Type 2 diabetes is known to be associated with increased breast cancer risk, especially in postmenopausal women. The high level of circulating insulin (hyperinsulinemia) found in type 2 diabetes appears to promote breast cancer both indirectly and directly by acting as a growth promoter. For example, elevated circulating insulin levels were found to result in larger, more aggressive mammary tumors with more numerous lung metastases in a mouse model of HER2+ breast cancer. Now a new study has reported that heightened circulating insulin levels are associated with poorer prognosis even among non-diabetic breast cancer patients.

High blood sugar and insulin resistance are also risk factors for breast cancer and metastasis. Diets with high glycemic load (heavy in foods that cause a rapid jump in blood sugar, such as processed foods made with refined starches, potatoes and sweets) appear to increase the risk of breast cancer even in women who are not overweight, in part through higher levels of insulin. This means women should avoid a high glycemic load diet. However, it also means avoiding individual meals or snacks that cause spikes in circulating blood sugar and insulin. Regular exercise also appears to lower breast cancer risk, in part by inducing beneficial changes in insulin levels and insulin-related pathways.

Latest research finds link between insulin level and prognosis

The prospective study referenced at the beginning of this news story was designed to investigate the hypothesis that impaired glucose metabolism could influence survival even among those without type 2 diabetes. The study included 286 non-diabetic women with breast cancer (37 of whom had stage IV disease) in which fasting blood glucose, HbA1c levels, insulin, and insulin resistance were assessed at diagnosis. The study also included 143 healthy controls matched for age, blood lipid levels, and body mass index (BMI). The average fasting blood glucose level was higher among the breast cancer patients (99 mg/dL) than the healthy controls (85 mg/dL). The median circulating insulin level was also higher among the women with breast cancer (10.0 vs. 6.8 IU/mL), as was insulin resistance. However, HbA1c level was not significantly elevated in the breast cancer patients compared to the controls.

Analysis of the relationships between blood glucose, insulin levels, and insulin resistance and disease stage demonstrated that insulin had the best specificity (92%) and sensitivity (41%). Insulin was found to be a negative prognostic marker of progression-free survival even when adjusting for other prognostic factors such as menopausal status, disease stage, hormone receptor status, HER2 expression, and proliferation (Ki-67). The authors conclude that measuring insulin levels could provide prognostic information in breast cancer patients. The results also suggest that lifestyle and/or pharmacological interventions that target glucose metabolism have the potential to improve the outcomes of selected breast cancer patients.