Its authors say that doctors treating women with breast cancer should be alert to the possibility their patients may develop diabetes and urge them to take steps to lower their risk where needed.
The effect, if real, does not appear to be huge and it was not the same across the board for all breast cancer survivors, according to the study by researchers from Women's College Hospital and the Institute for Clinical Evaluative Sciences, both of which are in Toronto.
The size of the increased risk seems to vary depending on whether the women have chemotherapy for their breast cancer, says the study, which was published this week by the journal Lancet.
"One important message is that because of the fact the association was not that strong, I really believe that it's likely not an issue for all breast cancer patients but rather for certain women who have other risk factors for diabetes," says lead author Dr. Lorraine Lipscombe, an endocrinologist and research scientist at Women's College Hospital.
"I think we need to confirm that in other studies, but I think we need to have a better, closer look at other risk factors."
In the past few years a number of studies have shown that women with Type 2 diabetes seem to be at a higher risk of getting breast cancer. So Lipscombe and her co-authors wanted to see if the possible association between the two diseases worked the other way as well.
This study only looked at women with breast cancer. But Lipscombe says the findings could apply to men who have breast cancer as well.
She and her co-authors used anonymized medical data from Ontario to study the question.
They looked at the records of nearly 25,000 post-menopausal women — aged 55 and older — diagnosed with breast cancer and comparing them to the records of nearly 125,000 age-matched women without breast cancer.
Women who had chemotherapy for breast cancer appeared to have about a 24 per cent higher chance of developing diabetes in the first two years after their diagnosis than women of the same age who didn't have breast cancer.
For the chemotherapy group, the risk peaked at two years and started to decline thereafter.
The early increased risk was small for women who didn't have chemotherapy, about seven per cent at two years post-diagnosis. If they survived to 10 years after their cancer diagnosis, these women seemed to face an increased risk of about 20 per cent.
This type of study, which observes what happens in two or more groups of presumably similar women, cannot prove that something causes something else. But it can suggest two things — in this case, two conditions — may be linked or associated.
Dr. Michael Pollak, a professor of oncology at McGill University in Montreal, says the findings are plausible and a novel twist on the diabetes and breast cancer issue.
Pollak specializes in breast cancer metabolism — the inter-relationship between endocrinology and oncology.
He wonders, from looking at the data, whether the increased risk comes from some of the drugs used to treat breast cancer, or whether the weight of the women who had cancer may have influenced their risk of developing diabetes.
"A very plausible explanation is that being overweight would tend to increase the risk of both diseases," says Pollak, who was not involved in the study.
"And therefore you would tend to find that diabetics have more breast cancer and breast cancer patients have more diabetes."
He suggests women who want to reduce their risk of developing breast cancer, diabetes or both should try to maintain a healthy body weight.
Because the researchers used anonymous data, they didn't have access to the kind of detailed information that would have allowed them to explore the question of weight.
But Lipscombe too wonders if the risk is due to the fact that carrying excess weight puts women at risk of developing both Type 2 diabetes and breast cancer, or whether weight gain that sometimes follows a diagnosis of breast cancer might be playing a role.
"That's something I think we need to look at more closely for breast cancer survivors," she says.
"There's some evidence to suggest that women who have breast cancer have this increased weight gain. And we don't know why. Could be the treatments we use. So that could be, over time, promoting their risk of diabetes."
The research was funded by Cancer Care Ontario, the Ontario Institute for Cancer Research and the Canadian Diabetes Association.