High body fat levels are associated with an increased breast cancer risk among postmenopausal women with a normal boxy mass index (BMI), according to data presented at the American Association for Cancer Research’s “Obesity and Cancer: Mechanisms Underlying Etiology and Outcomes” conference.

“It was previously unknown whether individuals who have a normal BMI but increased body fat have an increased risk of developing cancer,” lead author Neil Iyengar, MD, of the Memorial Sloan Kettering Cancer Center in New York City, said in a prepared statement. "Our findings show that the risk of invasive breast cancer is increased in postmenopausal women with normal BMI and higher levels of body fat, meaning that a large proportion of the population has an unrecognized risk of developing cancer.”

The authors studied more than 3,400 postmenopausal women with BMI ranging from 18.5 to less than 25 kg/m² with baseline dual energy X-ray absorptiometry measurements and no history of breast cancer. After a median follow-up of 16 years, 182 of the women had developed invasive breast cancer. And the cancer in 146 of those cases were estrogen-receptor positive.

“It is also notable that the level of physical activity was lower in women with higher amounts of body fat,” Iyengar said in the same statement. “This suggests that physical activity may be important even for those who are not obese or overweight.”

This research may change the way specialists view BMI statistics going forward. As the study’s authors wrote, normal BMI should now be seen as “an inadequate proxy for the risk of breast cancer associated with body fatness in postmenopausal women.”

The “Obesity and Cancer: Mechanisms Underlying Etiology and Outcomes” conference runs Jan. 27-30 in Austin, Texas. More information on the conference is available here. Abstracts of the studies featured at the conference can be read here.