Postmenopausal women with normal body mass index (BMI) have an increased risk for invasive breast cancer in case of high body fat level, according to data presented at the American Association for Cancer Research (AACR) Special Conference on Obesity and Cancer: Mechanisms Underlying Etiology and Outcomes, held 27 to 30 January 2018 in Austin, Texas, US. The results suggest that normal BMI is an inadequate proxy for the risk of breast cancer associated with body fatness.
Obesity, defined as BMI >30 kg/m2, is associated with increased risk of oestrogen receptor (ER)-positive breast cancer in postmenopausal women. However, BMI is not exact measure of body fat. The authors wrote in study background that it is unknown in individuals with normal BMI if body fat level contribute to breast cancer risk.
The study team examined the association between body fat level, measured by dual energy X-ray absorptiometry (DXA), and breast cancer risk among women with normal BMI. The study, conducted in the Women’s Health Initiative, included 3460 postmenopausal women with BMI 18.5 to <25.0 kg/m2 who had baseline DXA measurements and no history of breast cancer. By the median follow-up of 16 years, 182 incident invasive breast cancers had been recorded, of which 146 were ER-positive.
The authors also analysed mean blood concentrations of insulin, C-reactive protein (CRP), interleukin-6 (IL-6), leptin, SHBG, HDL-cholesterol and triglycerides.
Multivariable-adjusted hazard ratios (HRs) for invasive breast cancer were 1.70 and 1.75 in the highest versus lowest quartile of whole body fat mass and trunk fat mass. Adjusted HRs for ER-positive breast cancer were 2.10 and 1.91 in the highest versus lowest quartiles of whole body fat mass and trunk fat mass. The associations remained statistically significant after additional adjustment for waist-hip ratio.
Age and race/ethnicity-adjusted circulating levels of metabolic and inflammatory factors including insulin, CRP, IL-6, leptin and triglycerides were higher in the upper versus lower quartiles of trunk fat mass (p < 0.01). Levels of HDL-cholesterol and SHBG were lower in the upper versus lower quartiles of trunk fat mass (p < 0.01).
Dr Neil Iyengar of the Memorial Sloan Kettering Cancer Center and colleagues reported that the level of physical activity was lower in women with higher levels of body fat, suggesting that physical activity may be important even in those who are not obese or overweight.
The researchers were unable to analyze how changes in body fat over time related to breast cancer risk. The study team noted that thier findings are limited to postmenopausal women and are not generalisable to other populations or other cancers.
The authors concluded that in postmenopausal women with normal BMI, high body fat level is associated with elevated risk of ER-positive breast cancer and altered levels of circulating metabolic and inflammatory factors.
Iyengar N, Arthur R, Manson JE, et al. Body fat and risk of breast cancer in normal-size postmenopausal women. Abstract PR06; Presented at AACR Special Conference on Obesity and Cancer: Mechanisms Underlying Etiology and Outcomes, 27-30 January 2018, Austin, Texas, US.