Dr. Alison Stuebe suspected that something good was happening to her patients who breastfed their children, something beyond the usual health benefits that have cropped up in the medical literature.

One thing she noticed: Diabetic mothers who nursed their infants required less insulin than others who bottle-fed. Perhaps, she thought, the act of producing milk protects women in general from type 2 diabetes, a disease that is dangerously on the rise across America.

To find out if her theory was solid, the Boston physician turned to the Harvard-run Nurses Health Study, which has tracked thousands of nurses over the past 29 years -recording information about diet, exercise, health and illness. For the diabetes study, researchers were able to mine data on 150,000 women who had given birth.

"Lo and behold, when we plugged in lactation duration and the risk of diabetes, there was a clear dose response - the longer a woman breastfed, the lower her risk of diabetes," said Stuebe, a maternal-fetal specialist at the Brigham and Women's Hospital.

To Stuebe, who breastfed her son, Noah, while toiling through her medical residency, the study carries a strong message - not only for new mothers but for society as well.

"This isn't just about breastfeeding women, but their families and friends and people in the park who give you dirty looks when you breastfeed your baby," she said.

Stuebe, whose study appeared in the Journal of the American Medical Association in late November, cautioned that the study needs to be replicated in a broader population before the link between nursing and diabetes can be considered fact. There is always the possibility, she said, that nurses -who tend to be more health-conscious than average Americans -are doing something else to prevent diabetes that escaped the researchers' notice.

It will also be important to study the possible biological mechanisms behind the phenomenon, she said. That could be done by collecting blood samples from mothers and looking for differences among women who breastfeed for shorter or longer durations - or not at all.

Type 2 diabetes is a condition in which the body doesn't produce enough insulin or its cells don't use the insulin the body does make. Insulin is a chemical needed to break down the sugar glucose into a usable form. Abnormally high glucose levels in the blood can damage the eyes, heart, kidneys and circulatory system.

In her study, Stuebe and her Boston colleagues found that the risk of developing type 2 diabetes dropped by 15 percent for every year a women breastfed. That meant, for instance, that a mother who nursed two babies for six months each lowered her risk by that amount.

And the benefit lasted years after delivery. Researchers measured the protective effect among women who had delivered at least one child during the preceding 15 years. Whether the benefits last longer than that is open to question, though Stuebe suspects they wear off over time.

During the first half of the 20th century, breastfeeding fell victim to modern notions of convenience and propriety. By mid-century, only a quarter of American mothers left the hospital nursing their babies; the majority turned to infant formula.

The trend reversed later in the century, with 70 percent of today's mothers breastfeeding, at least initially, and 37 percent for the six months recommended by obstetricians, according to the U.S. Centers for Disease Control and Prevention.

Credit for the change goes in large part to the natural-childbirth movement, which stressed benefits to babies and mothers alike.

Perhaps better known are breastfeeding's advantages to babies, which include a healthier immune system and, consequently, fewer ear infections, rashes, allergies and diarrheal illnesses. Over the years, doctors have documented benefits to the mothers, too, which include a lowered risk of breast cancer, less postpartum bleeding and an easier time losing weight after pregnancy.

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