Type 2 diabetes ("diabetes") is associated with more advanced stage at diagnosis and shortened survival among breast cancer patients. Breast cancer patients with high fasting blood sugar levels at diagnosis are almost twice as likely to experience a recurrence as patients with normal levels, according to one study.

The high levels of circulating insulin (hyperinsulinemia) found in type 2 diabetes appear to promote breast cancer both indirectly and directly by promoting growth. For example, high circulating insulin levels were found to increase both tumor size and number of lung metastases in a mouse model of HER2+ breast cancer in one study. There is also some evidence that high circulating glucose promotes breast cancer even when insulin resistance is not present. High blood sugar has been shown to increase breast cancer risk even in lean women. Now a large new study has reported that the risk of breast cancer-specific death is 36% higher among Finnish breast cancer patients with diabetes compared to patients without diabetes.

Reducing diabetes-associated risk of recurrence

Although diabetes cannot be cured in most cases, there are steps women can take to reduce the heightened risk of breast cancer relapse associated with this disease:

  • Maintain strict blood sugar control, avoiding spikes in blood sugar caused by large meals or snacks high in simple carbohydrates in addition to reducing average blood glucose levels as measured by hemoglobin A1c.
  • Increase level of physical activity, performing at least half an hour per day of brisk walking or its equivalent.
  • Lose weight if overweight. Consider undergoing bariatric surgery if obese, otherwise in good health, and finished with treatment.
  • Avoid using insulin to treat diabetes, if possible. There is some evidence that long-acting insulins such as Lantus might promote breast cancer, although these findings are controversial. On the other hand, metformin appears to reduce risk of breast cancer and its recurrence. However, it is more important to control blood sugar levels than to avoid using insulin.
Please see our article on type 2 diabetes for more information.

Latest research finds diabetes increases risk of breast cancer-specific death

The study referenced above was designed to investigate the association between diabetes and breast cancer stage at diagnosis, as well as breast cancer-specific survival. To conduct the study, the authors used data from the Finnish Cancer Registry concerning all 73,170 Finnish breast cancer patients diagnosed between 1995 and 2013. A total of 11,676, or 16.0%, of the breast cancer patients had diabetes.

The authors obtained additional data concerning these patients using (1) the Mass Inspection Registry (participation in mammography screening); (2) the national Care Register for Health Care (information concerning diabetes diagnoses and other medical conditions); and (3) the national Prescription Register (medication use). All analyses were adjusted for age, other medical conditions and mammography screening (participation in mammogram screening did not differ according to diabetes status). Survival analyses were also adjusted for tumor size, breast cancer type, and treatment. The women were followed for a median of 5.8 years after diagnosis.

Diabetic women were found to be 26% more likely to have often locally advanced cancer and 59% more likely to have metastatic breast cancer at diagnosis than non-diabetic women. A total of 10,900, or 14.9%, of the women died of breast cancer during follow-up. Risk of breast cancer-specific death was found to be 36% higher among those with diabetes. Furthermore, risk of breast cancer-specific death increased with duration of diabetes. The authors conclude that diabetes is a risk factor for fatal breast cancer.