It appears those green leafy vegetables may have some benefit in preventing Type 2 Diabetes:
Increasing the amount of green leafy vegetables in an individual’s diet could help to reduce the risk of type 2 diabetes. An increase of 1.15 servings a day was associated with a 14% decrease in incidence. The data did not show any significant relations between the consumption of fruits, vegetables, or fruit and vegetables combined on the incidence of diabetes. As there was significant heterogeneity between studies, however, we carried out a sensitivity analysis. This showed no significant interactions between the outcomes of variables examined, and thus could not identify differences that occurred between the studies.