Long-term consumption of non-fermented and fermented dairy products and risk of breast cancer by estrogen receptor status - Population-based prospective cohort study

Background & aims: The impact of dairy consumption on breast cancer development is unclear. We sought to examine associations between long-term consumption of milk and fermented dairy products and risk of breast cancer by estrogen (ER) and progesterone receptor (PR) status and assess whether these associations varied by body weight.

Methods: The population-based Swedish Mammography Cohort included 33,780 women (88.2% postmenopausal), with no history of cancer or diabetes at baseline (1997). Long-term consumption of dairy products was assessed using a self-administered food-frequency questionnaire in 1987 and 1997. Cox proportional hazard regression models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs).

Results: During 16.6 years of follow-up (559,286 person-years), 1870 total breast cancer cases were diagnosed (1162 ER+/PR+; 195 ER-/PR-). High long-term non-fermented milk consumption was associated with increased ER+/PR+ breast cancer incidence, HR = 1.30, 95%CI:1.02-1.65 for the average of 1987 and 1997 intake ≥2 vs. 0 servings/day and this increased risk was limited to women with BMI<25 kg/m² HR = 1.55, 95%CI:1.08-2.21, while no significant associations with milk consumption were observed with ER-/PR- breast cancer. In contrast, consumption of fermented dairy products was inversely associated with ER-/PR- breast cancer (for consistently high intake ≥3 vs. <1 servings/day HR = 0.28, 95%CI:0.10-0.78), but not clear association was observed for ER+/PR+ (HR = 0.89, 95%CI:0.69-1.14).

Conclusions: In this cohort of mainly postmenopausal women, high long-term consumption of milk was associated with increased risk of ER+/PR+ breast cancer. In contrast, high long-term consumption of fermented dairy products was associated with decreased risk of ER-/PR- breast cancer.