Eating more whole fruits, particularly blueberries, grapes and apples, showed a “significant” link to a lower risk of type 2 diabetes, according to a new study from Harvard School of Public Health (HSPH) researchers. Greater consumption of fruit juices, however, was associated with a higher risk of type 2 diabetes.

Researchers said the study, which was published online in the British Medical Journal (BMJ) on 29 August 2013, was the first to look at the effects of individual fruits on diabetes risk.

“While fruits are recommended as a measure for diabetes prevention, previous studies have found mixed results for total fruit consumption,” said Qi Sun, senior author and Assistant Professor in the Department of Nutrition at HSPH and Assistant Professor at the Channing Division of Network Medicine, Brigham and Women’s Hospital.

Research method

The researchers examined data gathered between 1984 and 2008 from 187,382 participants in three long-running studies (Nurses’ Health Study, Nurses’ Health Study 2, and Health Professionals Follow-up Study). Participants who reported a diagnosis of diabetes, cardiovascular disease or cancer at enrolment were excluded. Results showed that 12,198 participants (6.5 per cent) developed diabetes during the study period.

The researchers looked at overall fruit consumption, as well as consumption of individual fruits: grapes or raisins, peaches, plums or apricots, prunes, bananas, cantaloupe or rockmelon, apples or pears, oranges, grapefruit, strawberries and blueberries. They also looked at consumption of apple, orange, grapefruit and “other” fruit juices.

People who ate at least two servings each week of certain whole fruits – particularly blueberries, grapes and apples – reduced their risk for type 2 diabetes by as much as 23 per cent in comparison to those who ate less than one serving per month. Conversely, those who consumed one or more servings of fruit juice each day increased their risk of developing type 2 diabetes by as much as 21 per cent. The researchers found that swapping three servings of juice per week for whole fruits would result in a 7 per cent reduction in diabetes risk.

GI not a factor for whole fruits

Researchers said the fruits’ glycemic index (GI) – a measure of how rapidly carbohydrates in a food boost blood sugar – did not prove to be a significant factor in determining a fruit’s association with type 2 diabetes risk. However, the high GI of fruit juice, which passes through the digestive system more rapidly than fibre-rich fruit, may explain the positive link between juice consumption and increased diabetes risk.

Individual fruit component may be at work

The researchers theorised that the beneficial effects of certain individual fruits could be the result of a particular component. Previous studies have linked anthocynanins, found in berries and grapes, to lowered heart attack risk, for example. But the authors said more research is necessary to determine which components in the more beneficial fruits influence diabetes risk.

“Our data further endorse current recommendations on increasing whole fruits, but not fruit juice, as a measure for diabetes prevention,” said lead author Isao Muraki, Research Fellow in the Department of Nutrition at HSPH. “And our novel findings may help refine this recommendation to facilitate diabetes prevention,” he said.

Other HSPH authors included JoAnn Manson, Professor in the Department of Epidemiology; Frank Hu, Professor of Nutrition and Epidemiology; Walter Willett, Fredrick John Stare Professor of Epidemiology and Nutrition Department Chair; and Rob van Dam, Adjunct Associate Professor of Nutrition and Epidemiology.

Support for the study came from research grants CA87969, CA17626, CA55075, CA50385, CA167552, DK58845, and DK082486 from the National Institutes of Health. Sun was supported by a career development award R00HL098459 from the National Heart, Lung, and Blood Institute.

Fruit juice under fire

The results of the HSPH study come as the sugar content of fruit juices in Australia gains attention.

Australian Food News reported in August 2013 that the Australian fruit juice industry had responded to criticisms from Australian nutritionists.

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