Obesity exerts adverse effects on breast cancer survival, but the means have not been fully elucidated. We evaluated obesity as a contributor to breast cancer survival according to tumor molecular subtypes in a population-based case–cohort study using data from the Surveillance Epidemiology and End Results (SEER) program. We determined whether obese women were more likely to be diagnosed with poor prognosis tumor characteristics and quantified the contribution of obesity to survival. Hazard ratios (HRs) and 95% confidence intervals (CI) were calculated via Cox multivariate models. The effect of obesity on survival was evaluated among 859 incident breast cancers (subcohort; 15% random sample; median survival 7.8 years) and 697 deaths from breast cancer (cases; 100% sample). Obese women had a 1.7- and 1.8-fold increased risk of stage III/IV disease and grade 3/4 tumors, respectively. Obese women with Luminal A- and Luminal B-like breast cancer were 1.8 (95% CI 1.3–2.5) and 2.2 (95% CI 0.9–5.0) times more likely to die from their cancer compared to normal weight women. In mediation analyses, the proportion of excess mortality attributable to tumor characteristics was 36.1% overall and 41% and 38% for Luminal A- and Luminal B-like disease, respectively. Obesity was not associated with breast cancer-specific mortality among women who had Her2-overexpressing or triple-negative tumors. Obesity may influence hormone-positive breast cancer-specific mortality in part through fostering poor prognosis tumors. When tumor biology is considered as part of the causal pathway, the public health impact of obesity on breast cancer survival may be greater than previously estimated.

肥胖对乳腺癌的生存有不利影响，但是这种方法尚未得到充分阐明。在一项基于人群的病例队列研究中，我们使用监测流行病学和最终结果（SEER）计划中的数据，根据肿瘤分子亚型评估了肥胖对乳腺癌存活的影响。我们确定了肥胖女性是否更有可能被诊断为预后不良的肿瘤特征，并量化了肥胖对生存的影响。通过Cox多元模型计算危险比（HRs）和95％置信区间（CI）。在859例乳腺癌（亚队列； 15％随机样本；中位生存期7.8年）和697例乳腺癌死亡（病例； 100％样本）中评估了肥胖对生存的影响。肥胖女性的得分为1.7和1。III / IV期疾病和3/4级肿瘤的风险分别增加了8倍。与正常体重的女性相比，患有Luminal A型和Luminal B型乳腺癌的肥胖女性死于癌症的可能性高1.8倍（95％CI 1.3–2.5）和2.2（95％CI 0.9–5.0）。在调解分析中，归因于肿瘤特征的额外死亡率总体上占Luminal A型和Luminal B类疾病的36.1％，分别为41％和38％。肥胖与Her2过表达或三阴性肿瘤的女性的乳腺癌特异性死亡率无关。肥胖可能部分通过促进预后不良的肿瘤而影响激素阳性的乳腺癌特异性死亡率。当将肿瘤生物学视为病因途径的一部分时，