High body fat, not weight, associated with increased breast cancer risk

By Liz Meszaros, MDLinx
Published January 30, 2018

Key Takeaways

Higher body fat levels in postmenopausal women with a normal body mass index (BMI) may increase their risk for invasive breast cancer, according to study results presented at the American Association for Cancer Research Special Conference, Obesity and Cancer: Mechanisms Underlying Etiology and Outcomes.

“It was previously unknown whether individuals who have a normal BMI but increased body fat have an increased risk of developing cancer,” said Neil Iyengar, MD, medical oncologist, Memorial Sloan Kettering Cancer Center, New York, NY. “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.”

“Body fat levels are typically measured via BMI, which is a ratio of weight to height. While BMI may be a convenient method to estimate body fat, it is not an exact way to determine whole body fat levels, as muscle mass and bone density cannot be distinguished from fat mass,” said Thomas Rohan, MBBS, PhD, DHSc, professor and chair, Department of Epidemiology and Population Health, Albert Einstein College of Medicine, Bronx, NY. Dual energy X-ray absorptiometry (DXA) is a technology that can specifically measure fat content, resulting in a more accurate assessment of total body fat levels, he explained.

Researchers used data from the Women’s Health Initiative (WHI) and included 3,460 subjects with a normal BMI (18.5 to < 25.0) who underwent baseline DXA measurements and had no history of breast cancer. Over a median of 16 years of follow-up, subjects were evaluated for invasive breast cancer, and those with cancer were assessed for estrogen receptor (ER) positive status.

During follow-up, 182 subjects, including 146 who were ER-positive, developed invasive breast cancer. Upon multivariable analysis, researchers found that women in the highest quartile of whole body fat mass had approximately twice the risk for ER-positive breast cancer than those in the lowest quartile of whole body fat mass. Multivariable-adjusted hazard ratios (HRs) for ER-positive breast cancer were 2.10 (95% CI: 1.23-3.58) vs 1.91 (95% CI: 1.13-3.23), respectively.

In addition, multivariable-adjusted HRs for invasive breast cancer were 1.70 (95% CI: 1.06-2.72) in the highest quartile vs 1.75 (95% CI: 1.09-2.81) in the lowest quartile. Both of these associations remained statistically significant even after adjusting for waist-hip ratio.

Further, Dr. Iyengar and colleagues found that the risk of ER-positive breast cancer increased by 35% for each 5-kilogram increase in whole body fat, whether or not subjects had a normal BMI.

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

Clinicians, therefore, should not be lulled into complacency regarding a woman’s risk for cancer based on normal body weight.

“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,” said Andrew Dannenberg, MD, associate director, Cancer Prevention, Sandra and Edward Meyer Cancer Center of Weill Cornell Medicine, New York, NY. “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.”

Several authors of this study are supported by the Breast Cancer Research Foundation. The WHI program is funded by the National Heart, Lung, and Blood Institute, National Institutes of Health, US Department of Health and Human Services.

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