Breast milk doesn't pass cancer virus from Mom to baby

Though several animal studies have suggested that breast cancer might in certain cases be triggered by a virus—perhaps one transmitted in breast milk—this does not appear to hold for people, a new study finds.

Until now, the idea that Mom could pass on some infectious cancer agent while nursing has received little scrutiny in studies of people. That possibility, however, would go a long way toward explaining why an elevated risk of breast cancer, especially early-onset cancer, tends to run in families. Moreover, it’s bolstered by a recent finding that genetic material isolated from the tumors of many breast cancer patients resemble the gene for a virus that causes mammary cancers in mice.

Several demographic trends, however, have strongly argued against the link. For instance, breast cancer risk tends to be low in nations where breast-feeding rates are high, while the malignancy is common in the United States and other Western nations where breast-feeding rates have been low and are waning.

To try to resolve the conundrum, researchers from Dartmouth Medical School, the Harvard School of Public Health, and the University of Wisconsin Comprehensive Cancer Center teamed up to investigate the provocative thesis in a group of 8,300 women, age 50 years or older. Each had participated in a tri-state breast cancer study. Nearly half of the women had been treated for the malignancy. The rest, who were matched to those women by age and state of residence, remained cancerfree.

Linda Titus-Ernstoff of Dartmouth and her colleagues found that 77 percent of the women from each group said they knew whether they had been breast-fed as infants. Based on the information that each of these women provided, the researchers found no evidence that a daughter’s cancer risk increases if she was breast-fed by a mother who would later go on to develop breast cancer. "We found no evidence of a positive association between having been breast-fed and risk of cancer," they report.

Indeed, they conclude in the June 17 Journal of the National Cancer Institute, "These findings argue against the presence of a transmissible agent in breast milk that increases risk of human breast cancer."

That study did not address whether breast-feeding affected the nursing mother’s cancer risk. In an earlier investigation headed by several of the new study’s authors, women who breast-fed their infants appeared to face a 20 percent lower risk of breast cancer than women who bottle-fed their children. The benefits were even higher for women who nursed infants during their teenage years.

The latest study’s findings also do not rule out the possibility that chemical carcinogens might be passed to the next generation through breast milk. Our bodies store in fat a number of toxic estrogen-mimicking chemicals—including dioxins, polychlorinated biphenyls, and the breakdown products of DDT. During lactation, a woman’s body uses this fat to enrich the energy content of breast milk. Any pollutants stored in the fat also pass to the baby with the fat in that milk. Because a woman’s cumulative lifetime exposure to estrogen is one of the best established risk factors for breast cancer, there is a growing suspicion that exposure to estrogen mimics may also pose a cancer risk.

To date, the jury remains out on the carcinogenic potential of those pollutants. However, there’s no doubt that babies are being exposed. Numerous studies have detected these compounds in the fat of people throughout the industrial world—and in breast milk.