Type 2 diabetes has been reported to increase the risk of breast cancer, especially in postmenopausal women. Many women with type 2 diabetes are obese, which also increases risk among postmenopausal women. However, type 2 diabetes increases breast cancer risk even in normal weight women. Women with type 2 diabetes also tend to be diagnosed with breast cancer at later stages than non-diabetics. Type 1 diabetes does not appear to be associated with increased 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.
The combination of high total cholesterol and type 2 diabetes appears to be more potentially harmful than either condition alone. High-density lipoprotein (HDL, the “good” cholesterol) from breast cancer patients with type 2 diabetes has been shown to promote one of the first steps in metastasis in cell and animal studies.
Hyperinsulinemia, insulin resistance, and diabetes have all been found to be associated with poor breast cancer outcomes. High levels of fasting glucose at diagnosis have also been found to be associated with increased risk of recurrence compared to normal levels. One large Finnish study reported that the risk of breast cancer-specific death was 36% higher among women with diabetes compared to those without diabetes. Women with type 2 diabetes can potentially reduce their risk of recurrence by taking steps to reduce insulin resistance and circulating insulin, as well as keeping their blood sugar under tight control. This can be accomplished in part 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 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).

Diabetes treatments

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, although there is some evidence that using metformin could increase the likelihood that any breast cancer that does develop is triple negative (ER-/PR-/HER2-).
The prognosis of diabetic women has been found to be improved by taking metformin, including among those with triple negative and HER2 overexpressing (HER2+) breast cancer. Metformin has been shown to inhibit aromatase activity, thereby inhibiting the production of estrogens from androgens within the body, which is likely to be beneficial for postmenopausal women with ER+ disease.
Metformin was found to increase the effectiveness of radiotherapy 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. One 2019 study reported that metformin counteracted resistance to Adriamycin chemotherapy in both hormone receptor positive (ER+/PR+) and triple negative breast cancer cells. However, one 2020 study reported that metformin had the potential to interfere with the effectiveness of both Adriamycin and Taxol. Metformin has complex interactions with breast cancer and there is some evidence that it might not be helpful in women without insulin resistance.

Insulin glargine (Lantus) and insulin

Some studies have reported that long-term use of insulin glargine (Lantus) is associated with increased risk of breast cancer and breast cancer recurrence. The association between diabetes and increased risk of recurrence also appears to be greater among short-acting insulin users. One study reported that use of medications such as metformin to treat type 2 diabetes appears to reduce breast density, whereas taking insulin might increase it.
However, this is a controversial topic and not settled. For example, one 2016 study using a mouse model of type 2 diabetes and breast cancer reported that insulin glargine at high concentrations did not promote the progression of breast tumors.

Sulfonylurea derivatives (Glipizide)

A major 2017 study reported that use of sulfonylureas such as Glipizide by women on Medicare was associated with 1.49 times higher risk of breast cancer-specific death, whereas use of Metformin was associated with 40% lower risk. Another study reported that use of sulfonylureas was associated with increased breast cancer-specific mortality, especially among women who developed type 2 diabetes after breast cancer.

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 with blood sugar, insulin sensitivity and/or circulating insulin

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:

Foods that may increase insulin resistance or increase the risk of diabetes

The following foods have been shown to increase insulin resistance or increase the risk of diabetes:
Bacon
Beef
Butter
Lamb
Lard
Milk, whole
Pork
Salt

Foods with a high glycemic index

The following are a few of the foods with a high glycemic index that may promote diabetes and breast cancer in some women:
Bread, white
Cookies, candy and cake
Cornflakes and similar breakfast cereals
Granola
Plantains, fried ripe
Potatoes, white
Rice, white
Sugar
Sweet desserts
A 2011 study found a link between increased starch intake after a diagnosis of early-stage breast cancer and a greater risk of recurrence.
Below are links to recent studies on this topic. For a more complete list of studies, please click on type 2 diabetes.

Selected breast cancer studies

Breast cancer risk factors and their effects on survival: a Mendelian randomisation study Metformin induced lactic acidosis impaired response of cancer cells towards paclitaxel and doxorubicin: Role of monocarboxylate transporter Metformin modulates oncogenic expression of HOTAIR gene via promoter methylation and reverses epithelial–mesenchymal transition in MDA‐MB‐231 cells Combining naringenin and metformin with doxorubicin enhances anticancer activity against triple-negative breast cancer in vitro and in vivo Metformin partially reverses the inhibitory effect of co-culture with ER-/PR-/HER2+ breast cancer cells on biomarkers of monocyte antitumor activity Use of metformin and aspirin is associated with delayed cancer incidence High-sensitivity C-reactive protein, hemoglobin A1c and breast cancer risk: a nested case–control study from Alberta’s Tomorrow Project cohort Patterns of cell death induced by Metformin in human MCF-7 breast cancer cells Effects of metformin on adipose-derived stromal cell (ADSC) - Breast cancer cell lines interaction Oxidative stress and TGF-β1 induction by metformin in MCF-7 and MDA-MB-231 human breast cancer cells are accompanied with the downregulation of genes related to cell proliferation, invasion and metastasis Insulin Resistance and Breast Cancer Incidence and Mortality in Postmenopausal Women in the Women's Health Initiative Insulin resistance contributes to racial disparities in breast cancer prognosis in US women Metformin Is Distributed to Tumor Tissue in Breast Cancer Patients in Vivo: A 11 C-metformin PET/CT Study History of Diabetes Mellitus and Risk of Breast Cancer in Asian Women: A Meta-Epidemiological Analysis of Population-Based Cohort Studies Diabetes and Metformin Association With Recurrence Score in a Large Oncotype Database of Breast Cancer Patients Diabetes Mellitus and Metformin Are Not Associated With Breast Cancer Pathologic Complete Response Metformin overcomes resistance to cisplatin in triple-negative breast cancer (TNBC) cells by targeting RAD51 Knockout model reveals the role of Nischarin in mammary gland development, breast tumorigenesis and response to metformin treatment Relationship between diabetes and diabetes medications and risk of different molecular subtypes of breast cancer Metformin prevention of doxorubicin resistance in MCF-7 and MDA-MB-231 involves oxidative stress generation and modulation of cell adaptation genes