Over a 24-year period, the study examined the diets of over 187,000 participants, mostly women, who were free of chronic disease at the baseline of the research.
In contrast to fruit juice consumption, certain whole fruits were shown to have a downward effect on type 2 diabetes.
"Greater consumption of specific whole fruits, particularly blueberries, grapes, and apples, is significantly associated with a lower risk of type 2 diabetes, whereas greater consumption of fruit juice is associated with a higher risk," the study concluded.
Not all fruits were equally connected with lower risk, however, due to highly variable content of fiber, antioxidants and other nutrients.
Through periodical questionnaires, the study examined consumption of 118 foods, including consumption habits for 11 categories of fruit and various types of fruit juices. Fruit analysis included grapes and raisins, peaches, plums and apricots, prunes, bananas, cantaloupe, apples and pears, oranges, grapefruit, strawberries, blueberries and sporadic analysis of watermelon.
The researchers then sent supplementary questionnaires to those who reported physician-diagnosed diabetes. A type 2 diabetes diagnosis was confirmed if respondents reported positively to set diagnostic standards.
During the study, over 12,000 people from the group developed type 2 diabetes.
"In three prospective cohorts of US men and women, we found that the associations with risk of type 2 diabetes differed significantly among individual fruits: greater consumption of blueberries, grapes, apples, bananas, and grapefruit were significantly associated with a reduced risk of type 2 diabetes," the study found.
"Additionally, differences in the glycemic index/glycemic load values of fruits did not account for the association of specific fruits with risk of type 2 diabetes. Moreover, greater fruit juice consumption was associated with an increased risk, and substitution of whole fruits for fruit juice was associated with a lower risk, except for strawberries and cantaloupe."