Breast-Feeding Cuts ER-Neg Breast Cancer Risk in Black Women

Roxanne Nelson

September 24, 2014

Breast-feeding might lower the risk for aggressive breast cancer in black women in the United States. A new study shows that parous women are at higher risk for estrogen-receptor (ER)-negative and triple-negative breast cancer, but that lactation might ameliorate the effects of pregnancy and childbirth.

Parous women had a 33% higher risk for ER-negative breast cancer than those who had never given birth, and a 37% higher risk for triple-negative breast cancer. However, breast-feeding lowered the risk for both ER-negative and triple-negative disease.

The study was published online September 15 in the Journal of the National Cancer Institute.

These data, note the researchers, might partially explain why black women are disproportionately affected by ER-negative breast cancer; although they typically have more children than white women, they have a lower prevalence of lactation. In addition, for every age category in the United States, the incidence of triple-negative breast cancer is higher in black women than in non-Hispanic white women.

These results are "exciting because this is something that can be acted upon, where we can actually prevent some cases of these often-deadly cancers," said researcher Christine Ambrosone, PhD, chair of the Department of Cancer Prevention and Control at the Roswell Park Cancer Institute in Buffalo, New York, in a statement.

"We already know that breast-feeding has so many benefits to babies and their mothers," said Dr. Ambrosone. "This is one more reason to encourage and support breast-feeding, particularly in African-American women."

However, in an accompanying editorial, Amanda Phipps, PhD, and Christopher Li, MD, PhD, from the University of Washington and the Fred Hutchinson Cancer Center in Seattle, make the point that this is based on the assumption that breast-feeding is causally protective against triple-negative breast cancer, which is not supported by biologic evidence.

Parity Linked to Risk

Some data suggest that parous women have a lower risk for breast cancer, but this appears to primarily apply to ER-positive breast cancer. Dr. Ambrosone and colleagues point to emerging evidence that suggests the opposite for ER-negative breast cancer — that pregnancy and childbirth might actually increase the incidence, but that breast-feeding might lower the risk.

They note that although the rates of breast-feeding have improved in the United States during the past 3 decades, black women as a group have had the smallest increase. For example, in the most recent report on breast-feeding prevalence, the percentage of white infants who had ever breast-fed was 75.2%, whereas the percentage of black infants was 58.9%.

The researchers investigated the relation of parity and lactation to specific subtypes of breast cancer in black women living in the United States. Their data came from the African American Breast Cancer Epidemiology and Risk (AMBER) Consortium, a collaboration of 4 epidemiologic studies.

Their cohort involved 14,180 control subjects, 2446 ER-positive breast cancer case patients, and 1252 ER-negative breast cancer case patients (567 of whom could be classified as triple-negative).

Overall, 43% of the control subjects who had given birth had breast-fed at least 1 baby, with similar proportions in the 4 studies. The multivariable odds ratio (OR) for ER-positive breast cancer in parous women, compared with nonparous women, was 0.92. There was no significant trend with increasing number of births, and breast-feeding was not associated with a reduced risk for ER-positive disease.

Conversely, parity was associated with an increased risk for ER-negative breast cancer (multivariable OR, 1.33). The risk increased as the number of births increased (P trend = .04), but lactation was associated with a lower risk (OR, 0.81).

The OR for triple-negative breast cancer in parous women, compared with nonparous women, was 1.37. The OR for ER-negative disease in women who breast-fed, compared with those who did not, was 0.81.

Effects of Parity and Lactation

The researchers evaluated the joint effects of parity and lactation in an analysis that was restricted to women who had given birth. The reference category was women who had delivered only 1 child and who had breast-fed.

In women who had never breast-fed, the risk for ER-negative breast cancer increased markedly as the number of deliveries increased. The ORs ranged from 1.22 for 1 delivery to 1.68 for 4 or more deliveries. Conversely, the ORs for women who had breast-fed were considerably lower, and were below 1.2 for all categories except women who had delivered 4 or more children (OR, 1.33).

There was no evidence of an association between triple-negative breast cancer and breast-feeding, but the OR for women with 4 or more deliveries who had not breast-fed was 1.51.

Lower Risk by Two-Thirds

In their editorial, Drs. Phipps and Li suggest that if black women breast-fed at the same rate as white women, the incidence of triple-negative breast cancer in parous women could be lowered by almost two-thirds.

"Of course, the assumption that breast-feeding is causally protective against triple-negative breast cancer is a nontrivial assumption not yet supported by biological evidence," they write.

They point out that the vast majority of triple-negative breast cancers, including all such cancers in nulliparous women, are likely to be unrelated to breast-feeding history. "However, given that the majority of African American women are parous, and given that age-specific incidence rates of triple-negative breast cancer are 50% to 100% greater in African American women than in non-Hispanic white women, an impact fraction of 66% has the potential to be of considerable public health significance," they explain.

There are already numerous reasons to encourage breast-feeding, and work-place and healthcare infrastructure that better supports breast-feeding in new mothers should be encouraged, the editorialists add. "Perhaps additional motivation can be found in the goal of diminishing disparities in triple-negative breast cancer incidence."

The study was supported by grants from the National Institutes of Health

J Natl Cancer Inst. Published online September 15, 2014. Abstract, Editorial

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