This looks promising especially since I drink tea all the time:

Drinking black tea could help prevent diabetes, according to new findings by scientists at Dundee University.
The researchers said black tea may have the potential to combat type 2 diabetes, the most common form of the disease.
They believe certain constituents of tea could act as an insulin substitute.
[…]

Dr Rena stressed that further research was needed.
“People shouldn’t be rushing to drink masses of black tea thinking it will cure them of diabetes,” he said. “We are still some way from this leading to new treatments or dietary advice.