An apple a day may keep the doctor away. But a few glasses of apple juice a day may have the opposite effect.

Harvard researchers found that eating whole fruits, specifically blueberries, grapes and apples, "significantly" reduced the risk of type-2 diabetes. But "greater consumption" of fruit juices, specifically apple, orange and grapefruit, increased diabetes risk.

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The reason appears to be differences in how the body metabolizes whole fruits vs. fruit juices. Researchers wrote that "fruit juices lead to more rapid and larger changes in serum levels of glucose and insulin than whole fruits."

The researchers concluded it took only three servings per week of whole fruits to reduce the risk of type 2 diabetes. In addition to apples, blueberries and grapes, other fruits that seemed to help ward of diabetes are raisins and pears, researchers said.

The research was published in August in the online edition of BMJ (formerly known as the British Medical Journal).