Preventing Diabetes: Not All Fruits Are Created Equal

SAHAR ASHRAFZADEH

study bar graph
How diabetes risk changes when you replace three servings of fruit juice with three servings of whole fruit (table published courtesy of the study).

Scientists have long known that type II diabetes—a disease in which the body does not produce enough insulin to be able to regulate blood sugar levels which naturally rise after someone consumes a meal—is often the long-term result of a poor diet. “The question is whether dietary sugar can increase or decrease your risk for developing diabetes,” says Dr. Clifford Lo, an associate professor at Harvard Medical School.  Fruit, while typically thought of as a healthy food, actually includes relatively large concentrations of naturally occurring sugars, and over time repeat exposure to high doses of sugar may dull the body’s natural insulin response mechanism, producing type II diabetes. “Large studies from the past 5-10 years from HSPH have made a strong case for dietary sugars increasing your risk for developing obesity and diabetes,” says Lo, who was not a member of the research team that published these findings.

The team at the Harvard School of Public Health consisting of Isao Muraki, JoAnn E. Manson, Frank B. Hu, Walter C. Willett, Rob M. Van Dam, and Qi Sun, in collaboration with Fumiaki Imamura at the University of Cambridge, designed a long-term study to find the answer. Their findings were published in August in the British Medical Journal. For over two decades, they have tracked the food choices of over 180,000 subjects, determining not only how many servings of fruit they consumed per day but also which specific kinds of fruit they consumed most. By the end of the study, over 12,000 of the participants had been diagnosed with type II diabetes. After taking into account the various lifestyle factors that would influence diabetes risk, the researchers identified which fruits most significantly reduced the chance of getting diabetes.

Grapes, raisins, prunes, apples, pears, bananas, and grapefruit were all found to decrease diabetes risk when three or more servings were consumed per week. While eating strawberries, oranges, peaches, plums, and apricots also showed a decrease in diabetes risk, their results were not as significant. Unlike the other fruits studied, the study found a slight increase in diabetes risk for those who ate cantaloupe, though the findings were within the margin of error.

Although the consumption of most fruits analyzed in the study was negatively correlated to diabetes risk, one superfruit stood out from the rest: blueberries.

Although the consumption of most fruits analyzed in the study was negatively correlated to diabetes risk, one superfruit stood out from the rest: blueberries. According to Lo, blueberries are especially high in polyphenol antioxidants called anthocyanins—molecules containing several phenol groups which inhibit the oxidation of other molecules. Previous studies have shown that oxidation of molecules in the body may be involved in the molecular process of diabetes pathogenesis. Whatever the role of antioxidants may be, blueberries were determined to decrease one’s risk of type II diabetes by an extraordinary 26 percent when consumed at least twice a week relative to people who did not consume them.

But the juicy details aren’t so optimistic. These findings only correspond to eating whole fruits. In fact, it was found that drinking fruit juice actually increases the risk of developing type II diabetes by up to 21 percent. This is partly due to the loss of beneficial phytochemicals and fiber during the juicing process and partly due the negative effects of added sugar.  What’s more, sugars from liquids are absorbed more quickly into the bloodstream than sugars from fiber-rich whole fruits, increasing the rate at which blood sugar rises and the urgency with which insulin responds to regulate blood sugar levels.

Future research remains to be done concerning the underlying mechanisms of this phenomenon. The study results suggest that it may be time to re-evaluate the foods we typically think of as “healthy” and think twice before drinking juices that are lacking fiber and full of added sugars.

What you should know next time you get your daily dose of fruit:

  • Overall, increased whole fruit consumption is associated with a decrease in type II diabetes risk.
  • The correlation between fruit and diabetes risk differs considerably among individual fruits.
  • Blueberries are optimal for reducing diabetes risk, followed by grapes, prunes, apples, and pears.
  • Drinking fruit juice can significantly increase one’s risk of getting diabetes. Skip the juice in exchange for whole fruits for a longer, healthier life.
Sahar Ashrafzadeh is a Brevia staff writer. She can be reached at sashrafzadeh@college.harvard.edu.