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.
In a study published in the British Medical Journal, nutrition experts report that consumption of certain fruits—especially blueberries—cut people’s risk of type 2 diabetes by as much as 26 percent in a survey of more than 180,000 subjects over two and a half decades.
Study participants were asked about their consumption of grapes or raisins, prunes, bananas, cantaloupe, apples or pears, oranges, grapefruit, blueberries, strawberries, and stone fruits (peaches, plums, or apricots).
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.
Sun and his collaborators based their research on data from 151,209 female participants in the long-running Nurses' Health Studies, which have tracked the lifestyles and health of participating nurses since 1976 through questionnaires and medical testing. They also included a cohort of 36,173 men from a similar survey of male medical health professionals, the Health Professionals Follow-Up Study, conducted from 1986 to 2008.
The study also revealed an interesting twist. 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.
The questionnaires did not specifically ask whether the juice people were consuming was pasteurized or sweetened, although many store-bought juices are.
It’s hard to get that kind of specific data from large epidemiological studies, said Sun: “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.”
Rather than simply counting carbs, people should pay attention to the quality of the carbohydrates in their diet, Sun said. Whole grains are a healthier choice than refined carbohydrates such as white bread, and whole fruits are a healthier choice than processed juices.