According to the World Health Organization, more than 1.9 billion adults age 18 and older were either overweight or obese in 2016. This excess weight can increase the risk factors of many illnesses, including heart disease and cancers like ovarian and colorectal cancer. Research indicates that a higher body weight can also translate into a higher risk for breast cancer.

What Is BMI?

The terms “overweight” and “obese” describe an excessive accumulation of fat (adipose tissue). Health care professionals use a measurement called body mass index (BMI) to assess the amount of body fat a person has. A person’s BMI is calculated by dividing their weight in kilograms by their height in meters squared. The result is a score that can be classified into the following categories:

• Underweight — Below 18.5
• Normal weight — 18.5 to 24.9
• Overweight — 25 to 29.9
• Obese — 30 and above

Obesity is further divided into subtypes:

• Class 1 — BMI of 30 to 34.9
• Class 2 — BMI of 35 to 39.9
• Class 3 — BMI of 40 or higher (also known as severe obesity)

How Does a Higher BMI Affect the Risk of Breast Cancer?

Adipose tissue is considered an active endocrine organ. In other words, it not only responds to hormones such as estrogen from other body systems, but the tissue itself also produces hormones. If a person has an excess amount of adipose tissue, the tissue can act as an estrogen receptor and secrete estrogen. Too much estrogen and other hormones in the body can cause the adipose tissue to dysfunction and release even more hormones. These excess hormones can increase inflammation levels throughout the body — and can lead to a higher risk of disease.

In some circumstances, these excess hormones could also potentially play a more direct role in the development of breast cancer. Breast tissue sometimes has hormone receptor cells that are strongly influenced by the signals that come from estrogen. These cells may interpret too much estrogen as an indication to grow, which raises the risk of cells growing out of control and into breast cancer.

Another effect of obesity or being overweight is the increased risk of insulin resistance. Insulin is a hormone released by the pancreas to help your body carry glucose from the bloodstream into the liver, muscle, and fat cells. When levels of insulin are not sufficient to manage the glucose, the glucose can remain in the bloodstream. High glucose levels may lead to a higher prevalence of metabolic inflammation, which can play a role in the development of breast cancer.

Does Menopause Change the Risk?

The link between BMI and breast cancer becomes more complex when comparing premenopausal and postmenopausal people.

Premenopausal Breast Cancer Risk

Researchers have found that being overweight before menopause may have a very slight protective effect. A 1997 study concluded that in premenopausal women, having a higher BMI was associated with lower breast cancer incidence. A 2017 study also associated a higher BMI in premenopausal women with a lower risk of estrogen-receptive positive breast cancer.

However, this same study also found that premenopausal women with a higher BMI had an increased risk of triple-negative breast cancer. This type of breast cancer does not have any of the hormone receptors (also known as biomarkers) normally associated with breast cancer: estrogen receptor, progesterone receptor, or human epidermal growth factor receptor 2.

Postmenopausal Breast Cancer Risk

Being overweight after menopause is believed to increase the risk of developing breast cancer. A 2018 study indicated that gaining weight after menopause is associated with a higher risk of developing breast cancer. The risk was stronger in women with a higher waist circumference and women who had never used hormone replacement therapy.

A 2019 meta-analysis of 16 previous studies found that after age 18, breast cancer risk increased 7 percent for every 11 pounds gained.

Does Having a Low BMI Affect the Risk of Breast Cancer?

Researchers have not found evidence to support that being underweight increases the risk of breast cancer. One study did uncover a possible connection between having a BMI that is considered underweight and the local recurrence of breast cancer in certain cases.

What Do MyBCTeam Members Say About Weight and Breast Cancer?

MyBCTeam members have mentioned extra weight as a factor in their experience with breast cancer. “I want you all to know that I blame obesity for my inflammatory cancer,” one member wrote. “Just prior to being discovered, like a year or so ago, I was pretty hefty. I’m not telling anyone to go on a strict diet, but please listen to your doctor. A friend who has been in remission for eight years told me that diet and exercise is what she believes in.”

Another member agreed: “Obesity is certainly a contributing factor for some of us. Making healthy choices in foods is advisable.”

Does Reducing Your BMI Reduce Your Risk?

If you’re concerned about your BMI and its correlation to breast cancer risk, you may be wondering what would happen if you lowered your BMI.

A 2021 review of clinical studies compared the effects of different diet and physical activity interventions on estrogen levels. Overweight and obese women who took part in these interventions showed a decrease in estrogen levels, which may reduce breast cancer risk.

For postmenopausal women age 50 years or older, losing weight and keeping it off may lead to a lower risk of developing breast cancer.

The National Cancer Institute recommends exercise to reduce the risk of breast cancer. Regular exercise is associated with an average 20 percent risk reduction in both postmenopausal and premenopausal women.

If you have concerns about your BMI, talk with your doctor or health care professional about achieving or maintaining a healthy weight.

Talk With Others Who Understand

MyBCTeam is the social network for people with breast cancer and their loved ones. On MyBCTeam, more than 54,300 members come together to ask questions, give advice, and share their stories with others who understand life with breast cancer.

Are you living with breast cancer and curious about how your BMI influences it? Share your experience in the comments below, or start a conversation by posting on your Activities page.