Obesity Changes Breast Tissue, Makes it Easier for Tumors to Grow Save as Favorite Sign in to receive recommendations Learn more) Sign in to receive recommendations

Overweight and obese women -- defined as having a BMI (body mass index) higher than 25 -- have a higher risk of being diagnosed with breast cancer compared to women who maintain a healthy weight, especially after menopause. Being overweight also can increase the risk of breast cancer coming back (recurrence) in women who’ve been diagnosed with the disease.

This higher risk is partially because fat cells make estrogen; extra fat cells mean more estrogen in the body and estrogen can make hormone-receptor-positive breast cancers develop and grow. Scientists also have recently found that extra fat cells can trigger long-term, low-grade inflammation in the body. Chronic inflammation has been linked to a higher risk of breast cancer recurrence; the proteins secreted by the immune system seem to stimulate breast cancer cells to grow, especially estrogen-receptor-positive breast cancer in postmenopausal women.

Researchers have wondered if there were other reasons obese women have a higher risk of breast cancer.

A study suggests that obesity changes the consistency of breast tissue in ways that promote the development and growth of breast cancer.

The research was published in the Aug. 19, 2015 issue of Science Translational Medicine. Read the abstract of “Obesity-dependent changes in interstitial ECM mechanics promote breast tumorigenesis.”

Looking at breast tissue in both women and mice, the researchers found that obesity causes the framework that surrounds fat cells in the breasts to become stiffer. This framework is called the extracellular matrix. The extracellular matrix is a collection of molecules that provide structural and biochemical support to the cells it surrounds.

When the extracellular matrix is stiffer, it creates the right conditions for cancer tumors to grow.

Fat tissue in obese women has more of a type of cell called a myofibroblast compared to fat tissue in women at a healthy weight. Myofibroblast cells help heal wounds and decide whether a scar will form. All cells secrete molecules to create an extracellular matrix. The cells remodel and grab onto the matrix to make tissue. But when myofibroblast cells make an extracellular matrix, they pull together, which is the action needed to close a wound. This pulling together stiffens the tissue.

Myofibroblasts are in everyone’s body, whether we are injured or not. Because obese women have so many more myofibroblast cells than women at a healthy weight, they have tissue scarring and stiffening in the extracellular matrix even though they haven’t been injured.

Besides causing the extracellular matrix to become stiffer, myofibroblasts also give off specific compounds that can make it easier for cancer to develop and grow.

"We all know that obesity is bad; the metabolism changes and hormones change, so when looking for links to breast cancer, researchers almost exclusively have focused on the biochemical changes happening," said Claudia Fischbach, associate professor of biomedical engineering at Cornell University and the study's senior author. "But what these findings show is that there are also biophysical changes that are important."

Right now, mammograms can’t detect any stiffness or increased density in the extracellular matrix of breast fat cells. Detecting that stiffness requires a test with a finer-scale resolution. The researchers suggested that their results may inspire the use of higher-resolution imaging techniques to detect stiffness in the extracellular matrix.

Until that imaging is available, it makes sense to do everything you can to keep your risk of breast cancer or breast cancer recurrence as low as it can be -- including getting to and maintaining a healthy weight.

Losing weight can be harder as you get older, but it can be done with careful changes to your diet and daily exercise. The first thing to do is talk to your doctor about a healthy weight for you based on your age, height, body type, and activity level. Next, talk to your doctor about a safe and sensible plan to lose weight designed specifically for you and your needs.

For more information on breast cancer risk and weight, as well as steps you can take to lose weight, visit the Breast Cancer Risk Factors: Being Overweight page in the Breastcancer.org Lower Your risk section.