- Analysis of the Nurses' Health Studies (NHS and NHS-II) finds that breastfeeding is inversely associated with estrogren receptor (ER)-negative breast cancers.
Why this matters
- One of the largest cohorts, and the first prospective cohort, suggests that breastfeeding is a risk-reduction strategy for aggressive breast cancer subtypes.
- Prospective cohort (n=199,514) that examined parity, breastfeeding, and breast cancer risk by hormone receptor (ER+/ER−) and molecular subtypes (luminal A, luminal B, HER2+, and basal-like).
- Funding: NIH.
- Parous (vs nulliparous) women had 18% reduced risk of ER+ breast cancer (HR, 0.82; 95% CI, 0.77-0.88) but not ER− breast cancer.
- Among parous women, breastfeeding (ever vs never) was associated with 18% lower risk of ER− (HR, 0.82; 95% CI, 0.74-0.91), but not ER+ breast cancer.
- Higher parity (≥3 children vs nulliparous) was associated with: 1) lower risk of luminal B breast cancer irrespective of breastfeeding (ever: HR, 0.78; 95% CI, 0.62-0.98; never: HR, 0.76; 95% CI, 0.58-1.00) and 2) lower risk of luminal A breast cancer only among those who ever breastfed (HR, 0.84; 95% CI, 0.71-0.99) but not among those who never breastfed.
- Tumor tissue only available for a subset (n=4058) of breast cancers.
- Observational design.