Women diagnosed with breast cancer who previously breastfed their babies had a 30 percent overall decreased risk of the disease coming back, as reported in a new Kaiser Permanente study published in the Journal of the National Cancer Institute, and also written up in a press release.
Researchers also discovered that the protective effect of breastfeeding was more evident for tumors of particular genetic subtypes, including the most commonly diagnosed of all breast cancers.
"This is the first study we're aware of that examined the role of breastfeeding history in cancer recurrence, and by tumor subtype," said Marilyn L. Kwan, PhD, research scientist with the Kaiser Permanente Division of Research and lead author of the study.
The study followed 1,636 women with breast cancer who completed a questionnaire that included breastfeeding history, with additional medical data obtained from medical chart reviews and electronic health records. Researchers found breastfeeding to be beneficial among women who were diagnosed with the luminal A subtype of breast cancer, while no significant associations were observed for those with other subtypes. Luminal A tumors include the estrogen-receptor positive (or ER+) breast tumors — the most commonly diagnosed of all breast cancers.
Aside from the decreased risk of breast cancer recurrence for women who breastfed, the study also found that they were 28 percent less likely to die from the disease.
"Women who breastfeed are more likely to get the luminal A subtype of breast cancer, which is less aggressive, and breastfeeding may set up a molecular environment that makes the tumor more responsive to anti-estrogen therapy," Kwan said.
Still, it's not entirely clear why women who breastfeed develop less aggressive tumors than those who don't.
"Breastfeeding may increase the maturation of ductal cells in the breast, making them less susceptible to carcinogens or facilitate the excretion of carcinogens, and lead to slower growing tumors," explained Bette J. Caan, DrPH, senior research scientist with the Kaiser Permanente Division of Research and a senior author of the study.
So what's the takeaway of all this? Breastfeed your kids if you can. It's good for the both of you!