Lifetime dairy product consumption and breast cancer risk: a prospective cohort study by tumor subtypes.

BACKGROUND: Previous literature on dairy products and risk of breast cancer is inconsistent, and the relationship may depend on the life-period of dietary assessment.

OBJECTIVE: We examined dairy consumption from adolescence through later adulthood and incidence of breast cancer by menopausal status and tumor molecular subtypes in the Nurses' Health Study (NHS), a prospective cohort study.

DESIGN: We analyzed data from 63,847 females in the NHS collected from 1980 to 2018. Average intake of dairy products during adulthood was assessed by validated semi-quantitative food frequency questionnaires throughout follow-up. Participants recalled adolescent dietary intake in 1986. Multivariable Cox proportional hazards models were used to estimate hazard ratios (HRs) relating dairy product consumption to breast cancer risk overall, by menopausal status, and by subtypes.

RESULTS: We documented 5733 incident cases of invasive breast cancer during 32 years of follow-up (n=5298 postmenopausal). Lifetime, adolescent, adulthood, and postmenopausal total dairy and milk intakes were not associated with overall breast cancer risk (nonsignificant HRs comparing highest to lowest quintile range=0.97-1.08) although there was a suggestive positive association between adolescent milk intake and breast cancer risk (HR=1.09, 95% CI=1.00, 1.18). Higher lifetime and premenopausal cheese intakes were associated with modestly lower risks of breast cancer (comparing highest to lowest quintile, HR for lifetime cheese intake=0.90, 95% CI=0.82, 0.98; HR for premenopausal cheese intake=0.89, 95% CI=0.79, 1.00). Results varied by tumor subtype, and some evidence for heterogeneity was observed for an association between premenopausal milk intake and breast cancer (HR for ER-positive=0.84, 95% CI=0.72, 0.99; ER-negative=1.36, 95% CI=1.00, 1.84; P heterogeneity=0.04).

CONCLUSIONS: These findings suggest that overall dairy consumption was not associated with risk of breast cancer. However, heterogeneity was observed for type of dairy food, period of life, and tumor subtypes.