People with type 2 diabetes who require insulin injections to control their disease may be at increased risk for colorectal cancer, according to findings from a newly published study. But a diabetes expert who spoke with WebMD remains unconvinced.

Researchers from the University of Pennsylvania School of Medicine concluded that people with diabetes who had been taking insulin for three to five years had almost a threefold increase in colorectal cancer risk compared with those who did not take insulin. This risk is similar to someone with a strong family history of the cancer. The risk was even higher in people who took insulin for more than five years.

The study was published in the October issue of the journal Gastroenterology.

Since type 2 diabetes and colorectal cancer share many of the same risk factors, the findings do not prove that insulin promotes the growth of tumors. But lead researcher Yu-Xiao Yang, MD, says the study offers compelling evidence that this may be the case.

He tells WebMD that the increase in colon cancer risk seen among long-time insulin users was much greater than has been seen among people with type 2 diabetes overall.

Previous lab studies also suggest that insulin may promote the growth of colon cancer.

But American Diabetes Association immediate past-president Gene Barrett, MD, says the study was too small to prove the association. The study included just 10 people with type 2 diabetes and colorectal cancer who took insulin for three or more years.

“These authors conclude that long-term insulin therapy significantly increases the risk of colorectal cancer, but they don’t have the data to support that claim,” says Barrett, who is professor of medicine at the University of Virginia.

Screening Is Key

Colorectal cancer is the second leading cause of cancer-related deaths in the U.S., but it is also one of the most highly treatable cancers when detected early. Like type 2 diabetes, risk factors that have been implicated as promoters of colorectal cancer include western diets high in fat and refined grains (such as white bread and white rice), being overweight, and leading a sedentary lifestyle.

Barrett and Yang agree that it is particularly important for people with type 2 diabetes to strictly follow guidelines regarding colorectal cancer screening, but both say it is too soon to recommend more aggressive cancer screening for this population.

Guidelines for the population at large call for regular colorectal cancer screening beginning at age 50, with subsequent screenings determined by the screening method chosen and a person’s individual risk factors. People considered at high risk for the colorectal cancer are usually advised to begin screening earlier.

“At this point we have no evidence that more aggressive screening is beneficial,” Yang says. “The colorectal cancer screening procedures that are now in place are very, very effective. They essentially reduce risk by about 70 percent.”

An Aspirin a Day

Because screening is so effective, Yang says people with diabetes who take insulin and who get screened when they should have little cause for concern.

“Nobody should discontinue insulin therapy or change their therapy based on this study,” he says. “There is absolutely no need for that.”

Barrett says the findings, if confirmed, also give people with type 2 diabetes another reason to take a daily low-dose of aspirin if their doctor advises them to do so. Aspirin therapy is widely recommended for people with type 2 diabetes because they are at increased risk for heart disease.

“Aspirin has been shown to protect the heart, and there is also good evidence that it reduces colorectal cancer risk,” he says. “Most of my [type 2 diabetes] patients ... are taking aspirin.”

SOURCES: Yang, Y. Gastroenterology, October 2004