The study, by Harvard School of Public Health, was published online in the British Medical Journal.
According to the study, eating more whole fruits--particularly blueberries, grapes, and apples, was associated with a significantly lower risk of type 2 diabetes. However, increased fruit juice consumption was linked with an increased risk of type 2 diabetes.
"Whole fruits are recommended as a measure for diabetes prevention, previous studies have found mixed results for total fruit consumption," Qi Sun, study lead author, said in a press release. "Our findings provide novel evidence suggesting that certain fruits may be especially beneficial for lowering diabetes risk."
Study researchers reviewed data gathered between 1984 and 2008 from three long-running studies which included 187,382 participants. Although diabetics were excluded, 12,198 participants, or 6.5 percent, developed diabetes during the study period.
Study researchers then examined overall fruit consumption, as well as consumption of individual fruits: grapes or raisins; peaches, plums, or apricots; prunes; bananas; cantaloupe; apples or pears; oranges; grapefruit; strawberries; and blueberries. They also examined consumption of apple, orange, grapefruit, and "other" fruit juices.
According to the study, participants who ate at least two servings each week of certain whole fruits —blueberries, grapes, and apples—reduced their risk for type 2 diabetes by as much as 23 percent as compared to those who ate less than one serving per month. However, participants who consumed one or more servings of fruit juice on a daily basis increased their risk of type 2 diabetes by as much as 21 percent. Researchers found that exchanging three servings of juice per week for whole fruits would result in a 7 percent reduction in diabetes risk.
The fruits' glycemic index was not a significant factor in determining a fruit's association with type 2 diabetes risk. However, the high glycemic index of fruit juice may explain why there is a link between increased fruit juice consumption and increased risk of type 2 diabetes. Researchers speculate that anthocyanins, which are found in berries and grapes, may be responsible for the reduced risk. More research is needed, however.
"Our data further endorse current recommendations on increasing whole fruits, but not fruit juice, as a measure for diabetes prevention," said lead author Isao Muraki. "And our novel findings may help refine this recommendation to facilitate diabetes prevention."