Duration of Lactation and Incidence of Type 2 Diabetes

Abstract

ABSTRACT

Lactation places a considerable metabolic burden on mothers through increasing energy requirements, but there are reports that lactation may lower the risk that type 2 diabetes will develop in the future. The authors report a prospective observational cohort study examining the association between type 2 diabetes and the history of lactation in 83,585 parous women enrolled in the Nurses’ Health Study (NHS). In addition, a retrospective observational cohort study was undertaken in 73,418 parous women taking part in the Nurses’ Health Study II (NHS II). A history of breast feeding was obtained from 64% of women in the NHS cohort and 85% of those in the NHS II cohort. In both groups, the lifetime duration of breast feeding increased with parity. Women who breastfed for longer times were relatively less likely to have a family history of diabetes.

In the NHS cohort, there were 5145 cases of type 2 diabetes during 1,239,709 person-years of follow up in the years 1986–2002. The NHS II cohort included 1132 women diagnosed as having type 2 diabetes during 778,876 person-years of follow up in 1989–2001. An increased duration of lactation was associated with a lower risk of type 2 diabetes. Women in the NHS group who gave birth in the past 15 years had a 15% reduction in the risk of diabetes for each additional year of lactation. The corresponding risk reduction in the NHS II cohort was 14%. These estimates were made while controlling for current body mass index and other risk factors relevant to type 2 diabetes. Women in the NHS II cohort who had a history of gestational diabetes were at markedly increased risk of developing type 2 diabetes, but lactation had no influence on the risk of diabetes in these women.

In these large cohorts of parous women, those reporting an increased duration of breast feeding were at reduced risk of developing type 2 diabetes. Along with clinical reports of improved glucose metabolism in lactating women, these findings suggest that lactation may lower the risk of type 2 diabetes in young and middle-aged women. The physiological factors underlying this association require further study.