A study presented at the Endocrine Society's 93rd annual meeting has reported that being obese near the time of diagnosis is associated with heightened risk of death from breast cancer among women with estrogen receptor positive (ER+) tumors, but not among those with ER- disease. However, women who had been overweight at age 18 were more likely to die from ER- disease if they subsequently developed it than women who had not been overweight at 18. The study was designed to investigate the association between obesity and risk of breast cancer mortality according to disease stage and estrogen receptor status. The study included 3,995 participants in the California Teachers Study who were diagnosed with invasive breast cancer during the period 1995-2006. Height and weight were reported by participants on a questionnaire at baseline when they enrolled in the study, as was weight at age 18. Body mass index (BMI) at baseline and change in BMI between age 18 and study enrollment were calculated. Obesity was defined as having a BMI of at least 30kg/m2. A total of 583 deaths took place among participants through 2007, of which 262 were due to breast cancer and 321 were from other causes.

Obesity at baseline was found to be associated with a 69% increased risk of dying from breast cancer in the study group overall. However, when the analysis was performed according to disease stage and ER status, some groups of women were found to be more susceptible to unfavorable effects of obesity than others. Study participants who had originally been diagnosed with regional (chest wall or lymph nodes) or distant disease (metastasis) had a 91% increased risk of dying from breast cancer compared to normal weight women. No link was found between baseline obesity and breast cancer mortality among women with disease that was localized in the breast. Higher baseline BMI was also found to be associated with higher breast cancer mortality risk among women with ER+ tumors but not among those with ER- disease. Women who had been overweight (BMI between 25 and 29) at age 18 had a higher breast cancer mortality risk than normal weight women. However, unlike obesity near time of diagnosis, being overweight at age 18 was found to be associated with increased breast cancer mortality risk among women with ER- breast cancer. In fact, women who had been overweight 18 year olds had more than three times the risk of dying from breast cancer if they developed triple negative (ER-/PR-/HER2-) disease than women who had not been overweight at 18. Results for analyses of death from any cause were similar to the breast cancer-specific mortality findings, but the associations were somewhat weaker.