Post-menopausal breast cancer survivors have an estimated 20 percent higher risk of diabetes according to new research published in the journal Diabetologia. For six years, the Canadian study looked at nearly 150,000 women ages 55 and over, of which 25,000 were breast cancer survivors.

The study showed that breast cancer survivors, when compared to women without breast cancer, have a seven percent increased risk of diabetes within the first 2 years after breast cancer diagnosis; risks rose to 21 percent ten years after diagnosis.

As more U.S. women survive breast cancer, understanding the long term health risks for survivors is becoming more and more important.

Although studies have shown that women with diabetes have higher risks of postmenopausal breast cancer and up to 50 percent increased mortality risks following a breast cancer diagnosis, there is little evidence to show how breast cancer affects diabetes.

Diabetes affects 8.7 percent Americans, and nearly 19 percent of all black American adults are diabetic. It causes complications such as heart disease, hypertension, amputations and eye problems.

“There is evidence of an association between diabetes and cancer, which may be due to risk factors common to both conditions,” said lead author Lorraine Lipscombe of the Women’s College Research Institute in Canada.  “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,” Lipscombe explained.  “However, diabetes only occurs many years later when insulin levels start to decline.”

Other cancer therapies like tamoxifen may also increase the risk of diabetes, according to another study published by Lipscombe showing higher diabetes risks for women with breast cancer who were treated with tamoxifen.

The new study showed that for women who had received chemotherapy to treat breast cancer, the trend for increased diabetes risk with time was reversed. Instead, diabetes rates were higher within two years after diagnosis and rates fell by ten years post-diagnosis.

“It is possible that chemotherapy treatment may bring out diabetes earlier in susceptible women,” explained Lipscombe, adding that weight gain – which has been associated with diabetes in general – has been observed in association with breast cancer chemotherapy.

Suppression of estrogen by chemotherapy could also promote diabetes, added Lipscombe, but this would be less important for post-menopausal women.

As women undergoing chemotherapy are more likely to be more closely monitored, this could mean that diabetes was detected sooner for at-risk women. In addition, glucocorticoid drugs – used to treat nausea during chemotherapy – are known to cause spikes in blood sugar and although these effects that wear off over time, they may contribute to the higher diabetes rates observed soon after treatment.

Overall, the associations made in this study between breast cancer, chemotherapy and diabetes have left researchers unsure about why they observed such different long-term risk profiles for breast cancer survivors who did and did not receive chemotherapy. But researchers do remain certain of the importance of observation.

For Lipscombe, the findings should make closer monitoring of diabetes among breast cancer survivors a priority.

Onome is a London-based freelance journalist, focusing on public health, infectious disease and international development. She has written for The Lancet, The Global Health Magazine and The Faster Times. Onome was awarded a Bachelor of Scence from the University of St Andrews, Scotland and a PhD from the London School of Hygiene and Tropical Medicine.