Harriet Hall, M.D., looked at the evidence for Plavinol as a diabetes treatment. She’s skeptical about it:
“In a recent article on SBM, Scott Gavura quoted a pharmacy customer who said “I don’t want to take any drugs. Do you have something natural I can use to cut my blood sugar?” Scott went on to cover the questionable evidence for cinnamon in that article. Many other “natural” remedies have been proposed. Here’s an alphabetical list: acetyl L-carnitine, aloe, alpha-lipoic acid, banaba leaf (not banana!), basil, berberine, bilberry, biotin, bitter melon, cinnamon, chromium, coQ10, crepe myrtle, fenugreek, fish oil, fructo-oligosaccharides, green tea, ginseng, glucomannan, gymnema, hibiscus, Indian kino tree extract, magnesium, mistletoe, olive leaf, onion, psyllium, purslane, resveratrol, starch blockers, thiamine, vanadium, and vitamins. I compiled that list from just three websites; I’m sure there are many more natural remedies that I missed. These natural remedies have been recommended on the basis of rather shaky preliminary evidence that they lower blood sugar, usually by only a small amount. Even the CAM-friendly National Center for Complementary and Integrative Health (NCCIH) concluded: “There is not enough scientific evidence to suggest that any dietary supplements can help prevent or manage type 2 diabetes.”
Source: Plavinol and Other Natural Remedies for Diabetes: “Condimentary Medicine”? « Science-Based Medicine
Diabetic Mediterranean Diet 