In women with a first-degree family history of breast cancer, breastfeeding may protect against the development of premenopausal breast cancer, according to results from a prospective cohort study of more than 60,000 women.
Analysis of data from the Nurses' Health Study II revealed that ever having breastfed appeared to reduce the risk of breast cancer by 59% in women with a family history of breast cancer, study investigators reported.
This reduction is comparable to that seen with hormonal treatments such as tamoxifen for these high-risk women, wrote Dr. Alison M. Stuebe of Brigham and Women's Hospital, Boston, and her associates. Only women who had a first-degree relative with breast cancer showed the benefit of breastfeeding.
“These data suggest that women with a family history of breast cancer should be strongly encouraged to breastfeed,” they noted.
The investigators assessed the relationship between breastfeeding intensity and later development of breast cancer in a subset of 60,075 subjects from the Nurses' Health Study II. These women had completed a detailed questionnaire on lactation in a 1997 assessment of the longitudinal study, and then continued to be followed every 2 years through 2007. A total of 68 cases of premenopausal breast cancer were diagnosed, with a mean age at diagnosis of 46 years.
Women who had ever breastfed for at least 1 month showed a lower risk of developing breast cancer than those who had never breastfed, but this benefit was restricted to those who had a first-degree relative with the disease.
“We found a 59% reduction in incidence of premenopausal breast cancer” in this subgroup of study subjects.
“No other prospective study to our knowledge has examined whether family history modifies the association between breastfeeding and breast cancer risk,” the researchers said.
“As in any observational study, we cannot exclude the possibility that unmeasured confounding explains the observed difference in incident disease, but such a confounder would have to be strongly associated with both breastfeeding and breast cancer risk to produce an association of this magnitude,” Dr. Stuebe and her colleagues wrote (Arch. Intern. Med. 2009;169:1364–71).
There was no association between incident breast cancer and duration of lactation, use of supplemental feedings, number of children who were breastfed, or maternal age at first birth.
The use of medication to suppress lactation modified the association between breastfeeding and incident cancer.
“We found a lower incidence of breast cancer among women who had never breastfed but had suppressed lactation than among those who had neither breastfed nor suppressed lactation,” the researchers said.
They attributed this finding to disordered involution in women who neither breastfed nor suppressed lactation. “During involution, a highly coordinated process of apoptosis, remodeling, and inflammation returns mammary tissue to its prepregnant state. In physiologic weaning, this process occurs over weeks or months.
“By contrast, if a woman does not breastfeed, she experiences abrupt engorgement, and mammary tissue may become progressively inflamed. We hypothesize that both breastfeeding and use of suppressive medications prevent this inflammation, thereby preventing disordered involution,” they explained.
No financial conflicts of interest were reported for this study.