US experts who reviewed the emerging evidence on the links between cancer and diabetes have reported their findings in a consensus statement on the state of science linking both diseases and highlighted key questions that remain unanswered.

You can read the consensus report by experts representing the American Diabetes Association and the American Cancer Society, in the 16 June online before print issue of CA: A Cancer Journal for Clinicians; the print version will appear in the July/August paper edition.

Some evidence suggests that people with diabetes are at a higher risk of developing many types of cancer, and type 2 diabetes and cancer share some risk factors as well as treatments; however, the underlying biology of this overlap is somewhat elusive.

This gap in understanding spurred the American Diabetes Association and the American Cancer Society to bring together experts to address some key questions, for instance, what risk factors are common to both cancer and diabetes, is there a meaningful link between their incidence and prognosis, what might the underlying biological mechanisms be, and do treatments for diabetes affect cancer risk and prognosis?

The experts were asked to investigate each question, identify current gaps in evidence and suggest future strategies so researchers and epidemiologists can work toward closing the gaps.

Here are some points they reached consensus on, and some recommendations they made:

  • People with diabetes (mostly type 2), are likely to be at increased risk of developing cancer of the liver, pancreas, endometrium (lining of the uterus), colon, rectum, breast and bladder.
  • The evidence, some of which is inconclusive, does not point to a link between diabetes and increased risk of cancer in other parts of the body, and for prostate cancer, having diabetes appears to lower the risk.
  • Shared risk factors such as age, obesity, diet and lack of physical activity may explain the link between diabetes and some cancers.
  • Biological mechanisms that directly link diabetes and cancer include hyperinsulinemia (too much insulin in the blood), hyperglycemia (too much sugar in the blood), and inflammation.
  • Being physically active, managing weight, and keeping to a healthy diet, things that we should all do for sake of our health anyway, reduce the risk and improve outcomes of type 2 diabetes and some cancers.Doctors and healthcare professionals should encourage all patients with diabetes to have the recommended cancer screening for their age and sex.
  • On the whole, cancer risk should not be a major factor affecting choice of diabetes therapy for most patients. However, some specific cases, such as patients at higher risk of cancer, or at higher risk of recurrence of certain cancers, may need more careful consideration.
  • There is limited evidence that specific drugs influence cancer risk. Suggestions that they do could be a result of certain side effects such as impact on body weight and hyperinsulinemia, as well as the complex and progressive way that hyperglycemia and drugs interact in type 2 diabetes.
  • The evidence on metformin (an oral anti-diabetic drug) is still patchy, but what there is suggests it is linked to a lower risk of cancer.
  • Similarly, early evidence would suggest that exogenous insulin (that is introduced into the body) is linked to a higher risk of cancer.
  • More research is needed to confirm or clarify this early evidence, and to evaluate the possible links between insulin and other diabetes medications and cancer risk.

In many respects the report points to a multitude of uncertain issues, and possibly furnishes more questions than answers, for reasons that Dr Edward Giovannucci, co-chair of the consensus report group, attempted to explain:

“Traditionally there hasn’t been much overlap between research in cancer and in diabetes.”

“But recently it’s become clearer that there are fascinating links between the two. Our summary may raise more questions than provide answers, but we hope that it will spur additional studies,” he added.

His colleague and co-chair Dr David M. Harlan said that for the vast majority of diabetes patients, worrying about which diabetes treatment is better or worse as far as cancer is concerned will not be an issue.

“Only patients with a very high risk for cancer occurrence, or re-occurrence, may wish to carefully consider their options. Even then, the association appears to exist for some cancer types, and not for others,” said Harlan, concluding that “we have much to learn”.

“Diabetes and Cancer: A Consensus Report.”
Edward Giovannucci, David M. Harlan, Michael C. Archer, Richard M. Bergenstal, Susan M. Gapstur, Laurel A. Habel, Michael Pollak, Judith G. Regensteiner, and Douglas Yee.
CA Cancer J Clin, published online before print 16 June 2010.

Source: American Cancer Society.

Written by: Catharine Paddock, PhD