Women who breastfeed longer have a lower risk of developing type 2 diabetes, according to a study in the November 23/30 issue of JAMA.

Type 2 diabetes mellitus affects about 9 million adult women in the United States, according to background information in the article. The disease and its complications impose a considerable burden on the health care system, absorbing $1 of every $10 spent on health care. Multiple lifestyle factors, including diet, exercise, and obesity, are associated with risk of diabetes.

Previous studies have demonstrated improved insulin sensitivity and glucose tolerance during lactation compared with nonlactating mothers. Although these and other findings have suggested that maternal lactation may reduce future risk of type 2 diabetes, no study has directly examined this association.

Alison M. Stuebe, M.D., of Brigham and Women’s Hospital and Harvard Medical School, Boston, and colleagues studied the association between lactation duration and development of type 2 diabetes. The researchers analyzed data on 83,585 parous (having given birth) women who were part of the Nurses’ Health Study (NHS) and 73,418 parous women who were in the Nurses’ Health Study II (NHS II).

In the NHS, 5,145 cases of type 2 diabetes were diagnosed between 1986 and 2002, and in the NHS II, 1,132 cases were diagnosed between 1989 and 2001. In analyses restricted to women who reported a birth in the past 15 years and controlling for current body mass index and other relevant risk factors for type 2 diabetes, there was a 15 percent reduced risk of diabetes in the NHS and 14 percent reduced risk in the NHS II per additional year of breastfeeding. Among women who reported their last birth more than 15 years ago, there was no association between duration of lactation and type 2 diabetes in the NHS II and a substantially reduced association in the NHS.

“In conclusion, increased duration of breastfeeding was associated with reduced risk of type 2 diabetes in 2 large cohorts of women. Together with clinical evidence of improved glucose homeostasis [equilibrium] in lactating women, these data suggest that lactation may reduce the risk of type 2 diabetes in young and middle-aged women. Further clinical studies are needed to confirm this finding and to elucidate the physiologic mechanisms for an inverse association between duration of breastfeeding and risk of type 2 diabetes,” the authors write.

(JAMA.2005; 294:2601-2610. Available pre-embargo to the media at http://www.jamamedia.org)