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.

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. Therefore, 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.

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.

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. The prognosis of diabetic women has also been found to be improved by taking metformin, including among those with triple negative (ER-/PR-/HER2-) or HER2 overexpressing (HER2+) breast cancer. 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. Metformin has complex interactions with breast cancer and there is some evidence that it might be unhelpful 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:

Basil
Beans, dry
Buckwheat
Flaxseed
Garlic
Hot peppers
Mushrooms

Olives & olive oil
Pumpkins
Rice, brown
Soybeans
Turmeric
Walnuts

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.