Risk of colon cancers very high in diabetic women

Insulin and low motility may be the culprits

Recent studies that show an elevated risk of colorectal cancer among diabetic women may soon prompt a change in recommendations for early screenings for those at risk.

The Harvard School of Public Health study based on data from the 18-year, 118,000-participant Nurses’ Health Study shows diabetic women have a 43% higher risk of colorectal cancer and a 49% higher risk of colon cancer than nondiabetic women.

"I am surprised. This is not something I have ever considered myself," says John H. Bond, MD, chairman of the American Digestive Health Foundation’s colorectal cancer awareness campaign.

He says he is particularly surprised that even after correcting for obesity and low exercise levels, known to be associated with both cancer and diabetes, there was still a positive association. "That is statistically significant," Bond says.

"Diabetics are clearly at higher risk for colorectal cancer, so they should clearly raise the radar screen for physicians seeing them," says Michael Thun, MD, MS, head of epidemiology at the American Cancer Society in Atlanta.

He recommends all women over 50 get an annual fecal occult exam and a sigmoidoscopy every five years or, in the alternative, a barium enema and colonoscopy.

In diabetic women, he says, physicians need to be particularly alert and perform those screenings earlier if there are any reasons for concern. "Colorectal cancer deaths are almost all avoidable by doing the appropriate screening," Thun says. "We can identify benign polyps and prevent the disease or cure it if we catch it early enough."

Bernard Levin, MD, a gastroenterologist at the MD Anderson Cancer Center at the University of Texas in Houston, says the elevated risk among diabetic women is "worth noting," and "physicians taking care of diabetic patients need to be aware of it."

Researchers, led by Frank B. Hu, PhD, a nutritionist at the Harvard School of Public Health in Boston, theorize that hyperinsulinemia may promote the growth of cancer cells in the colon. They also note that the elevated fecal bile acid may cause slower bowel transit, exposing diabetics to more toxic substances and carcinogenic bile acids.

Thun adds that several studies show that hormone replacement therapy for post-menopausal women decreases the risk of colon cancer.

There is no explanation for the higher risk among women, but Bond suggests that may simply be a factor of the heavily female-weighted Nurses’ Health Study, the source of the data for Hu’s analysis.

However, Bond cautions that the correlation showing women diabetics are susceptible to colorectal cancer is a "relatively weak one" and notes the data from the Nurses’ Health Study "may or may not be generalizeable to the general population."

[Michael Thun can be reached at (404) 329-5747 and John Bond at (612) 725-2000.]