Science is finding more health benefits from blueberries—but raising more concerns about fruit juice. According to a new study by Harvard University researchers, eating whole fruits helps ward off diabetes, while drinking juice can actually raise the risk of developing the disease.
Blueberries had the strongest effect on cutting diabetes risk, followed by grapes and apples, especially when three or more servings a week were eaten. A standard serving of blueberries was half a cup.
Prunes, pears, bananas, and grapefruit also helped lower diabetes risk, while the other fruits did not.
The difference is something called polyphenols, said study co-author Qi Sun, an assistant professor of nutrition at Brigham and Women’s Hospital and Harvard School of Public Health. Some of these plant-based chemical compounds—including anthocyanins, chlorogenic acid, and resveratrol, all powerful antioxidants—may help the body process glucose. Blueberries, grapes, and apples are all rich in these beneficial polyphenols.
Consumption of fruit juices—including apple, orange, and grapefruit—not only failed to deliver the same benefits as whole fruit but even appeared to raise the risk of diabetes. People who drank at least one serving a day of juice had a 21 percent higher risk of developing diabetes than those who did not.
There are a few possible reasons, said Sun.
"During juicing processes, some phytochemicals and dietary fiber are lost,” said Sun. And since fluids are more rapidly absorbed than solids, drinking juice brings on a “more rapid and more dramatic glucose and insulin response” than eating whole fruits.
“Participants often are not aware of how much sugar is added to the juices that they typically drink.” But, he added, “One can reasonably assume that juices with added sugar may be more strongly associated with diabetes risk.”