The investigators analyzed data from a prospective observational cohort of 83,585 parous women in the Nurses' Health Study (NHS) and from a retrospective observational cohort of 73,418 parous women in the Nurses' Health Study II (NHS II). The primary outcome was incident cases of type 2 DM.

In the NHS, 5,145 cases of type 2 DM were diagnosed between 1986 and 2002 during 1,239,709 person-years of follow-up. In the NHS II, 1,132 cases were diagnosed during 778,876 person-years of follow-up between 1989 and 2001.

Increasing duration of lactation was associated with a reduced risk for type 2 DM. In the NHS, for each additional year of lactation, women with a birth in the prior 15 years had a decrease in diabetes risk of 15%, after adjustment for current body mass index (BMI) and other relevant risk factors for type 2 DM. In the NHS II, the corresponding decrease in risk was 14%.
Our data on exclusive breastfeeding and duration stratified by parity suggest that the length and intensity of breastfeeding with each pregnancy affect the association with diabetes risk," Dr. Michels and colleagues wrote.

"We found that each year of exclusive breastfeeding was associated with a greater risk reduction than total breastfeeding," they continued. "This may reflect the greater metabolic burden imposed by exclusive breastfeeding."

They also found that longer duration of breastfeeding per pregnancy was associated with a greater benefit. For example: women who breastfed one child for a total of one year had a 44% reduction in age-adjusted risk for diabetes, whereas one-year total breastfeeding duration divided between two children was associated with only a 24% reduction in risk.

"Together with clinical evidence of improved glucose homeostasis in lactating women, these data suggest that lactation may reduce the risk of type 2 diabetes in young and middle-aged women," the investigators concluded.

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