapy to mange their disease is also likely to rise.

Seeking to tease out the insulin effect on colorectal cancer, Yang and colleagues studied a group of 24,918 type 2 diabetics in a United Kingdom database. The analysis included people with at least three years of follow-up data after their initial diagnosis. The authors excluded colorectal cancer cases diagnosed within the first year of insulin therapy.

One important caveat: The authors had no access to patients' insulin use prior to the three-year study period.

What the authors don't know is whether the increased risk of colorectal cancer seen among patients on long-term insulin therapy is the result of the insulin, the severity of their illness, or both. Yang suspects the insulin itself is stimulating cancer-cell growth.

This study shouldn't change how physicians treat diabetes patients, Barrett said. And Yang agrees: Patients shouldn't be alarmed by the cancer finding, he said. "They should not change their insulin regimen."

Both agree that doctors should at least be following existing guidelines for identifying potential cancers of the colon or rectum. A new study from the ACS shows colorectal cancer screening is still widely underused in the United States, with just 58 percent of men and 51 percent of women reporting that they ever had either a sigmoidoscopy or colonoscopy.

Yang advises people with type 2 diabetes to ask their physician about current screening options.

More information

Click on the American Cancer Society to learn more about the risk factors for colorectal cancer.

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