Breastfeeding Lowers Mom's Diabetes Risk
By Neil Osterweil, Senior Associate Editor, MedPage Today
Reviewed by Zalman S. Agus, MD; Emeritus Professor at the University of Pennsylvania School of Medicine.
November 22, 2005
BOSTON, Nov. 22 - In addition to all the advantages for infants, breastfeeding may protect mothers against type 2 diabetes later in life, according to a pair of large studies.
Each additional year of breastfeeding reduces the mother's chance of developing type 2 diabetes later on by about 15%, according to Karin Michels, Sc.D., Ph.D., and colleagues at Brigham and Women's Hospital and Harvard Medical School.
"We found that duration of lactation was inversely associated with risk of type 2 diabetes in young and middle-aged women, independent of other diabetes risk factors, including body mass index, diet, exercise, and smoking status," they wrote in the Nov. 23 issue of Journal of the American Medical Association. This association appeared to wane with time after the last birth.
The investigators drew their conclusions by studying data on 157,003 women enrolled in the Nurses' Health Study (NHS) and NHS II, two large-scale, prospective longitudinal studies of women's health.
Evidence for a possible protective effect of breastfeeding against type 2 diabetes had come from human and animal studies showing that both insulin sensitivity and glucose tolerance are improved among lactating mother compared with controls, and these differences were independent of weight loss among the mothers, the authors noted.
To see whether there is an association between lactation history and the rate of type 2 diabetes, the authors looked prospectively at 83,585 women in the NHS, and retrospectively at 73,418 women in the NHS II.
The NHS, started in 1976, enrolled 121,700 women between the age of 30 and 55 years in 11. Participants completed detailed baseline questionnaires regarding diseases and health-related topics, and then every two years they completed follow-up questionnaires regarding medical diagnoses and health-related topics, including pregnancy history, diet, exercise, and smoking.
The NHS II began enrolling women in 1989. It includes 116,671 women between the ages of 25 and 42 years from 14 states. As in the first study, participants completed a baseline questionnaire and fill out followup questionnaires every two years. This study also included detailed data on lactation history, allowing for closer analysis of responses.
The authors found that there were 5,145 cases of type 2 diabetes diagnosed during 1,239,709 person-years of follow-up, and 1,132 during 778,876 cumulative years of followup in the NHS II.
In an analysis of only those women who had given birth within the previous 15 years, the investigators found that the covariate adjusted hazard ratio for diabetes in the NHS was 0.85 (95% CI, 0.73-0.99), and in NHS II was 0.86 (95% CI, 0.79-0.93) for each additional year of breastfeeding, when body mass index and other diabetes risk factors were taken into account.
Among women who last gave birth more than 15 years before, however, the covariate adjusted analysis in the NHS found a "substantially reduced association, and in the NHS II no association between breast-feeding and diabetes risk reduction.
The authors noted that breastfeeding places a large metabolic burden on the body, resulting in increases in insulin sensitivity and glucose tolerance, and higher carbohydrate use and total energy expenditure, suggesting mechanisms for its protective effects.
"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.
Primary source: Journal of the American Medical Association
Stuebe AM et al. Duration of Lactation and Incidence of Type 2 Diabetes JAMA. 2005;294:2601-2610