Breastfeeding may cut a mother's risk of type 2 diabetes, research suggests.
Breast milk is highly nutritious
A team from Harvard Medical School found one year of breastfeeding was associated with a 15% drop in a woman's risk of developing type 2 diabetes.
Each additional year of breastfeeding was associated with a greater reduction in risk.
The study, published in the Journal of the American Medical Association, found the protective effect lasted for up to 15 years after a woman's last birth.
The findings suggest that if a woman had two children and breastfed each child for a year, her risk of diabetes would be reduced by a third.
However, women with gestational diabetes did not appear to lower their risk of type 2 diabetes, even if they breastfed intensely.
The study also suggested that women who used medications to prevent lactation had an increased risk for type 2 diabetes.
Previous studies have suggested a link between breastfeeding and improved insulin sensitivity and glucose tolerance - both traits which reduce the risk of diabetes.
The Harvard team analysed data on more than 157,000 mothers enrolled in two separate studies.
Lead researcher Dr Alison Stuebe said: "We've known for a long time that breastfeeding is good for babies.
"In this study, we found that it's good for mums too.
"Based on these findings, we have one more reason to encourage mothers to breastfeed." The production of milk requires a breastfeeding mother to use an average of 500 calories each day - the equivalent of running four to five miles.
The additional energy required for lactation is associated with short-term changes in insulin and glucose.
Dr Stuebe said: "Our study supports the theory that breastfeeding may be associated with important metabolic changes that influence diabetes risk.
"However, more research is needed to determine what hormonal and biological factors are involved."
Roopinder Brar, a care advisor at Diabetes UK, said breastfeeding had been shown to help the health of the baby and mother in a variety of ways.
"Women who give birth to large babies and those who develop gestational diabetes during pregnancy have an increased risk of developing Type 2 diabetes later in life.
"A balanced diet and regular physical activity have already been proven to reduce this risk, but we will need to wait for the results of further research before we can see if breastfeeding also plays a part."
The World Health Organization recommends that mothers feed their babies exclusively on breast milk for six months, and continue to offer it alongside other food for at least two years.