New Study Links Body Fat with Increased Breast Cancer Risk in Postmenopausal Women with Normal BMI

BCRF investigators report that women at a healthy weight but with high levels of body fat may have an increased risk of breast cancer after menopause.

A healthy weight is typically defined using a measure known as the body mass index (BMI). While this measurement is a widely used tool to determine a person’s body fat, it is not an exact measure. With this in mind, BCRF investigators Drs. Neil Iyengar, Andrew Dannenberg and Thomas Rohan conducted a study that looked at a person’s body fat using a different method to determine its association with breast cancer risk in postmenopausal women.

The study, to be presented at the AACR Special Conference: Obesity and Cancer: Mechanisms Underlying Etiology and Outcomes on January 27-30, revealed a high body fat mass measured by dual energy X-ray absorptiometry (DXA) was associated with increased risk of invasive breast cancer in postmenopausal women, regardless of a normal BMI measurement.

DXA is commonly used to measure bone density but can also specifically measure fat, making it a more accurate measure of body fat than BMI, which cannot separate fat from muscle and bone.

“These findings will probably be surprising to many doctors and patients alike, as BMI is the current standard method to assess the risks for diseases related to body weight,” Dr. Dannenberg said. “We hope that our findings will alert women to the possibility of increased breast cancer risk related to body fat, even if they have a healthy weight.”

Commenting on the study, Dr. Iyengar emphasized that while its results indicate “a large proportion of the population has an unrecognized risk of developing cancer” researchers hope the findings will make DXA measurement of body composition more available to patients.

“Monitoring body fat levels is a more accurate way to keep tabs on one’s health risks than monitoring BMI,” Dr. Iyengar added.

Body fat, not BMI is a better assessment of breast cancer risk

To conduct the study, the researchers used data from the Women’s Health Initiative, a long-term observational study of postmenopausal women between the ages 50-79. Researchers compared body fat measured by DXA and incidence of breast cancer in 3,460 normal weight women.

Over a 16-year period, 182 women developed invasive breast cancer; the majority (146) of these being estrogen receptor (ER)-positive breast cancers.

When the researchers compared breast cancer incidence in women with highest levels of body fat mass to those with the lowest body fat mass, they found that women in the highest fat group (who had total body fat levels greater than 41%) were twice as likely to develop ER-positive breast cancer than those in the lowest fat group (who had total body fat levels less than 34%).

Circulating levels of metabolic and inflammatory factors were also higher in the highest fat mass group compared to the lowest fat mass group.

What should women take away from the study?

According to Dr. Iyengar, another important observation from the study was that women with a lower level of physical activity also had higher levels of body fat mass. “This suggests that physical activity may be important even for those who are not obese or overweight,” he said.

The next steps are to conduct clinical trials of interventions that include specific diets and/or exercise programs to develop recommendations and guidelines for women at increased risk of breast cancer due to body fat. For now, the findings from this study underscore the importance of maintaining a healthy diet and engaging in exercise – even for individuals who have a normal weight.

To learn more about maintaining a healthy weight read our Prevention series.

Read more about the BCRF research of Drs. Iyengar, Dannenberg and Rohan on our Meet the Researchers page.