According to a study published in the journal Breast Cancer Research and Treatment, women with diabetes are more likely than women without diabetes to report a history of breast cancer.

To further understand the causes of breast cancer and to improve breast cancer prevention, researchers have explored a variety of factors that may increase or decrease the risk of developing breast cancer. These factors include diet, body weight, physical activity, reproductive factors, and specific hormonal exposures.

Among potential hormonal risk factors, a majority of research has focused on the role of estrogen. Researchers have speculated, however, that the high levels of insulin (hyperinsulinemia) seen in individuals with insulin resistance may also contribute to some cases of breast cancer. Insulin resistance and hyperinsulinemia commonly precede a diagnosis of type-2 diabetes.

To explore the frequency of breast cancer before a diabetes diagnosis, researchers conducted a study among more than 82,000 women with newly diagnosed diabetes and more than 400,000 women without diabetes. All women were between the ages of 55 and 79 and were residents of Ontario, Canada.

  • A history of breast cancer was reported by 3.7% of the women with diabetes and 3.1% of the women without diabetes.
  • After accounting for other factors that may influence breast cancer risk, women with diabetes were 13% more likely to report a history of breast cancer than women without diabetes.

The researchers note that there at least two possible explanations for these findings. First, it’s possible that risk of breast cancer is increased in the pre-diabetes time period, when insulin levels are at their highest. If this is the case, interventions to manage insulin resistance may decrease a woman’s risk of both diabetes and breast cancer.

Second, it’s possible that breast cancer or its treatment increases the risk of developing diabetes. If this is the case, it suggests that diabetes screening may be important for breast cancer survivors.

Additional studies will be necessary to distinguish between these possible explanations, and to better understand the hormonal underpinnings of breast cancer. In the meantime, this study demonstrates that breast cancer is more common in women who go on to develop diabetes than in women who do not.

Reference: Lipscombe LL, Goodwin PJ, Zinman B, McLaughlin JR, Hux JE. Increased Prevalence of Prior Breast Cancer in Women with Newly Diagnosed Diabetes. Breast Cancer Research and Treatment. 2006;98:303-309.

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