The longer a woman breast-feeds her offspring, the less likely she is to develop type 2 diabetes, Dr. Alison M. Stuebe, and her colleagues have reported.
Their analysis of more than 157,000 women concluded that every year of lactation results in a decreased risk of up to 15% for developing the disease. However, the association wasn't significant for women whose last child was born more than 15 years ago, said Dr. Stuebe of Brigham and Women's Hospital, Boston (JAMA 2005;294:2601-10)
The researchers examined lactation and disease occurrence in women enrolled in the two national Nurses' Health Studies. The studies provided more than 2 million person-years of follow-up from 1986 to 2002.
The Nurses' Health Study (NHS I) included 83,585 women; there were 5,145 incident cases of type 2 diabetes. The NHS II included 73,418 women; there were 1,132 incident cases of type 2 diabetes. In both studies, there was a slight, but significant, inverse association between length of lactation and disease risk.
However, when the researchers analyzed the groups by length of time since giving birth, the impact of breast-feeding became more apparent. Among 857 women who had given birth within the last 15 years, the risk reduction for each year of lactation was 15% in the NHS I and 14% in the NHS II. This association was stronger when women breast-fed for a total of at least 6 months. In the NHS I, the risk reduction was 26% for a cumulative lactation of up to 6 months, 36% for cumulative lactation of up to 1 year, and 53% for cumulative lactation of 23 months or more. The numbers were similar for women in the NHS II.
However, among women who reported their last birth more than 15 years ago, there was no association between the duration of lactation and diabetes. Likewise, there was no association between lactation and disease risk in women with gestational diabetes.
The researchers also found that pharmacological suppression of lactation was associated with an increased risk of developing type 2 diabetes. There was a 46% increased risk of disease in women who used medication to suppress lactation, compared with women who never breast-fed and did not use this type of medication.
The cause of this association is unknown. The investigators suggested that bromocriptine may disrupt mechanisms of appetite regulation or that a choice to suppress lactation may be associated with other behaviors that increase the risk of diabetes.
The decrease in development of diabetes among women who have breast-fed appears linked to lactation's effect on insulin resistance, the researchers said. Studies have shown that lactation is associated with improved glucose tolerance and fasting glucose levels.
