Obesity increases a woman’s chance of developing breast cancer, as well as the risk of relapse and death upon diagnosis. But a new study shows that even women with normal BMI may be at risk.

Researchers at New York’s Memorial Sloan Kettering Cancer Center have found that some women of healthy weight may have obesity-related disorders, including inflammation in breast fat tissue, increased levels of aromatase and high metabolic markers like insulin and glucose — all of which increase a women’s risk for breast cancer.

The study, “Metabolic Obesity, Adipose Inflammation and Elevated Breast Aromatase in Women with Normal Body Mass Index,” appeared in Cancer Prevention Research. It suggests that the impact of obesity on cancer risk and mortality may be far greater than originally believed.

In women with excessive body fat, adipocytes — the cells that uptake and store fat in the body — may become too large and get sick, causing them to release substances into the fat tissue and blood that help recruit the immune system’s scavengers, macrophages, which clear the abnormal adipocytes.

But while “these macrophages eat the dead or sick adipocyte and clear it,” the inflammatory process that increases a women’s risk of cancer has already started, senior author Dr. Andrew Dannenberg said in a press release. This inflammation state is linked to molecular changes in the breast and blood, including increased production of aromatase — a key enzyme in estrogen synthesis. Estrogen is often used as fuel for the growth of hormone-sensitive breast cancers.

In addition, inflammation has been implicated in the onset of obesity-related diseases that promote breast cancer, including insulin resistance and high levels of glucose in the blood.

To understand if some women with normal BMI also experience breast inflammation and so-called “metabolic obesity,” Sloan Kettering researchers examined breast samples from 72 women with healthy BMI (below 25 kg/m²) who were undergoing mastectomies to reduce their risk for breast cancer, or as part of their breast cancer treatment.

Results showed that 39 percent of the women had inflammation in breast fat tissue, as well as increased leve , aromatase levels and activity were increased, compared to those without inflammation. They also had higher levels of insulin, glucose and other markers of metabolic syndrome.

“It is similar to pre-diabetes, which is traditionally considered to be associated with overweight or obesity,” said study lead author Dr. Neil Iyengar, a medical oncologist at Sloan Kettering and an assistant professor of medicine at Weill Cornell Medicine. “We call it metabo-inflammation, which means there’s inflammation in the fat that has metabolic consequences even in these normal-weight women.”

The team does not know yet why some women have breast inflammation while others don’t. But diet and exercise may be key factors in preventing women from developing metabolic obesity.