Exposure to breast milk in infancy and adult breast cancer risk.

Background: There is considerable interest in the possibility of an infectious etiology for human breast cancer. Although studies have shown that certain strains of mice transmit mammary tumor virus via breast milk, few epidemiologic studies have addressed this topic in humans. Methods: We evaluated the relationship between having been breast-fed as an infant and breast cancer risk among 8299 women who participated in a population-based, case-control study of breast cancer in women aged 50 years or more. Case women were identified through cancer registries in three states (Massachusetts, New Hampshire, and Wisconsin); control women were identified through statewide driver's license lists (age <65 years) or Medicare lists (ages 65-79 years). Information on epidemiologic risk factors was obtained through telephone interview. We used multiple logistic regression to assess having been breast-fed and maternal history of breast cancer in relation to breast cancer occurrence both in premenopausal women (205 case women; 220 control women) and in postmenopausal women (3803 case women; 4071 control women). Results: We found no evidence that having been breast-fed increased breast cancer risk in either premenopausal women (odds ratio [OR] = 0.65; 95% confidence interval [CI] = 0.41-1.04) or postmenopausal women (OR = 0.95; 95% CI = 0.85-1.07). In addition, breast cancer risk was not increased by having been breast-fed by a mother who later developed breast cancer. Conclusion: Our results do not support the hypothesis that a transmissible agent in breast milk increases breast cancer risk. Because premenopausal women were not well represented in our study population, our findings with regard to this group may not be generalizable and should be viewed with caution.