he more a woman breast-feeds, the lower her risk of diabetes, according to a study published last week in the Journal of the American Medical Association.

This is the first time a link has been found between breast-feeding and health benefits for the mother, said Karin B. Michels, the study’s senior author and associate professor in the Department of Epidemiology at the Harvard School of Public Health (HSPH).

“We know that breast-feeding is good in particular for the child, but this study adds evidence that it is also good for the mother,” she said.

The study found that, after data had been controlled, each additional year of breast-feeding brought a drop of about 15 percent in the risk of type 2 diabetes.

The paper did not attempt to explain what produced the apparent decrease in diabetes risk. Michels postulated that perhaps lactation has a beneficial effect on insulin resistance, which is a key factor in the development of diabetes. Further study of hormonal and insulin mechanisms will be necessary to determine the exact cause, she said.

The study’s authors arrived at their conclusion after analyzing existing data from two major women’s health studies, both of which used nurses as subjects. The first of these studies, called the Nurses’ Health Study I, began in 1976 with 121,700 women. The second, the Nurses’ Health Study II, began with 116,671 in 1989. The two studies were observational, relying on participants to self-report data about their health and breast-feeding habits.

Michels acknowledged that “confounding variables”—other influences that can lead to false conclusions—are always a concern in observational research. She said, though, that she was confident in the integrity of her conclusions.

“We controlled for lifestyle factors—a fairly large number of variables that indicated particular lifestyles, such as physical activity and body-mass index,” she said. She said the alternative to an observational study, a randomized trial, would be impractical given the nature of breast-feeding.

Meir Stampfer, chair of HSPH’s Department of Epidemiology and a member of Michels’ research group, said that he found the study’s findings credible.

“I think it’s a sound finding,” he said. “We know a lot now about prevention of diabetes. It’s interesting to have another benefit for breast-feeding in terms of diabetes prevention.”

According to the American Diabetes Association, type 2 is the most common form of diabetes. It generally occurs in adults, but has recently been diagnosed in younger patients as well. Those diagnosed with the disease tend to be obese.

The study comes at a crucial time: 9 million women currently suffer from type II diabetes, according to the authors.

“Right now in the U.S., there’s an epidemic of diabetes that’s entirely preventable,” said Stampfer.

The study’s implications are also timely given the growing proportion of breast-feeders nationwide. While Americans breast-fed least during the 1960s, the past decade has brought a resurgence in its popularity, according to Michels.

In the study, the authors cited a recommendation from the American Academy of Pediatrics that women breast-feed for a minimum of a year. Michels said she did not see a reason to stop there.

“[Women] should breast-feed for as long as they possibly can,” she said.

Although she said she hoped women would take the study’s findings to heart, she said she realized that jobs and other obstacles can prevent some women from breast-feeding as long as they should, she said.