Nursing protects moms from breast cancer.

What's good for baby may be good for mom as well. Mother's milk is known to provide newborns with crucial immunity to disease-causing microbes. Now, a pair of studies suggests that mothers who choose to breast-feed their infants may gain an important edge against breast cancer that strikes before menopause.

Previous research had shown no consistent link between nursing and a woman's chance of developing breast cancer. Some studies had demonstrated that women who nurse gain protection against breast cancer; others failed to show that benefit. The inconsistent results might be explained by the low statistical power of some of the studies, which included only small numbers of premenopausal women.

To clear up the confusion, Polly A. Newcomb of the University of Wisconsin Comprehensive Cancer Center in Madison and her colleagues decided to conduct a large-scale study of breast-feeding and breast cancer risk. Twenty-five percent of the women studied by Newcomb's group were premenopausal.

The multicenter team first searched tumor registries in Wisconsin, Massachusetts, Maine, and New Hampshire for women age 74 or younger who had been diagnosed with breast cancer. The researchers then recruited women in the same age range who had never suffered from the disease. After excluding women who had never had a child, the team focused on 5,878 women with breast cancer and 8,216 controls.

Interviewers then asked participants a series of questions about their reproductive history.

A statistical analysis of the data revealed that women who had nursed their infants had, on average, a 20 percent lower risk of getting breast cancer before menopause than mothers who had not nursed. That reduction held true even when the team adjusted for other factors linked to breast cancer, such as a family history of the disease.

A 20 percent risk reduction may appear modest, but it could contribute significantly to the fight against premenopausal breast cancer, which accounts for slightly less than one-quarter of all U.S. breast cancer cases.

"Any reduction in risk could be important," Newcomb says. Knowing that breast-feeding offers some security against breast cancer might make a big difference to young women, who currently have very few options for preventing or reducing their risk of dying from breast cancer, adds coauthor Walter C. Willett of the Harvard School of Public Health in Boston.

The study finds that very young women gain the most protection from breastfeeding a child. A woman age 19 or younger who nursed her infant for a period of six months had about a 50 percent reduction in premenopausal breast cancer risk. The researchers describe their findings in the Jan. 13 NEW ENGLAND JOURNAL OF MEDiCiNE (NEJM).

For women, such as working mothers, who can't breast-feed for extended periods, the study offers some reassuring news. Even women who nursed for a relatively short period (from four to six months) had a "substantial" reduction in their breast cancer risk, Newcomb says.

A second study, this one by Richard P. Gallagher of Canada's British Columbia Cancer Agency in Vancouver and his coworkers, indicates that women who breast-feed for two months or longer are shielded from premenopausal breast cancer. GaIlagher's team interviewed 1,018 women with breast cancer and 1,025 controls; over 30 percent of the women were premenopausal. They present their findings in the just-released Dec. 15 AMERICAN JOURNAL OF EPIDEMIOLOGY.

Data from both studies fit a controversial theory that nursing may guard against breast cancer by interrupting ovulation, a process marked by the release of powerful hormones (SN: 10/31/92, p.298). Or, milk production may cause physiological changes that leave breast tissue resistant to carcinogens, speculates Esther M. John, an epidemiologist at Stanford University School of Medicine. John, along with Stanford's Jennifer L. Kelsey, wrote an editorial that accompanies the NEJM report.

Breast-feeding appears to protect women only against breast cancer that strikes before menopause. Both studies report no link between nursing and the risk of postmenopausal breast cancer. Thus, it is important for women age 50 and older to get regular mammograms, which can reduce their risk of dying from breast cancer, Newcomb points out.

Why would breast-feeding protect against premenopausal breast cancer and not against breast cancer that hits a woman after menopause? Unfortunately, no one knows the answer to that question, Newcomb says.