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At the University of California, Berkeley, researchers were so surprised with their findings that they rechecked their results several times. In a study of animals fed a high-fat diet, grapefruit juice reduced weight gain by 18 percent, compared to water. Plus, it lowered unhealthy levels of blood sugar as effectively as a prescription diabetes drug. With a low-fat diet, the juice had no effect on weight but it did improve insulin function, which could reduce risk for diabetes.