Duration of lactation and incidence of type 2 diabetes.

CONTEXT Lactation is associated with improved glucose and insulin homeostasis, independent of weight change. OBJECTIVE To evaluate the association between lactation history and incidence of type 2 diabetes. DESIGN, SETTING, AND PARTICIPANTS Prospective observational cohort study of 83,585 parous women in the Nurses' Health Study (NHS) and retrospective observational cohort study of 73,418 parous women in the Nurses' Health Study II (NHS II). MAIN OUTCOME MEASURE Incident cases of type 2 diabetes mellitus. RESULTS In the NHS, 5145 cases of type 2 diabetes were diagnosed during 1,239,709 person-years of follow-up between 1986 and 2002, and in the NHS II, 1132 cases were diagnosed during 778,876 person-years of follow-up between 1989 and 2001. Among parous women, increasing duration of lactation was associated with a reduced risk of type 2 diabetes. For each additional year of lactation, women with a birth in the prior 15 years had a decrease in the risk of diabetes of 15% (95% confidence interval, 1%-27%) among NHS participants and of 14% (95% confidence interval, 7%-21%) among NHS II participants, controlling for current body mass index and other relevant risk factors for type 2 diabetes. CONCLUSIONS Longer duration of breastfeeding was associated with reduced incidence of type 2 diabetes in 2 large US cohorts of women. Lactation may reduce risk of type 2 diabetes in young and middle-aged women by improving glucose homeostasis.