Obesity and Breast Cancer

People who are considered obese or overweight have a higher risk of being diagnosed with breast cancer compared to people who maintain a healthy weight, especially after menopause. Being overweight also can increase the risk of breast cancer coming back (recurrence). 

Typically doctors use body mass index (BMI) — which estimates body fat based on weight and height — to assess if a person is overweight or has obesity. People with a BMI of 25 or higher are considered overweight, while people with a BMI of 30 or higher are considered obese. 

But BMI isn’t a perfect measurement. For example, someone with a lot of muscle may have the same BMI as someone with less muscle and more body fat. In 2025, a group of global experts set a new definition of obesity that goes beyond BMI to also include waist measurements. According to these experts, the change could allow for more tailored interventions to protect against diseases like breast cancer. 

Obesity is on the rise around the world, including in the United States. The Centers for Disease Control and Prevention (CDC) estimate that 2 in 5 Americans are considered obese. Breast cancer is one of several cancers associated with obesity. And obesity-linked cancers have more than tripled in the U.S. from 1999 to 2020, according to one estimate. 

Excess weight can affect breast cancer risk differently for different people. For example, being overweight is known to increase the risk of hormone receptor-positive breast cancer in women who have gone through menopause. For younger pre-menopausal women, obesity may increase risk of estrogen receptor-negative or triple-negative breast cancer. Obesity may also increase breast cancer risk for women with a BRCA mutation. 

Obesity may increase the risk of a second cancer for people who are considered obese. Cancer survivors with excess weight had a 40% higher risk of developing a second obesity-related cancer like breast cancer, according to one study. That number jumped to 78% for people with obesity. 

The link between extra weight and breast cancer is complicated and may be affected by multiple factors. 

Extra estrogen 

One theory is that excess weight increases breast risk because fat cells make estrogen, so extra fat cells means extra estrogen. Estrogen can help hormone receptor-positive breast cancers to grow and spread. 

Chronic inflammation

Research shows that extra fat cells can trigger long-term inflammation in the body. Chronic inflammation has been linked to a higher risk of breast cancer recurrence since it may create the perfect environment for tumors to grow.  This is especially true for postmenopausal women who have estrogen receptor-positive breast cancer. 

Changes in breast tissue 

Obesity may change the consistency of breast tissue, making it stiffer. These changes may promote the development and growth of breast cancer, according to a 2015 study. 

More mutations 

Obesity also leaves its mark on breast cancer cells. In women who are considered obese, estrogen receptor-positive breast cancer cells have different mutations than women at a healthy weight, according to a 2023 study. Women who are considered obese had mutations in genes that control cell growth and suppress tumors. When these genes are mutated, breast cancer cells can grow easier.  

Some research suggests excess weight may increase the risk of breast cancer coming back. One study estimates that recurrence risk increases by about 2% with every increase in a BMI unit.  Based on a review of several studies, people who are considered obese have an increased risk of recurrence of about 35% to 40%, regardless of menopausal or hormone receptor status.  

Postmenopausal women who are considered obese who were treated with aromatase inhibitors for hormone receptor-positive breast cancer may be at a greater risk of breast cancer coming back, according to a study. 

Being overweight may also impact survival outcomes of people with breast cancer. Postmenopausal women who are considered severely obese (BMI higher than 35) may be more likely to die from breast cancer than women with lower BMIs, according to a 2024 study.

Some studies suggest that sustained weight loss in women aged 50 or older lowers breast cancer risk compared to women whose weight remained the same. Even losing a modest amount of weight may lower breast cancer risk — as little as 10 or 15 pounds — for at least 10 years in postmenopausal women with obesity. Research is lacking on how losing weight impacts recurrence risk. Large studies assessing weight loss and breast cancer recurrence are currently underway,  including one at Dana-Farber Cancer Institute.  

If you’re living with excess weight and are concerned about your risk of breast cancer, there are several steps you can take. First, talk to your doctors to figure out a goal weight and different strategies you can try. Proven strategies include gradual changes to your diet, regular exercise, and medical interventions. It may help to start making changes slowly, so you’re more likely to stick with them. 

Diet 

If you’re adjusting your diet to lose weight, try cutting down sugar, ultra-processed foods, and alcohol. You could also try introducing a variety of nutrient-dense food onto your plate, like fresh vegetables and fruits, whole grains, and lean proteins. If your insurance covers it, you may be able to work directly with a registered dietitian to create a plan that fits your lifestyle. 

Exercise 

Exercise is an important part of daily life. The CDC recommends 2.5 hours of aerobic activity every week and muscle strengthening activities twice a week. To start small, try incorporating a daily walk around your neighborhood into your routine. 

Medical interventions 

In addition to lifestyle changes, you can talk with your doctor about medical interventions, including weight loss medications like GLP-1s or weight loss surgery. GLP-1s have been shown to help people with breast cancer lose weight, lessen side effects for some treatments, and help people live longer.