Breast-feeding cuts moms’ risk of breast cancer, study shows

Left to right -- Taking advantage of the feeding rooms, Theresa McNulty breastfeeds 8 month old daughter, Ivy, and chats with new mom Jennifer Barulich who also feeds her 5 week old son, Jacob while another baby, 15 month old Brynn Waldman, plays nearby at Burlingame Breastfeeding Center in Burlingame, Calif., on Tuesday, April 28, 2015. less Left to right -- Taking advantage of the feeding rooms, Theresa McNulty breastfeeds 8 month old daughter, Ivy, and chats with new mom Jennifer Barulich who also feeds her 5 week old son, Jacob while another ... more Photo: Amy Osborne, The Chronicle Buy photo Photo: Amy Osborne, The Chronicle Image 1 of / 4 Caption Close Breast-feeding cuts moms’ risk of breast cancer, study shows 1 / 4 Back to Gallery

Breast-feeding is not only good for babies but appears to be beneficial to mothers who are diagnosed with breast cancer later in life, a new Kaiser Permanente study of hundreds of Northern California women has found.

Looking at the medical histories of more than 1,600 women diagnosed with breast cancer, researchers discovered that those who had breast-fed had a 30 percent lower risk of recurrence of the disease after effective treatment and tended to have less aggressive tumor types than those who had not breast-fed.

“We’re not looking at the association of breast-feeding on risk of breast cancer but on the (question): If you breast-fed early on and developed breast cancer, what is your prognosis?” said Marilyn Kwan, research scientist with Kaiser’s Division of Research in Oakland and lead author of the study.

The researchers saw improved breast-cancer outcomes among women who had breast-fed, but in addition found that those who breast-fed for a total of six months or longer fared even better throughout the nine-year follow-up period, which ended last year.

The study found women who had breast-fed for at least six months had a 37 percent reduced risk of breast-cancer recurrence after treatment. Researchers also observed that women who breast-fed were 28 percent less likely to die from the disease than those who never did. The rate jumped to 39 percent among those who breast-fed six months or more.

The study, published Tuesday in the Journal of the National Cancer Institute, builds on previous evidence that shows breast-feeding appears to have a protective effect against breast cancer.

The reasons why aren’t entirely clear, but health experts say breast-feeding delays the return of the mother’s menstrual cycle, which in turn reduces risk. It also fully develops the ductal cells, which help protect the breast.

Lower estrogen

“You’re not returning to your regular menstrual cycles, so you have lower levels of estrogen compared to a woman who is regularly menstruating,” said Kwan, who explained that the hormone can promote breast-cancer cell growth. “Breast-feeding also makes your ductal cells in the breast mature, and many have hypothesized that makes your breasts more resistance to carcinogens.”

Kwan’s study is considered the first to look at breast-feeding’s potential effect on recurrence and survival based on different subtypes of breast cancer.

Most of the women who breast-fed were diagnosed with breast cancers that feed off of hormones: estrogen, progesterone or both. About 80 percent of all women are diagnosed with this subtype, called luminal A, which generally has better outcomes than other forms of breast cancer because the tumors are less aggressive, and they respond well to hormonal therapy and other treatments.

The study did not find significant risk associations between breast-feeding and other types of breast cancers, including those that are hormone-responsive and feed off a protein known as HER2 or those that do not respond to any of those factors, a subtype known as basal-like or triple negative.

Of course, lower risk doesn’t mean no risk, as many women who have breast-fed their babies are eventually diagnosed with breast cancer.

Audrey Blaker of Mill Valley breast-fed her daughter for more than six months after her birth 30 years ago. In 2007, Blaker was diagnosed at age 57 with luminal A-type breast cancer, the most common type.

Now 65, Blaker said she was relieved to learn that something she did decades earlier for the health of her baby may be helping her now. She underwent treatment, which included surgery and chemotherapy, and the disease has not returned.

Living example

“This is very good news for me, because I do not want my breast cancer to come back,” said Blaker, who is participating in a long-term Kaiser study on breast-cancer patients and whose data was likely used in this study.

Lactation specialists and health experts say they will use the study results to encourage new mothers to breast-feed. About 77 percent of new U.S. mothers breast-feed, according to the U.S. Centers for Disease Control and Prevention. U.S. breast-feeding rates hit a low point in the 1970s and have generally been on the rise since then. The CDC’s 2014 report showed California with among the nation’s highest breast-feeding rates at nearly 93 percent, with 63 percent of moms in the state breastfeeding for at least six months.

“The message has been getting out about the value to babies of breast-feeding. It really needs to get out about the value to the mom,” said Dr. Susan Kutner, a breast surgeon at Kaiser’s San Jose hospital and chairwoman of Kaiser’s Northern California’s Breast Care Task Force. She was not directly involved in the study.

To Robbie Gonzalez-Dow, executive director of the California Breastfeeding Coalition, a lactation education nonprofit, the study results made biological sense.

“There’s so much we still don’t know,” she said. “But when breast cells are able to do what they’re supposed to do, they’re fulfilling their function.”

Avid breast-feeder

At 25, Maggie Kler’s concerns about breast cancer are far in the future, but she is breast-feeding her first baby, 9-month old Michael Janakos-Kler, and said she will until he stops naturally.

Kler, a certified lactation educator who works at Healthy Horizons Peninsula Breastfeeding Center in Burlingame, said she was aware that breast-feeding benefits the mom, but she was surprised to learn it could have such a dramatic effect on recurrence rates down the road.

“I know it’s a good thing to do,” she said. “It’s what you’re supposed to do.”

Victoria Colliver is a San Francisco Chronicle staff writer. E-mail: vcolliver@sfchronicle.com

Twitter: @vcolliver