Breastfeeding and Breast Cancer Risk

Not breastfeeding does increase the risk of breast cancer, particularly for triple-negative breast cancer and in women with BRCA1 mutations. 1

Evidence for Protective Effect of Breastfeeding

Breastfeeding is inversely associated with overall risk of breast cancer, with stronger protection observed for certain breast cancer subtypes 1
Among parous women, ever breastfeeding (versus never breastfeeding) is associated with a reduced risk of estrogen receptor-negative and progesterone receptor-negative breast cancers (combined OR 0.90; 95% CI 0.82–0.99) 1
Breastfeeding shows a stronger inverse association with triple-negative breast cancer (OR 0.78; 95% CI 0.66–0.91) among parous women 1
The protective effect appears to be strongest for women who had extended periods of breastfeeding during their lifetime 2
For BRCA1 mutation carriers, breastfeeding for at least one year is associated with a 32% reduction in breast cancer risk (OR = 0.68; 95% CI 0.52 to 0.91) 3
For BRCA1 carriers, breastfeeding for two or more years confers an even greater reduction in risk (OR = 0.51; 95% CI 0.35 to 0.74) 3

The protective effect of breastfeeding is hypothesized to work through two primary mechanisms 4:
1. Differentiation of breast tissue during lactation
2. Reduction in the lifetime number of ovulatory cycles
These mechanisms may explain why breastfeeding appears more protective against hormone receptor-negative cancers, as these represent different carcinogenic pathways 1

The protective effect of breastfeeding varies by breast cancer subtype, with stronger protection against estrogen receptor-negative cancers 1
Protection appears stronger for BRCA1 mutation carriers than for BRCA2 mutation carriers 1, 3
Among BRCA2 mutation carriers, there is no significant association between breastfeeding for at least one year and breast cancer risk (OR = 0.83; 95% CI 0.53 to 1.31) 3
The evidence for protection in the general population is less consistent, with some studies showing no significant overall association 5, 6

Longer duration of breastfeeding appears to confer greater protection 2, 3
For BRCA1 carriers, breastfeeding for two or more years provides greater risk reduction than shorter durations 3
Some studies in the general population have not found a clear dose-response relationship with duration 5

Women should be encouraged to breastfeed their children, as breast milk is the ideal nutrient for newborns and breastfeeding is a modifiable risk factor for breast cancer 2
Women with familial risks for breast cancer could potentially benefit most from breastfeeding 2
Physical exercise, maintaining healthy body weight, and limiting alcohol intake are additional recommended strategies to reduce breast cancer risk 1, 7
Women previously treated for breast cancer who do not show evidence of residual tumor should be encouraged to breastfeed their children, as there is no evidence that breastfeeding increases risk of recurrence 2

Not all studies have found consistent protection, suggesting that confounding factors may play a role 4, 6
The protective effect may be smaller in magnitude compared to other known risk factors for breast cancer 6
Some studies suggest the protective effect may be stronger in premenopausal women 6
The only prospective cohort study included in the evidence found no important overall association between breastfeeding and breast cancer occurrence 5