Some women who survive breast cancer may be more likely to develop Type 2 diabetes, say Ontario doctors who suggest prevention and screening for the metabolic disease.

Knowing that women with diabetes have a higher risk of postmenopausal breast cancer than those without diabetes, researchers set out to explore the reverse: are breast cancer survivors more likely to develop diabetes?

The increased diabetes risk seemed to differ with chemotherapy. (David Proeber/The Pantagraph/Associated Press)

Diabetes specialist Dr. Lorraine Lipscombe of Women's College Research Institute in Toronto and her co-authors looked at the incidence of diabetes among women in Ontario aged 55 years or older diagnosed with early stage breast cancer compared with women the same age without breast cancer.

The women were followed for an average of six years for the study published in Wednesday's issue of the journal Diabetologia, the journal of the European Association for the Study of Diabetes.

"The increase in diabetes we saw was modest," Lipscombe said.

"We certainly don't think this means that breast cancer causes diabetes or that all breast cancer survivors are at higher risk of diabetes. Rather, this is a study that raises awareness of a potential association between the two conditions."

The reasons why are unclear and more works needs to be done to determine which women who survive breast cancer could be at higher risk of getting diabetes, the researchers said.

"In the meantime, greater diabetes screening and prevention strategies among breast cancer survivors may be warranted," the study's authors concluded. The diabetes screening involves a simple blood test.

Chemotherapy questions

In most women, the diabetes risk began to increase two years after the cancer was diagnosed.

The risk was highest in the first two years among those who received chemotherapy, said Lipscombe, who is also a scientist at Toronto's Institute for Clinical Evaluative Sciences.

Something about the chemotherapy may have brought on diabetes earlier in susceptible women, Lipscombe said.

Screening "could be something that women could ask about and perhaps doctors who are administering chemotherapy can make sure to monitor for it," she suggested.

Overall, 14, 576 women were diagnosed with diabetes (9.7 per cent) during the follow-up.

Of those, 2,440 of 24, 976 or 9.8 per cent were among those with breast cancer and 12,126 of 124, 880 or 9.7 per cent were in the other group.

The investigators did not have details on the specific types of chemotherapies, prescriptions for hormone therapy and other medications. They also lacked Information on other factors that could make a difference such as body mass index and family history.

When women receive chemotherapy, they may be given other drugs such as steroids that could be contributing.

Over the long term, the diabetes risk might have been underestimated by the competing risk of death, although the researchers did consider this in their model.

Previously, the team found a similar association with diabetes risk in another population, namely women with the BRCA genetic risk factors for breast cancer.

Lipscombe said the findings likely apply in other jurisdictions.

Cancer Care Ontario, Ontario Institute for Cancer Research, and the Canadian Diabetes Association/Canadian Institute of Health Research funded the study.

With files from CBC's Amina Zafar