Cinnamon
(photo8, Cinnamon-other, Wikimedia Commons. CC: BY 2.0.)
Cinnamon appears to improve insulin receptor function, however studies have differing results. There is a meta-analysis from 2011 that found that cinnamon can reduce plasma blood glucose levels (1). However, a Cochrane meta-analysis of 10 RCTs in 2012 did not demonstrate the efficacy of Cinnamomum cassia in improving HbA1c levels (2).
It is reasonable to try, and is quite safe. Cinnamon can cause stomatitis and perioral dermatitis, but this is rare.
Common dosing is 1 to 5 g ground bark with meals or equivalent extract.
This is about 1/4 to 1/2 teaspoon ground cinnamon.
1. Davis, W. Cinnamon intake lowers fasting blood glucose: meta-analysis. J Med Food. 14 (9):884 2011.
2. Leach, S. Kumar: Cinnamon for diabetes mellitus. Cochrane Database Syst Rev. 2012