Cinnamon may improve blood sugar levels: Study
cinnamon may in very very small clinical trials. the same article discusses some of the metaanalysis trials showing no improvement by cinammon, but those are poor substitute for prospective, double blinded clinical trials. It takes a lot of money to do the latter so unless a drug company is heavily invested in this, it may be hard to find such research, this is true for almost all over the counter supplements.
with that said, it is no substitute for good low carb diet and excercise and weight control
in clinical practice, I have not seen patients taking the cinammon improve their hemoglobin a1cs, but these folks seem to depend soley on supplements and not healthy eating and excercise, so it may not be a good indication due to the sample size i have in my own practice.