Researchers analysed data from 1996 to 2008 from the province of Ontario, Canada, to determine the incidence of diabetes among nearly 25,000 breast cancer survivors aged 55 or older and nearly 125 000 age-matched women without breast cancer.
During a median follow-up of more than five years, nearly 10% of all the women in the study developed diabetes. Compared to those who had not had breast cancer, the risk of diabetes among breast cancer survivors was 7 percent higher two years after cancer diagnosis and 21% higher 10 years after cancer diagnosis, the investigators found.
The risk of diabetes, however, decreased over time among breast cancer survivors who had undergone chemotherapy. Their risk compared to women without breast cancer was 24% higher in the first two years after cancer diagnosis and 8% higher 10 years after cancer diagnosis, according to the study, which was published in the journal Diabetologia.
Chemo may be to blame
"It is possible that chemotherapy treatment may bring out diabetes earlier in susceptible women," study author Dr Lorraine Lipscombe, of Women's College Hospital and Women's College Research Institute in Toronto, said in a journal news release. "Increased weight gain has been noted [after receiving] chemotherapy for breast cancer, which may be a factor in the increased risk of diabetes in women receiving treatment."
"Oestrogen suppression as a result of chemotherapy may also promote diabetes," Lipscombe added. "However, this may have been less of a factor in this study where most women were already postmenopausal."
The study authors suggested that there may be other factors involved for women who received chemotherapy, including glucocorticoid drugs, which are used to treat nausea in patients receiving chemo and are known to cause spikes in blood sugar. In addition, breast cancer patients undergoing chemotherapy are monitored more closely and thus are more likely to have diabetes detected, they noted.
The researchers said it is unclear why diabetes risk increased over time among breast cancer survivors who did not receive chemotherapy.
"There is, however, evidence of an association between diabetes and cancer, which may be due to risk factors common to both conditions," Lipscombe said. "One such risk factor is insulin resistance, which predisposes to both diabetes and many types of cancer - initially insulin resistance is associated with high insulin levels and there is evidence that high circulating insulin may increase the risk of cancer."
"However, diabetes only occurs many years later when insulin levels start to decline," she said. "Therefore, it is possible that cancer risk occurs much earlier than diabetes in insulin-resistant individuals, when insulin levels are high."
Overall, the "findings support a need for closer monitoring of diabetes among breast cancer survivors," Lipscombe concluded.
Although the study found an association between diabetes and breast cancer, it did not prove a cause-and-effect relationship.
The American Cancer Society outlines what happens after breast cancer treatment.
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