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Why Does Diabetes Raise Cancer Risk?

More Questions Than Answers From Expert Panel on Diabetes, Cancer Link
WebMD Health News
Reviewed by Laura J. Martin, MD

June 16, 2010 -- People with diabetes are at increased risk of certain cancers -- but why?

Could it be that some diabetes treatments trigger or promote cancer? Or do the underlying causes of diabetes also underlie cancer?

These are the questions put before an expert panel from the American Diabetes Association and the American Cancer Society (ACS).

Their conclusion: We aren't sure.

Even so, lifestyle changes that prevent or reverse diabetes will certainly cut cancer risk, says panel member Susan M. Gapstur, PhD, ACS vice president of epidemiology.

"The full biologic link between diabetes and cancer has not been completely defined," Gapstur tells WebMD. "But first of all we should prevent diabetes. Then we can prevent some cancers. And for those who do have diabetes, it should be controlled as much as possible through a healthy lifestyle."

Diabetes doubles the risk of liver, pancreas, and endometrial cancer. It increases the risk of colorectal, breast, and bladder cancer by 20% to 50%. But it cuts men's risk of prostate cancer.

People with diabetes tend to have some known risk factors for cancer: older age, obesity, poor diet, and physical inactivity. And problems common in diabetes -- too-high insulin levels, too-high blood sugar levels, and inflammation -- increase cancer risk.

"No matter what science ultimately reveals ... we already know what we need to do to lower risk for both cancer and diabetes," Alice Bender, RD, of the American Institute for Cancer Research, says in a news release. "Eat a healthy, varied, predominantly plant-based diet, be physically active every day, and maintain a healthy body weight."

Do Diabetes Treatments Raise Cancer Risk?

There is evidence, but not definitive proof, that diabetes treatments affect cancer risk.

Metformin, the most commonly used diabetes drug, seems to lower cancer risk. But there's also evidence from some studies -- contradicted by others -- that insulin, particularly long-acting insulin glargine (Lantus), may increase cancer risk.

Moreover, there are at least theoretical concerns that other relatively new diabetes drugs may affect cancer risk. Unfortunately, the panel found too little data to form an opinion on this question.

Because there is no definitive link between diabetes treatment and cancer, the panel strongly advises people with diabetes -- except those at extremely high risk of cancer -- not to make treatment decisions based on fear of cancer.

"Clearly those being treated for diabetes need to be talking with their doctors about the importance of regular cancer screenings as recommended by the American Cancer Society," Gapstur says.

The consensus panel's report appears in the July/August issue of the ACS journal CA: A Cancer Journal for Clinicians.

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