Links Between Diabetes and Cancer -- ADA/ACS Consensus Report

Zosia Chustecka

June 16, 2010

June 16, 2010 — A consensus statement issued jointly by the American Diabetes Association (ADA) and the American Cancer Society (ACS) notes that there are some links between diabetes and cancer, but points out that there are also many unanswered questions.

The statement was written by a panel of experts assembled by the ADA and ACS and headed by Edward Giovannucci, MD, ScD, from the Harvard School of Public Health in Boston, Massachusetts. It is published online June 16 in Cancer.

"Probably the most important point to re-emphasize is that modifiable risk factors (overweight, obesity, lack of exercise, poor diet, and smoking) contribute to both diabetes and cancer risk, probably in part through similar mechanisms, such as insulin resistance and increased inflammation," Dr. Giovannucci told Medscape Oncology.

The report notes that diabetes, primarily type 2, is associated with an increased risk for some cancers, including liver, pancreas, endometrium, colorectal, breast, and bladder, but it is also associated with a decreased risk for prostate cancer. For other cancer types there is no association or the evidence is inconclusive, they note.

"The 2 diseases are diagnosed within the same individual more frequently than would be expected by chance, even after adjustment for age," the experts comment, but this association might not be causal, they point out.

"Both diseases are complex, with multiple subtypes," they note.

"It remains unclear whether the association between diabetes and cancer is direct (e.g., due to hypoglycemia), whether diabetes is a marker of underlying biological factors that alter cancer risk (e.g., insulin resistance and hyperinsulinemia), or whether the association between cancer and diabetes is indirect and due to common risk factors such as obesity," the authors write.

Some, but not all, epidemiologic studies have suggested that diabetes significantly increases mortality in patients with cancer, the report notes.

"Whether cancer risk is influenced by the duration of diabetes is a critical and complex issue, and may be complicated further by the multidrug therapy often necessary," they add.

Dr. Giovannucci told Medscape Oncology that diabetics have a higher mortality from all causes, including cancer. "In general, diabetics require careful scrutiny for treatment," he said. He also said that the treatment of cancer patients who are also diabetics "may be made more complex because of their diabetes," because they are more likely to have comorbidities and might already be taking drugs that could lead to interactions.

Action Points

Although the data so far suggest that there are links between diabetes and cancer, "it is too early to draw definite conclusions," said another of the authors, Michael Pollak, MD, professor of medicine and oncology at McGill University in Montreal, Quebec.

"It is certainly important for diabetics, especially diabetics on insulin, to receive at least routine screening for cancer according to standard recommendations," Dr. Pollak told Medscape Oncology.

In addition, he emphasized that "people with both diseases should try to maintain an ideal body weight . . . to improve the prognosis of both conditions."

Cancer Risk for Diabetes Drugs

The issue of diabetic treatments being associated with cancer risk has come up only recently, the experts note.

Thus far, early evidence suggests that metformin is associated with a lower risk for cancer and that exogenous insulin is associated with an increased risk (but whether insulin glargine is associated with more of a risk than other insulins is unclear).

However, these observed associations might be confounded by biologic changes that occur with diabetes, for which the drugs are being prescribed, the expert group points out. In addition, the association appears to exist for some types of cancer but not others.

Dr. Pollak added that the possibility that metformin actually reduces the risk for certain cancers in diabetics is being investigated. "Evidence for this is not strong enough at present to recommend the use of metformin for cancer prevention or cancer treatment," he said, "although this is an active research area."

When it comes to choosing between available diabetes therapies for the average patient, the experts state that "cancer risk should not be a major factor" in the choice, although "more careful consideration" might be required for selected patients who have a very high risk for cancer occurrence.

Summarizing the new report in a statement to the press, Dr. Giovannucci said: "Traditionally, there hasn't been much overlap between research in cancer and diabetes. But recently, it's become clearer that there are fascinating links between the 2. Our summary may raise more questions than answers, but we hope that it will spur additional studies."

Dr. Giovannucci has disclosed no relevant financial relationships. Dr. Pollak reports receiving research support from Pfizer and serving as a consultant to Sanofi-Aventis and Novo Nordisk. Many of the coauthors also report relationships with pharmaceutical industries; these are detailed in the paper. The conference at which this consensus statement was developed was supported by unrestricted grants from Amylin, Lilly, Merck & Co, Novo Nordisk, and Sanofi Aventis.

Cancer. Published online June 16, 2010.