Duration of Lactation and Incidence of Type 2 Diabetes

What is the problem and what is known about it so far?
Approximately 9 million American women have Type 2 Diabetes. It is a disease that affects many facets of life and if left unchecked can lead to blindness, limb amputations, kidney failure and an increased risk of cardiovascular disease. The cost is tremendous and almost $1 out of every $10 spent on health care is related to diabetes.

Why did the researchers do this particular study?
While it is well known that breastfeeding has many benefits for the child who is being nursed, there is less known about its health benefits for the lactating mother. We know that breastfeeding women have higher energy requirements and their metabolism functions at a higher rate. We also know that it is easier for lactating women to have their blood sugar levels controlled than it is for women who are not lactating. Since an inability to regulate blood sugar levels and weight gain both can lead to the development of diabetes, the researchers were interested in finding out if women who breastfeed have a lower risk of getting type 2 diabetes.

Who was studied?
121,700 women age 30 to 55 years from the Nurses Health Study begun in 1976 were studied. A second group of 116,761 women age 25 to 42 years who were part of the Nurses Health Study II which began in 1989 were also studied... Women in both of these groups filled out detailed questionnaires regarding disease and health-related topics at the beginning of their enrollment, every two years thereafter filled out follow-up questionnaires. Questions were related to medical diagnoses and health topics such as pregnancy history, diet, exercise and smoking.

How was the study done?
The questionnaires were examined for information about breastfeeding. The total duration of breastfeeding was calculated based on the number of months after birth that the woman reported stopping breastfeeding. If a woman had several children, the total number of months of lactation from each pregnancy was added together.

Women who reported the diagnosis of diabetes filled out an additional form with questions relating to symptoms, diagnostic tests and treatment. A case of diabetes was confirmed by the researchers if the women met criteria that had been established by the National Diabetes Data Group. The investigators checked that those who reported having diabetes really did have it. The diagnosis was verified by an endocrinologist that 61 of the 62 women that reported having diabetes, indeed had it. Additional information was gathered from the questionnaires about diabetes occurring during pregnancy, diet, height and weight.

What did the researchers find?
The researchers found that there was a lower incidence of type two diabetes in women who had breastfed for longer time periods. They also found that in first time mothers this association occurred after six months of breastfeeding. In other words, breastfeeding reduced the likelihood of getting diabetes and the longer a mother breastfeed the lower the risk was for developing this disease. These results were independent of other risk factors for the development of diabetes such as body mass index, diet, exercise and smoking status.

What were the limitations of the study?
Since the data in this study is based on recall of past events, there is a possibility that women either under or overestimated the length of time that they breastfed. There is still a lot to be learned about the physiology of breastfeeding and its effects on the body, this study because of its design could not account for all the factors that may play a role in the development of diabetes in women.

What were the implications of the study?
The results of this study provide American women with just one more reason why they should breastfeed their children. Based on the results of this study, it is likely that if more women breastfed there would be a lower incidence of diabetes among American women. This would in turn improve the quality of life for those women who might otherwise develop diabetes and save millions of health care dollars. It also provides more evidence to support the recommendations by many national and international health organizations for women to breastfeed their children exclusively for the first 6 months of their life and to continue breastfeeding for at least twelve months. Breastfeeding provides benefits for both the child and the mother and its advantages continue to be discovered.

Summarized by Jean Andersson Swayze, MD, Family Medicine Resident, Fletcher Allen Health Care.
Summarized from "Duration of Lactation and Incidence of Type 2 Diabetes". Stuebe A. M. et al., Journal of the American Medical Association, November 23, 2005, Volume 294, Number 20, pages 2601-2610.