Diabetes mellitus, often called diabetes, is a group of diseases characterized by high levels of glucose in the blood. Most of what we eat is broken down into glucose, a form of sugar in the blood that is the main source of fuel for our bodies.
When food is digested, glucose makes its way into the bloodstream and our cells use the glucose for energy and growth. However, glucose cannot enter cells without insulin being present.
Insulin is a hormone that is produced by the pancreas. After eating, the pancreas automatically makes and releases insulin which moves the glucose present in our blood into the cells. As soon as glucose enters the cells, blood glucose levels drop quickly. Diabetes occurs when insulin production is inadequate, or when the body's cells do not respond properly to insulin, or both.
Having diabetes causes damage to the blood vessels in the circulatory system. As a result, diabetes is the leading cause of kidney failure, lower limb amputations (other than those caused by injury) and vision loss due to damage to the retina.
So what does this all have to do with breast cancer? Studies show having diabetes is associated with an increased risk of developing cancer, such as breast cancer.. This is why maintaining the right amount of glucose in our bodies is so important.
Does Diabetes Affect My Risk for Breast Cancer?It was first observed 50 years ago that cancer, including breast cancer, is more commonly found in people with diabetes . More recent studies have reinforced a link between cancer and diabetes and have been able to specifically identify a link between breast cancer risk and type 2 diabetes.
There are three types of diabetes and a “pre” diabetes condition. For each type of diabetes, there is a different level of risk for breast cancer discussed below.
Type 2 diabetes
In type 2 diabetes, the body does not produce enough insulin to function properly, or the cells in the body do not react normally to the insulin it does produce (insulin resistance). Being overweight, physically inactive and eating an unhealthy diet all contribute to our risk of developing type 2 diabetes. Approximately 90-95 percent of all cases of diabetes worldwide are of this type .
Postmenopausal women 50 years or older who have type 2 diabetes have about a 20-27 percent increased risk of breast cancer .
It’s not entirely clear why people with type 2 diabetes are at increased risk for breast cancer. Type 2 diabetes causes several changes that could increase breast cancer risk such as high glucose levels, high insulin levels and increased inflammation. Studies have shown a link between all of these changes and the development of breast cancer.
In addition, many risk factors for developing type 2 diabetes and breast cancer overlap – such as being older, being overweight or obese and lack of regular physical activity. These risk factors may separately, and together, contribute to the increased risk of breast cancer in postmenopausal women with diabetes.
See Tables 1 (Risk Factors You Cannot Change) and 2 (Risk Factors You Can Change) below for more details on risk factors for type 2 diabetes and breast cancer.
Type 1 diabetes
In type 1 diabetes, also referred to as insulin-dependent diabetes, juvenile diabetes or early-onset diabetes, the body does not produce insulin. People usually develop type 1 diabetes before they are 40 years old, often in early adulthood or teenage years. Type 1 diabetics live with this disease for their entire lives and control their blood sugar levels with a combination of medication and proper diet.
Unlike type 2 diabetes, women with type 1 diabetes do not appear to have an increased risk of breast cancer .
Gestational diabetes affects women during pregnancy who have never had diabetes, but have high blood glucose levels. Some pregnant women are unable to produce enough insulin to transfer all of the glucose into their cells, resulting in high blood glucose levels. The majority of gestational diabetes patients can control their diabetes with exercise and diet. It is also important to know that blood glucose levels usually go back to normal after pregnancy.
Women with gestational diabetes do not appear to have an increased risk of breast cancer . While having gestational diabetes does not appear to be directly associated with increased risk of breast cancer, women who have had gestational diabetes have a 35 to 65 percent chance of developing type 2 diabetes in the next 10-20 years .
Pre-diabetes is a condition in which blood glucose levels are higher than normal, but not high enough to have diabetes. This condition is also commonly associated with obesity and can often lead to type 2 diabetes. However, pre-diabetes has not been shown to directly increase the risk of developing breast cancer. Does Having Type 2 Diabetes Impact Breast Cancer Prognosis?
People with breast cancer who also have type 2 diabetes have as much as a 50 percent increased chance of dying (mortality) from any cause .
Increased mortality could be due to a number of factors. For example, people with breast cancer who also have type 2 diabetes are more likely to be diagnosed with late stage breast cancer, when the disease is more difficult to treat. Differences in how treatment is given for diabetic patients have also been also observed. For example, diabetic patients are more likely to be treated with surgery and hormone therapy alone - and tend not to receive more aggressive treatments, such as the addition of chemotherapy. This may be due to their increased risk of having side effects from aggressive treatments .
Diabetes has many serious associated medical conditions (co-morbidities) which on their own negatively influence breast cancer outcomes and mortality – such as poor cardiovascular health, being overweight and a sedentary lifestyle. Researchers are working to determine the exact relationship between breast cancer and diabetes and how it influences risk.Are the Risk Factors for Type 2 Diabetes Similar to Breast Cancer?
As shown in Tables 1 and 2, type 2 diabetes and breast cancer share many risk factors – some that you can change and others you cannot. Because of the significant overlap, this adds to the challenge of knowing why women with diabetes have a higher risk for breast cancer.
Note: there are other known risk factors for breast cancer, such as age at first pregnancy and age at menopause. A complete list can be found in our Risk Factors web page
RISK FACTORS YOU CANNOT CHANGE
TYPE 2 DIABETES
26.9 percent of adults aged ≥65 yrs or older have been diagnosed or have undiagnosed
diabetes compared to 3.7% in adults aged 20-44.
The older an individual is, the more likely she or
he will develop breast cancer.
Rate of breast cancer increases after age 40 and are
highest over age 70.
11.8 percent of all men ≥ 20 yrs of age have diabetes
vs. 10.8 percent of all women.
Men are more likely to have undiagnosed diabetes.
Being female is the most common risk factor for
breast cancer. Breast cancer is 100 times more common in women than in men.
Having a family history of the disease
If your siblings or parents have diabetes, you have
a higher risk.
A family history of certain types of cancer can
increase your risk of breast cancer. This increased risk may be due to
genetic factors (known and unknown) and lifestyle.
Learn more about the role of family history.
Race plays a limited role
Compared to non-Hispanic whites, the risk of
diagnosed diabetes was 18 percent higher in Asian Americans, 66 percent
higher among Hispanics and 77 percent higher among non-Hispanic blacks. *age
adjusted figures. 
Non-Hispanic white women have the highest breast
cancer incidence overall. However, African American women have the highest
breast cancer mortality overall.
Learn more about the association of race & ethnicity and breast
RISK FACTORS YOU CAN CHANGE (MODIFIABLE)
Having high blood pressure
2 out of 3 people with diabetes report having high
blood pressure (140-90 or higher).
Not directly associated with breast cancer risk.
Being physically inactive
Being active helps maintain normal levels of blood sugar
and keeps your body sensitive to insulin.
appears to lower breast cancer risk by about 10-20 percent. This benefit is
most clearly seen in postmenopausal women.
Learn more about exercise and risk.
Being overweight or obese
raises your risk.
For most people, losing weight can reverse type 2 diabetes.
Before menopause, being overweight or obese modestly
After menopause, being overweight or obese
more about the effects of being overweight.
Eating an unhealthy diet
Diets low in red and processed meats and high in
vegetable, fruit, whole grains cereals and dietary fiber may protect against
type 2 diabetes by improving insulin sensitivity.
Note: While low carbohydrate, high protein and fat
diets have been shown to reduce weight and lower insulin levels, large
randomized clinical trials have only been performed on low fat, low calorie
diets to date.
Studies now show that eating vegetables may slightly
lower the risk of some breast cancers.
No link between high-fat diet in adulthood and an
increased risk of breast cancer has been shown. Eating a high fat diet
during adolescence may be associated with increased risk of premenopausal
Learn more about dietary fat and breast cancer risk.
May be an independent risk factor for developing
type 2 diabetes.
Smoking increases risk for complications from
diabetes, such as – cardiovascular disease, damage to the retina of the eye and
other diabetes-related health outcomes.
Smoking’s effect on breast cancer is still under
study. Although there is growing evidence that smoking may slightly increase
the risk of breast cancer, overall, study findings remain mixed.
Learn more about smoking and breast cancer risk.
Excessive alcohol consumption has been associated
with increased risk for type 2 diabetes. However, moderate alcohol (about 2
drinks per day) consumption appears to lower risk slightly.
Studies show that women who had more than two
alcoholic drinks per day had a 20 percent higher risk of breast cancer.
However, drinking low to moderate amounts of alcohol may lower the
risks of heart disease, high blood pressure and mortality.
Learn more about alcohol and breast cancer risk.
If pre-diabetes is left untreated, it often
progresses to type 2 diabetes.
Not directly associated with breast cancer risk
If you had gestational diabetes, your risk for
developing type 2 diabetes later on increases by about 35-65%. 
insulin resistance syndrome
A diagnosis of metabolic syndrome can result in up
to a 5 fold increase risk for developing type 2 diabetes. 
Associated with a moderately increased risk for
postmenopausal breast cancer.  This is still an area of active investigation.
American Cancer Society recommends about 150 minutes of physical activity a
week. Activity equal to walking for 30 minutes a day.
Overweight is defined as having a body mass index (BMI) value of 25.0 to 25.9.
Obese is defined as having a BMI of 30 or greater.
Metabolic Syndrome is also known
as insulin resistance syndrome, obesity syndrome, dysmetabolic syndrome, hypertryglyceridemic
waist. It is generally accepted that a
diagnosis of metabolic syndrome should include at least three of the following
conditions (or be on drug treatment for): increased waist circumference,
elevated trigycerides, reduced high density lipoprotein cholesterol levels
(HDL), elevated blood pressure, and elevated fasting glucose .
Metabolic Syndrome is also known as insulin resistance syndrome, obesity syndrome, dysmetabolic syndrome, hypertryglyceridemic waist. It is generally accepted that a diagnosis of metabolic syndrome should include at least three of the following conditions (or be on drug treatment for): increased waist circumference, elevated trigycerides, reduced high density lipoprotein cholesterol levels (HDL), elevated blood pressure, and elevated fasting glucose .
(also known as Glucophage®) is one of the most commonly prescribed oral
medicines used to treat type 2 diabetes.
Diabetic patients taking metformin have a lower incidence of invasive
breast cancer compared those who took other anti-diabetic medications, like
sulfonylureas (e.g. Glucotrol®, Micronase®) .
on the use of metformin and outcomes for breast cancer are mixed. One often referenced study found that
diabetic breast cancer patients that received chemotherapy and metformin were
much more likely to have a “complete response”, meaning they did not find an
remaining cancer in the affected
breast(s) or lymph node(s). However,
other studies found no link between improved survival and metformin use .
don’t yet know if people with breast cancer, who are not diabetic, would
benefit from taking metformin. Some studies suggest that breast cancer may
affect how a cell uses glucose and that treating with metformin may correct
this change. Currently a number of clinical trials are trying to determine if
metformin benefits breast cancer patients independent of diabetes.
insulins, like insulin glargine (Lantus®), are manufactured and have small
differences compared to insulin produced by our pancreas. Insulin replacements and breast cancer risk
is still an active area of investigation. Studies have found mixed results but
for now indicate there is little to no effect on breast cancer incidence with
use of insulin replacements. However, studies examining the long term (>6 yrs)
use of insulin replacements, such as glargine, and incidence of breast cancer
like glipizide (Glucotrol®) are another class of drugs that have been used to
treat type 2 diabetes for more than 50 years. They work by stimulating the
pancreas to produce more insulin. A slightly elevated risk of developing breast
cancer has been observed with the use of sulfonylurea. However, sulfonyureas are often prescribed with metformin
as a combination therapy. When used in
combination with metformin, the slightly elevated risk for breast cancer
associated with sulfonyureas is not seen .
Having type 2 diabetes
appears to increase your risk of breast cancer by about 20-27 percent.
Many risk factors
for type 2 diabetes, such as being overweight, being older and lacking regular
exercise overlap with risk factors for breast cancer.
It is difficult
to determine if diabetes alone is a risk factor for breast cancer since there
are many other conditions associated with diabetes that may also contribute to
breast cancer risk.
medication metformin appears to reduce risk and improve outcomes for diabetic
breast cancer patients, but more research is needed to confirm results and to
determine whether it has a positive benefit for non-diabetic breast cancer
Other common anti-diabetic
medications and their effect on breast cancer risk and prognosis is still an active
area of research.
Obesity is a strong, overlapping risk factor for
both type 2 diabetes and breast cancer in postmenopausal women.
✓ Read more about how body weight can affect your
breast cancer risk in our Komen Perspectives article.
about what Komen is doing to understand how body weight and weight gain
affect your risk of breast cancer by reading our Research Fast Facts on Obesity and Breast
Is Komen Doing?
1982, Komen has invested more than $14.4 million in grants to support research
related to the metabolic and hormonal changes that occur in diabetics, such as high
levels of insulin (hyperinsulimia), high blood sugar (hyperglycemia), and use
of anti-diabetic drugs and how they affect breast cancer risk. Examples of
research projects include:
diabetes affects the life expectancy of breast cancer patients from minority
populations or low socio-economic backgrounds.
laboratory research and clinical trials to determine if the anti-diabetic drug metformin
can improve outcomes for breast cancer patients without type 2 diabetes.
biomarkers to more accurately predict which diabetic patients are at an
increased risk of developing breast cancer.
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