The benefits of using insulin to treat diabetes far outweigh the risks, but a review just published online by IJCP, the International Journal of Clinical Practice, suggests that commonly used diabetes therapies may differ from each other when it comes to their influence on cancer risk.
Cancer expert Professor Michael Pollak from McGill University, Montreal, Canada, teamed up with diabetes expert Professor David Russell-Jones from The Royal Surrey County Hospital, Guildford, UK, to review more than three decades of laboratory and population studies.
They believe that their findings will be of special interest to clinicians advising diabetic patients who have also been diagnosed with cancer or have a strong family history of cancer.
"The inter-relationships between cancer and diabetes deserve more attention as both of these diseases are becoming more prevalent globally and it is increasingly more common to see patients with both conditions" says Professor Pollak.
The paper also reflects the views expressed by experts at scientific meetings held in 2009 to assess the relative risk of malignancy associated with diabetes itself and with the use of different insulin products and other diabetes treatments. These meetings followed the publication of a series of epidemiological studies in 2009 that raised questions concerning the relative risk of cancer incidence associated with the basal insulin analogue, insulin glargine.
"Recent publications have resurrected awareness and focused attention on an issue that first emerged more than a decade ago, when it was shown that artificial modification of the molecular structure of insulin could result in increased cell division" says Professor Pollak.
"Our review showed that people with diabetes, particularly those with type 2 diabetes, may face an increased risk of cancer and that their cancer may be modified by treatment choices.
"Research suggests that metformin, which is used to treat some patients with diabetes, may provide a protective effect, while insulin and/or certain insulin analogues may promote tumour growth."
The review found that diabetes appears to be associated with an intrinsic increase in cancer incidence. A number of meta-analyses have been carried out, showing that:
- Diabetic patients were 30 per cent more likely to develop colorectal cancer (15 studies covering 2.5 million patients).
- Women with diabetes had a 20 per cent greater risk of developing breast cancer, according to 20 studies.
- People with diabetes had an 82 per cent higher risk of developing pancreatic cancer (36 studies covering more than 9,000 patients).
"The excess risk of pancreatic cancer was highest among those who had only had diabetes a short time, suggesting that the cancer could cause the diabetes" says Professor Pollak. "However, the incidence of this cancer in people with a longer history of diabetes does suggest there may also be an intrinsic cancer risk from diabetes.
"It is also important to remember that diabetes is often associated with other illnesses and risk factors, such as obesity and physical inactivity, that may in themselves increase the cancer risk."
The authors are keen to point out that the current evidence is far from clear-cut and further research is needed to examine the risks and mechanisms that appear to link insulin with tumour growth.