Does type 2 diabetes increase the risk of breast cancer?

Type 2 diabetes has been reported to increase the risk of breast cancer, but population studies have not consistently found this association. Many women with type 2 diabetes are overweight, but obesity also is not a consistent marker of breast cancer risk. Circulating insulin appears to be an important factor common to both type 2 diabetes and obesity. High levels of circulating insulin (hyperinsulinemia) appear to promote breast cancer both indirectly and directly by acting as a growth promoter. Production and metabolism of insulin-like growth factors are also disturbed in diabetics, and may contribute to increased cancer risk and progression. Women with metabolic syndrome are more likely to have triple negative breast cancer upon diagnosis than women without it.

High levels of fasting glucose at diagnosis have been found to be associated with increased risk of recurrence compared to normal levels. Hyperinsulinemia, insulin resistance, and diabetes have all been found to be associated with poor breast cancer outcomes. Therefore, women with type 2 diabetes can potentially reduce their risk of breast cancer or recurrence by taking steps to reduce insulin resistance and circulating insulin. This can be accomplished through appropriate diet, regular exercise, and losing weight if overweight.

Metabolic syndrome

Metabolic syndrome is a state of insulin resistance characterized by high waist-to-hip ratio, high body mass index (BMI), high triglyceride level, low high-density lipoprotein (HDL) cholesterol, above normal fasting glucose, high blood pressure and polycystic ovarian syndrome. Not all of these factors have to be present for a diagnosis of metabolic syndrome. Elevated leptin (a fat metabolism hormone) levels are also associated with metabolic syndrome. People with the metabolic syndrome have a five-fold increased risk of developing type 2 diabetes.

Insulin resistance

People with obesity, metabolic syndrome, glucose intolerance, and/or type 2 diabetes normally are insulin resistant. This means that a given level of insulin produces a lower-than-expected biological effect. Therefore, the body requires more insulin to control blood sugar and the pancreas will respond by increasing insulin secretion as long as it is able (in later stages of type 2 diabetes, the pancreas starts to give out and insulin shots are required to obtain the level necessary to control blood sugar).

Metformin

Metformin, which is used to treat type 2 diabetes, is thought to influence cancer cells either through insulin-mediated effects or by directly affecting cell proliferation and apoptosis. Diabetic women taking metformin have been shown to have lower risk of breast cancer. The prognosis of diabetic women with HER2 overexpressing (HER2+) breast cancer has been found to be improved by taking metformin. Metformin was found to increase the effectiveness of radiation treatment by reducing the fraction of cancer stem cells in one cell study. Diabetics taking metformin have been reported to have better response to chemotherapy than other diabetics. However, it is not clear whether this is true of women with triple negative (ER-/PR-/HER2-) breast cancer.

Insulin glargine (Lantus)

Long-term use of insulin glargine (Lantus) has been found to be associated with increased risk of breast cancer in some studies.

Tamoxifen

One study has reported that tamoxifen use in older breast cancer survivors is associated with an increased risk of type 2 diabetes.

Diet

Foods that can help reduce or regulate blood sugar, increase insulin sensitivity and/or reduce circulating insulin or IGF levels

The following foods have been shown to help regulate glucose, increase insulin sensitivity and/or reduce circulating insulin or insulin-like growth factors levels while also protecting against breast cancer: