Breast-feeding may reduce a woman’s risk for Type 2 diabetes, a new study reports.

Researchers followed 1,238 women, average age 24 at the start, for up to 30 years. Each delivered at least one baby, and none had diabetes before the study began. The scientists collected data on health and lifestyle at interviews and physical examinations conducted seven times over the course of the project. The study is in JAMA Internal Medicine.

There were 182 cases of diabetes, and after adjusting for physical activity, smoking and other factors, the researchers found that breast-feeding for up to six months was associated with a reduced risk for diabetes of 25 percent; breast-feeding for six to 12 months was tied to a 48 percent reduced risk; and breast-feeding for 12 months or more with a 47 percent risk reduction. The associations held for obese women and for those who had had gestational diabetes, both strong risk factors for Type 2 diabetes.

There are several plausible mechanisms, the researchers say. For example, lactating women have lower circulating glucose, and lactation may help preserve the function of the cells that produce insulin.

“We’ve known for a long time that breast-feeding has major benefits for child health,” said the lead author, Erica P. Gunderson, an epidemiologist with Kaiser Permanente in Oakland, Calif. “The specific benefits for women’s health have been less recognized.”