How can the phytochemical berberine be used to treat diabetes?

Not as far as I know. It has a lot of drug interactions and if cautiously taken under supervision would be to improve strength of the heartbeat. It is not a supplement to be taken lightly or with ought doctor approval. It could do more harm than good, an I am all for supplements when helpful and good!
Great question! There is evidence that it can reduce blood sugar effectively! but research is early in development; no one knows correct dose, and long term safety.