LAUSR

Emerging evidence suggests that normal weight postmenopausal women with a relative excess of body fat are at increased breast cancer risk. However, little is known about the associations between obesity‐related blood markers and risk of breast cancer among these individuals. In this prospective study comprising 58 629 normal weight postmenopausal women (body mass index between 18.5 kg/m2 and 24.9 kg/m2) who were enrolled in the UK Biobank cohort between 2006 and 2010, we examined the associations of glycated hemoglobin, triglycerides, high‐density lipoprotein cholesterol, C‐reactive protein (CRP), testosterone and sex hormone‐binding globulin (SHBG) with risk of breast cancer. A total of 1268 postmenopausal breast cancer cases were ascertained during a median follow‐up period of 7 years. Women with CRP, total testosterone and free testosterone (FT) levels in the highest quintile had increased risk of breast cancer compared to those in the lowest quintile (HRQ5 vs Q1: 1.35, 95% confidence interval [CI]: 1.12‐1.63, HR Q5 vs Q1: 1.47, 95% CI: 1.20‐1.80 and HR Q5 vs Q1: 1.53, 95% CI: 1.23‐1.90, respectively), whereas those with SHBG in the highest quintile had reduced risk (HR Q5 vs Q1: 0.70, 95% CI: 0.56‐0.88). These associations were attenuated but persisted after additional adjustment for BMI, fat mass index (whole body fat mass [kg]/height [m2]) or waist circumference and after mutual adjustment for testosterone, CRP and/or SHBG. Our study suggests that the risk of postmenopausal breast cancer among normal weight women is increased in association with relatively high levels of CRP and testosterone and with relatively low levels of SHBG.