Women who survive breast cancer have an increased risk of developing diabetes, especially if they have been treated with chemotherapy, a new study shows.
The research, published in the medical journal Diabetologia, shows that women with breast cancer saw their risk of diabetes rise modestly (by 7 per cent) two years after diagnosis.
But, if they were treated with chemotherapy, their risk rose more sharply, by 24 per cent, in that same two-year time period.
"There is no reason for breast-cancer survivors to be alarmed," Dr. Lorraine Lipscombe, a research scientist at Women's College Research Institute in Toronto and lead researcher for the study, said in an interview.
"Rather, this is something that physicians – and women themselves – should be aware of so they can get appropriate treatment during follow-up cancer care," she said.
The findings were derived from a large health database, including 24,976 breast-cancer survivors and another 124,880 women who did not have cancer who served as a control group. All the women were over the age of 55 and post-menopausal. The risk of developing breast cancer and diabetes both increase with age.
Overall, one in 10 women developed diabetes over the study period, 1996 to 2008.
Because the research involved only an examination of health records, it does not explain why breast-cancer survivors had a higher risk of diabetes.
"It is possible that chemotherapy treatment may bring out diabetes earlier in susceptible women," said Lipscombe, who is also an adjunct scientist at the Institute for Clinical Evaluative Sciences in Toronto.
For example, the production of the hormone estrogen is suppressed by chemotherapy and that can promote diabetes, though that is not likely a major factor in women who are post-menopausal.
A more likely reason, the researcher said, is the common use of steroids to treat nausea in chemotherapy. In addition to causing weight gain, steroids can cause spikes in blood sugar and alter insulin production. (Diabetes is a condition in which the body does not produce or does not properly respond to insulin, a hormone produced by the pancreas.)
There is a growing body of evidence that shows that women with diabetes have a higher risk of breast cancer. Earlier research has shown that the risk of a woman with diabetes developing breast cancer is about 20 per cent higher than a woman without diabetes. Women with diabetes are also 50 per cent more likely to die of breast cancer.
"This suggests that there are common risk factors for both diabetes and cancer," she said. She said insulin resistance is a risk factor for both diabetes and cancer and that could help explain, the higher risk of diabetes in breast-cancer survivors.
The flip side of the equation is that there are interventions – like exercise and healthy diet – that can reduce the risk of both cancer and diabetes.
"This research actually provides a little more evidence that we need to focus on healthy lifestyles, that it can influence both your risk of diabetes and cancer," Lipscombe said.
About three million Canadians have diabetes, according to the Canadian Diabetes Association.
In 2012, an estimated 22,900 Canadians will be diagnosed with breast cancer and 5,155 will die of the disease, according to the Canadian Cancer Society.