Breast feeding cuts risk of diabetes

according to this article, summarized here:

Longer duration of breastfeeding was associated with reduced incidence of type 2 diabetes in 2 large US cohorts of women. Lactation may reduce risk of type 2 diabetes in young and middle-aged women by improving glucose homeostasis.

The weakness of the study is it is observational. In other words they just followed women who did or didn't breastfeed and then observed who developed diabetes. Ideally, you'd randomize women to breastfeed or not, but this is not really ethical. One has to watch out for the "healthy lifestyle" effect in which women who breastfeed also exercise, watch what they eat, don't smoke, etc. Perhaps it is one or more of those things, not breastfeeding itself that provides the protection. The authors try to control for this, but you never know for sure if you are missing something.

I'd say the results are intuitive since it an article of faith that women who breastfeed lose weight quicker after delivery than those who don't and obesity is definitely a major risk factor for type II (adult onset) diabetes.