Objective  To explore the relationships between plasma adiponectin levels and risk of breast cancer by molecular subtype.

  Methods  A case-control study including 437 histopathologic confirmed primary breast cancer cases and 469 healthy female controls was conducted between April 2014 and May 2015. Basic information of the participants were collected using a structured questionnaire. Blood samples were collected and the plasma adiponectin levels were measured by enzyme-linked immunosorbent assay (ELASA). Analysis of variance (ANOVA) was used to compare the differences of plasma adiponectin levels among the control group and the breast cancer groups with different molecular subtypes. Multinomial logistic regression was used to investigate the association between plasma adiponectin levels and risk of breast cancer by molecular subtypes. All the statistical analyses were stratified by menopausal status.

  Results  Among the 437 breast cancer cases, there were 310 Luminal breast cancer cases, 83 HER-2-enriched breast cancer cases and 44 basal-like breast cancer cases. The median (P₂₅, P₇₅) of plasma adiponectin level of the controls was 14.85 (9.69, 21.35) μg/mL. The medians (P₂₅, P₇₅) of plasma adiponectin levels of the cases were 11.74 (8.15, 16.14) μg/mL, 12.02(8.43, 16.96) μg/mL and 12.67(8.25, 17.27) μg/mL for Luminal, HER-2-enriched and basal-like subtype respectively, which were statistically different from the controls (P < 0.001). Multinomial logistic regression showed that, after adjustment for the confounders, the higher levels of plasma adiponectin were associated with the lower risks of pre-menopausal Luminal breast cancer (OR_{pre-menopausal Luminal}=0.50, 95%CI: 0.27-0.92, P_trend=0.001), post-menopausal Luminal breast cancer (OR_{post-menopausal Luminal}=0.06, 95%CI: 0.02-0.23, P_trend < 0.001) and post-menopausal HER-2-enriched breast cancer (OR_{post-menopausal HER-2-enriched}=0.06, 95%CI: 0.01-0.62, P_trend=0.001).

  Conclusion  Lower levels of plasma adiponectin may increase the risk of pre-menopausal and post-menopausal Luminal breast cancer and post-menopausal HER-2-enriched breast cancer.