On the flip side of the coin, however, an increased consumption of fruit was not associated with any benefits, according to the study with 64,191 middle-aged Chinese women published in the new issue of the Journal of Nutrition.
"Our study adds to the limited and conflicting data of the associations between fruit and vegetable intake and the risk of type-2 diabetes," wrote lead author Raquel Villegas from Vanderbilt Epidemiology Center.
"A higher intake of vegetables, rich in fibre, antioxidants, and magnesium and with a low glycemic index, was associated with a decreased risk of type-2 diabetes."
The authors, including researchers from the Shanghai Cancer Institute and the Diabetes Research and Training Center in Nashville, recruited the women (aged between 40 and 70) and, using a food-frequency questionnaire, report they had average fruit and vegetable intakes of 239.4 and 236.0 grams per day, respectively.
Dietary intakes were assessed at the start of the study (baseline) and again after 4.6 years. Vegetables were categorised according to cruciferous, green leafy, yellow, tomatoes, allium (onion), or 'other', and fruits into citrus, watermelon, or 'other'.
Over the course of the study, Villegas and co-workers documented 1,608 cases of type-2 diabetes. Consumption of the most vegetables, averaging 428 grams per day, was associated with a 28 per cent lower risk of developing the disease, compared to consumption of the lowest average amount (121.5 grams per day).
Putting this in context, a cup of cooked cauliflower weighs approximately 50 g, while a medium apple weighs in at around 140 g.
"There were [also] inverse associations across quintiles of intake of cruciferous vegetables, green leafy vegetables, yellow vegetables, allium vegetables, tomatoes, and other vegetables," added the researchers. "Although trend tests were significant, some of these inverse associations did not follow a linear dose-response relationship."
On the other hand, all fruit or specific types of fruit were not related to any risk of developing type-2 diabetes.
"The mechanism by which vegetables affect glucose tolerance has not been clearly defined but may be associated with the high content of antioxidants, fibre, and magnesium or the low glycemic index in vegetables," stated the authors.
They continue by noting that previous studies have reported diabetes-related benefits for both vitamins E and C, but not beta-carotene.
"In our study, the inverse association between vegetable intake and type-2 diabetes persisted after adjustment for vitamin C, vitamin E, carotene, and fibre intake. Further adjustment for magnesium intake did not alter the association," wrote Villegas.
"Taking this evidence into consideration, it appears that the beneficial effects of vegetable consumption on the risk of T2D cannot be entirely explained by antioxidant vitamins, magnesium, or fibre intake.
"Vegetables also contain other compounds such as phytates, lignans, and isoflavones that might have an additive or synergistic effect on lowering the risk of type-2 diabetes."
The study does have several limitations, most notably the use of FFQs that are subjects to some errors with regards to recalling dietary intakes, and the possibility that subjects changed their diets when pre-type-2 diabetes began to manifest itself. In addition, the study focussed solely on Chinese women, which limits our ability to generalise the results to other populations.
Despite these limitations, the study adds to the small but growing body of evidence linking vegetable intake to a reduced risk of diabetes, although it does not prove causality.
"Further follow-up of the cohort would provide a more definite assessment of the vegetable and type-2 diabetes association," concluded the researchers.
An estimated 19 million people are affected by diabetes in the EU 25, equal to four per cent of the total population. This figure is projected to increase to 26 million by 2030.
In the US, there are over 20 million people with diabetes, equal to seven per cent of the population. The total costs are thought to be as much as $132 billion, with $92 billion being direct costs from medication, according to 2002 American Diabetes Association figures.
Source: Journal of NutritionVolume 138, Pages 574-580"Vegetable but not fruit consumption reduces the risk of type 2 diabetes in Chinese women"Authors: R. Villegas, X.O. Shu, Y.-T. Gao, G. Yang, T. Elasy, H. Li, W. Zheng