Post-menopausal survivors of breast cancer are more likely to develop diabetes than women without breast cancer, according to a study.
Furthermore, the relationship between breast cancer and diabetes varies depending on whether a breast cancer survivor has undergone chemotherapy, Canadian researchers reported.
As breast cancer survival rates continue to improve, understanding the long-term health consequences for survivors as they age becomes increasingly important, according to background information for the study, which appears in the December issue of the journal Diabetologia. However, little research has been carried out on post-menopausal breast cancer survivors risk of developing diabetes.
In this population-based study, Lorraine Lipscombe, MD, MSc, FRCPC, of the Womens College Research Institute in Toronto, and colleagues used population-based data from Ontario, Canada, to compare the incidence of diabetes among women ages 55 and older with breast cancer, from 1996 to 2008, with that of age-matched women without breast cancer. They further explored this relationship based on whether the patient had undergone chemotherapy.
They found that of 24,976 breast cancer survivors and 124,880 controls, 9.7% developed diabetes over an average follow-up of 5.8 years. The risk of diabetes among breast cancer survivors compared with women without breast cancer began to increase two years after diagnosis, with a 7% increased risk that rose to 21% after 10 years.
Among those who received adjuvant chemotherapy (4,404 patients) almost the opposite relationship was found: risk was highest in the first two years after diagnosis (a 24% increased risk compared with controls) and then declined to an 8% increased risk after 10 years.
“It is possible that chemotherapy may bring out diabetes earlier in susceptible women,” Lipscombe said. “Increased weight gain has been noted in the setting for adjuvant chemotherapy for breast cancer, which may be a factor in the increased risk of diabetes in women receiving treatment.
“Estrogen suppression as a result of chemotherapy may also promote diabetes. However, this may have been less of factor in this study, where most women already were post-menopausal.”
Other factors that may affect the risk in women who have received chemotherapy are the glucocorticoid drugs used to treat nausea in chemotherapy, known to cause spikes in blood sugar; and the fact that women who undergo chemotherapy could be monitored more closely and thus are more likely to have diabetes detected, the researchers said.
A reason that risk decreased in the chemotherapy group over time could be that many of the at-risk women developed diabetes in the first two years, and thus no longer were followed up. In addition, the effects of glucocorticoids are known to wear off over time.
The researchers are unsure why the relative risk of diabetes increased for breast cancer survivors who did not receive chemotherapy. Lipscombe noted “evidence of an association between diabetes and cancer, which may be due to risk factors common to both conditions.
“One such risk factor is insulin resistance, which predisposes to 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. Therefore it is possible that cancer risk occurs much earlier than diabetes in insulin-resistant individuals, when insulin levels are high.
“These findings support a need for closer monitoring of diabetes among breast cancer survivors.”
The study abstract is available at http://link.springer.com/article/10.1007/s00125-012-2793-9.