Studies done on rats found that supplementing with soy isoflavones (specifically genistein) increased insulin levels and lowered HbA1c in diabetic rats.[1][2] A study in monkeys found that consuming soy protein high in soy isoflavones increased the insulin response to glucose.[3]
In human studies, supplementing with soy and soy isoflavones also seemed to improve insulin sensitivity, lowered HbA1c, and improved fasting insulin.[4][5] Although these results are promising, the changes in insulin and HbA1c levels were small, and the studies involved people with type 2 diabetes who were already taking antidiabetic medications. Further research into the appropriate dosage and the clinical relevance of the effect on insulin sensitivity is needed.