In an analysis of six studies into fruit and vegetable intake, only food including spinach and cabbage was found to have a significant positive effect.
A portion and a half a day was found to cut type 2 diabetes risk by 14%, the British Medical Journal (BMJ) reports.
But experts urged people to continue to aim for five portions of fruit and vegetables a day.
The researchers from Leicester University reviewed data from the studies of 220,000 adults in total.
They found that eating more fruit and vegetables in general was not strongly linked with a smaller chance of developing type 2 diabetes but