The Telegraph reports that breastfeeding lowers the risk of post-pregnancy metabolic syndrome, diabetes and heart disease.
Also nursing mothers who breastfeed for longer are helping to prevent their baby from becoming obese. See Breastfeeding for longer helps to protect your baby from obesity
The main reason for this is that human breast milk is low in salt/sodium. Sodium is more harmful to children than to adults because of their small size, and it is particularly harmful to babies.
See Children and Obesity
and advice for pregnant mothers