Diabetes is a major public health concern, as is breast cancer. Combined, their threat is significant. People with diabetes are at a 13 percent higher risk of developing all types of cancer than those without diabetes. Though the scientific evidence is inconclusive, research suggests diabetes may be a risk factor for breast cancer.

How high the risk and exactly how the two conditions affect one another is still under debate and in need of further research. Cancer research strongly suggests:

• There are higher rates of diabetes among women with breast cancer than in healthy women.
• Several other factors affect the rates of diabetes among those with breast cancer including breast cancer subtypes, age, and other health issues.
• Women with diabetes and breast cancer die at higher rates than women who do not have diabetes but who do have breast cancer.

Breast Cancer and Diabetes: By the Numbers

Per the journal Cancer Cell International, at least 422 million people worldwide — 16 million people in America alone — have diabetes. According to an article cited in the journal Cureus, more than 7 percent of adults in developed countries have type 2 diabetes. And it is the seventh leading cause of death globally.

Breast cancer is the most common type of cancer seen in women worldwide. Breast cancer is the fifth most common cause of death among women. Approximately 10 percent to 20 percent of people with breast cancer also have diabetes.

The Common Thread Between Breast Cancer and Diabetes

The relationship between diabetes and breast cancer is complex. Type 2 diabetes and breast cancer share several common risk factors, comorbidities (co-existing health conditions), and exacerbating factors (such as exposure to sex hormones like estrogen).

Age

Type 2 diabetes, which is much more prevalent than type 1 diabetes, usually occurs in adults. One’s risk of developing diabetes increases with age, as does the risk of developing breast cancer.

Breast cancer that develops after menopause is more common in women who have diabetes as well as in women who enter menopause after age 55. This risk may be higher depending on how severe one’s diabetes is and how long one has had diabetes.

Insulin Resistance

Insulin, the hormone that controls blood sugar levels, plays a large role in breast cancer as well as in diabetes. Insulin resistance occurs when muscle, fat, and liver cells don’t respond to insulin properly and don’t absorb enough glucose (sugar) from your blood. To make up for this problem, the pancreas makes more insulin.

Insulin resistance and high insulin levels have been linked to an increased risk of developing type 2 diabetes. When a person has diabetes, the pancreas cannot generate enough insulin to manage a person’s blood glucose levels. This can cause glucose to stay in your bloodstream instead of being absorbed by your cells, leading to hyperglycemia, or high blood sugar levels.

Insulin resistance may also raise breast cancer risk. Insulin resistance decreases levels of estrogen, a sex hormone that helps control the development and health of the female reproductive system. Low estrogen levels raise the risk of cancer in multiple tissues and organs, including the endometrium (lining of the uterus), ovaries, and breasts. Additionally, ongoing hyperglycemia can add to an increased risk of breast cancer.

Obesity

Obesity is defined as weighing more than what is deemed healthy for a person’s height. The height-weight ratio is measured by a screening known as body mass index (BMI).

Type 2 diabetes is generally linked to obesity, a sedentary (non-active) lifestyle, and an unhealthy diet. According to the journal Diabetes Care, diabetes diagnoses in the United States have risen by 33 percent over the past couple of decades, combined with the rapid increase of obesity in the population. Excess weight may lead to insulin resistance, which may lead to diabetes.

According to a study published in the British Journal of Cancer, women diagnosed with diabetes were 27 percent more likely to develop breast cancer. However, when researchers considered BMI when calculating their results, they found that women with diabetes were only 16 percent more likely to develop breast cancer. This indicates that weight plays a role in calculating breast cancer/diabetes risk.

How a Dual Diagnosis Affects Treatment and Prognosis

Per Diabetes Care, women living with both breast cancer and diabetes may receive different treatments and face worse outcomes than those with breast cancer alone. Women with diabetes are more likely to:

• Be diagnosed with a more advanced stage of breast cancer compared to women without diabetes
• Have a higher risk of death and die earlier from breast cancer than women who don’t have diabetes

Challenges With Cancer Treatments

Research from the American Diabetes Association suggests that diabetes may increase how toxic cancer treatment is. Toxic cancer treatments like chemotherapy may be a problem for people living with diabetes. As a result, women with the condition tend to have less aggressive breast cancer treatment plans. Some chemotherapy drugs may not be an option for some women with diabetes or heart disease. Diabetes also increases a person’s chances of developing heart problems.

Additionally, those with breast cancer who undergo chemotherapy may also be at a higher risk of developing diabetes.

Breast Cancer Severity

Women who have diabetes may be more likely to receive a breast cancer diagnosis of later-stage or more invasive breast cancer. They may also have a higher chance of experiencing metastasis (having breast cancer cells spread to other parts of the body).

Higher Mortality Rates

According to Diabetes Care, women with diabetes are 40 percent more likely to die after a breast cancer diagnosis than women without diabetes. That said, having diabetes — whether or not a person has cancer — seems to increase all-cause mortality, meaning a person living with diabetes is more likely to die sooner from any cause of death, not just from cancer. Women who have had diabetes for a longer duration also have higher rates of death than those who have been recently diagnosed.

Lowering Your Risk of Breast Cancer

People with diabetes can try a few things to reduce their chances of being diagnosed with cancer: more aggressive cancer screening, proper treatment of diabetes, and lifestyle changes.

Enhanced Cancer Screening

Women being treated for diabetes are more likely to undergo cancer screening more regularly. This step improves the likelihood that their cancer will be caught early and may lead to a better outlook. A more aggressive follow-up cancer screening plan might include screening for breast cancer cells more frequently (every six to 12 months), in addition to a mammogram and a breast MRI or ultrasound every year.

Diabetes Treatment

An article in U.S. News & World Report stated that women with diabetes who take metformin (a commonly prescribed drug that controls blood sugar levels) have a 25 percent lower risk of developing breast cancer than nondiabetic women who weren’t taking the medication.

Maintaining a Healthy Weight

Attaining and maintaining a healthy weight through physical activity and nutrition reduces breast cancer risk by 15 percent. People having trouble managing their weight may be able to work with a registered dietitian or nutritionist to come up with a balanced diet plan.

Talk With Others Who Understand

MyBCTeam is the social network for people with breast cancer and their loved ones. On MyBCTeam, more than 57,000 members come together to ask questions, give advice, and share their stories with others who understand life with breast cancer.

Are you living with diabetes and breast cancer? Share your experience in the comments below, or start a conversation by posting on your Activities page.