Dr. Mandell points to black tea but not to diets the decrease insulin like that latest flyer I found... which I'll cite here:Bottom Line Books, 30-day Diabetes Cure. Without ordering the book for $50, they give away one hint that reduced the problem by 50%: vitamin D. Vit D is more of a hormone than a vitamin, I've seen elsewhere. Getting back to Mandell, I haven't search his channel for more on this. It's the first time I've heard black tea helps.
https://m.youtube.com/watch?v=Bd_pwyQp0rQ