Breast milk may provide the ideal nourishment for an infant, but two recent studies are putting a different spin on the bottle-versus-breast debate, suggesting it is mothers, and not just babies, who may have much to gain from breast-feeding.

One study found that breast-feeding may help protect women from a particularly vicious type of breast cancer. The other suggests that breast-feeding may act as a sort of "reset” button for metabolism after pregnancy, helping women who had gestational diabetes avoid becoming lifelong diabetics.

The findings complement earlier research showing that women who breast-feed have a lower risk for breast and ovarian cancers, Type 2 diabetes, and rheumatoid arthritis. Breast-feeding may also promote cardiovascular health, including a healthy blood pressure.

"This is a win-win — it’s good for the baby, too,” said Dr. Eleanor Bimla Schwarz, a professor of medicine at the University of California, Davis. She called for paid maternity leave, citing earlier research showing that near-universal breast-feeding in the United States could spare an estimated 5,000 women a breast cancer diagnosis every year and cut nearly 14,000 heart attacks. "For all the women who want to do something to reduce breast cancer, this is doable. We need to make sure that’s part of the debate,” she said.

The latest report on the effects of breast-feeding on breast cancer analyzed dozens of studies encompassing nearly 40,000 cancer cases from around the world. The study, published late last month in Annals of Oncology, found that breast-feeding reduced the risk of hormone receptor negative tumors, a very aggressive type of breast cancer, by up to 20 percent. Even a brief period of breast-feeding reduced the risk of these hard-to-treat tumors, which are more common in African-Americans and younger women.

Dr. Marisa Weiss, the paper’s senior author, said that pregnancy and lactation are important steps on the breast’s decades-long path to maturation, with lactation triggering changes in milk duct cells that make the breast more resistant to cancer.

"The breast gland is immature and unable to do its job — which is to make milk — until it goes through the bat mitzvah of a full-term pregnancy,” Dr. Weiss said. "Breast-feeding forces the breasts to finally grow up and get a job, and make milk, and show up for work every day and every night, and stop fooling around.”

But the physiological changes of lactation extend beyond the breast. Some scientists have started referring to breast-feeding as the "fourth trimester” of pregnancy that completes the reproductive cycle, restoring a woman’s body to long-term metabolic and cardiac health.

Women who develop pregnancy-related diabetes, who are at seven times the normal risk of developing diabetes after pregnancy, are typically encouraged to breast-feed because lactation improves glucose metabolism and insulin sensitivity. Breast-feeding also improves lipid metabolism, burns calories and mobilizes the stores of fat that have accumulated during pregnancy (though it does not always lead to much weight loss).

But doctors were unsure whether breast-feeding reduced such women’s odds of developing diabetes long term. So to test that, researchers identified 1,010 women who had developed gestational diabetes during their pregnancies and monitored them closely for two years after the birth. They came from diverse backgrounds; one-third were of Asian heritage, one-third were Hispanic, about 9 percent were black, and 25 percent were white.

Of the 959 mothers who were evaluated, 113 — or nearly 12 percent — had gone on to develop Type 2 diabetes. But those who breast-fed cut their risk by half, and the longer they breast-fed and the more heavily they relied on milk rather than formula the more they lowered their risk.

Those who breast-fed for more than 10 months cut their risk of a diabetes diagnosis by almost 60 percent in the two years they were followed. Of the 205 women who only breast-fed and used no formula for the first two months of the baby’s life, 17 — or 3.9 percent – developed diabetes, compared with 27 women — or 8.79 percent — of the 153 mothers who did not breast-feed and only used formula. The study was published Monday in Annals of Internal Medicine.

"Pregnancy is a metabolic challenge,” said Erica P. Gunderson, the study’s lead author and a senior research scientist at Kaiser Permanente Northern California. During lactation, glucose levels are lower and there is less demand for insulin, she said. "One of the possible biologic explanations is that this gives the woman’s pancreatic beta cells a bit of a break, allowing for recovery from the demands of pregnancy,” she said.

This story originally appeared in The New York Times.