Type 2 diabetes and breast cancer are two health conditions that share some of the same risk factors like obesity, changes in insulin-like growth factors, steroid hormone changes, and changes in inflammatory chemicals produced by fat cells. Early research has suggested that type 2 diabetes might be associated with a 20-30% increased risk for developing breast cancer. However, the full relationship between breast cancer and diabetes is unclear and is actively being investigated. In fact, four new research studies published online ahead of print in the Journal of Clinical Oncology have examined different aspects of this relationship between breast cancer and diabetes.
In one study, investigators conducted a meta-analysis of previously published data in order to more closely examine the link between diabetes and breast cancer outcomes. In six of seven studies, breast cancer patients with diabetes were at increased risk for death due to any cause such that there was an overall 49% increased risk of dying due to any cause compared to non-diabetic breast cancer patients. Furthermore, breast cancer patients with diabetes were diagnosed with more advanced stages of breast cancer on average and were at greater risk for adverse side effects from chemotherapy.
In a separate study , researchers examined the link between breast cancer outcomes and diabetes as assessed by blood levels of hemoglobin A1c, a marker for chronic high blood glucose levels. Blood samples taken from 3,000 early stage breast cancer patients who participated in the Women's Healthy Eating and Living Study were analyzed for the hemoglobin A1c levels. Breast cancer patients in this study who had hemoglobin A1c levels of at least 6.5% were more likely to be diagnosed with advanced stages of breast cancer. Additionally, the risk of death due to any cause was more than doubled and the risk of breast cancer recurrences was increased by 26% in breast cancer patients with hemoglobin A1c levels of at least 7% compared to breast cancer patients with levels below 6.5%.
A third study looked at the association between blood levels of C-peptide and survival of breast cancer patients enrolled in the Health, Eating, Activity, and Lifestyle study. C-peptide is a marker often used to track insulin secretion. Among all women, those with and without diabetes, C-peptide levels were linked to an increased risk of death such that each 1-ng/ml increase in C-peptide increased the risk of death due to any cause by 31% and increased the risk of death due to breast cancer by 35%. This link between C-peptide levels and risk of dying was greater in women with type 2 diabetes and women diagnosed with more advanced breast cancer.
The same investigators who studied C-peptide levels and breast cancer also conducted a study on the link between breast cancer survival and insulin resistance in over 500 breast cancer patients enrolled in the Health, Eating, Activity, and Lifestyle study. Increasing insulin resistance was linked to about a 12% reduction in breast cancer survival and a 9% reduction in all-cause survival.
Each of these four studies clearly indicate a significant link between type 2 diabetes and breast cancer. In general, breast cancer patients with type 2 diabetes, insulin resistance or chronically high blood sugar levels have poorer breast cancer outcomes, are often diagnosed with more advanced stages of breast cancer and are at greater risk for adverse side effects from breast cancer treatments. Lifestyle changes generally recommended for individuals with type 2 diabetes (healthier eating, increased physical activity, etc.) have been reported to help improve breast cancer outcomes, so these lifestyle changes are an important part of breast cancer treatment strategies for patients with diabetes and breast cancer. The author of an associated editorial, "Insulin Breast Cancer Connection: Confirmatory Data Set the Stage for Better Care ", suggests that all breast cancer patients should be assessed for the metabolic syndrome and type 2 diabetes in order to better personalize breast cancer treatments for improved breast cancer outcomes.