Lead researcher Lucie Nield, of the Center for Food, Physical Activity & Obesity, University of Teesside, in Middlesbrough, England, set out to see if dietary advice alone could help a person with type 2 diabetes, but he was only able to identify two trials that together involved just 358 people.

"Considering the importance of this disorder, we were disappointed to find such a small amount of relevant data," Nield said in a statement. "We do need more well-designed, long-term studies before we can work out the best advice to give."

The two studies did, however, indicate that dietary advice alone could play an important role, Nield said. One study randomly assigned people to either a control group or a dietary advice group. After six years 67.7 percent of people in the control group had diabetes, compared with only 43.8 percent in the advice group -- a 33 percent reduction. In another study, 12 months of dietary advice led to significant reductions in many diabetes related factors, such as insulin resistance.