Intake of fruits and vegetables, and prevention of type 2 diabetes

The increase in the incidence of type 2 diabetes has been attributed in part to high-fat, high calorie diets, overweight and obesity – particularly excess abdominal fat, and lack of exercise^(1,2). These factors are associated with insulin resistance and metabolic syndrome- important risk factors for type 2 diabetes and cardiovascular disease⁽³⁾. Estimates suggest that up to a 75% reduction in risk for type 2 diabetes could be achieved by preventing obesity⁽⁴⁾.

Observations from population-based studies suggest that fruit and vegetable consumption may be associated with a reduced risk of diabetes^(5-8) or may be protective(9), although results have not always been consistent^(10-12). A positive association between fruits and vegetables and reduced risk is supported by an inverse association between serum carotenoids, a marker for fruit and vegetable intake, and type 2 diabetes and impaired glucose metabolism in adults⁽¹³⁾.

Dietary patterns that include fruits and vegetables have been associated with a reduction in fasting blood glucose^(14-16), improved results on glucose tolerance tests^(17,18) lower glycosylated hemoglobin values⁽¹⁹⁾ and enhanced insulin sensitivity⁽²⁰⁾. However, energy intake may modify these associations^(21). Taken together, these studies suggest that fruit and vegetable intake is potentially beneficial for prevention and management of diabetes.

Fruits and vegetables and weight management

The relationship of obesity and type 2 diabetes is well established⁽²²⁾. Further, weight reduction is an integral part of treatment for type 2 diabetes, leading to improved glycemic control and often a reduction or elimination of the need for medication⁽²²⁾. Weight management may also minimize the increased risk of people with diabetes for cardiovascular diseases.

Research suggests that advice to increase intake of fruits and vegetables coupled with advice to decrease energy intake is an effective recommendation for weight management^(23-25), as they are less calorically dense and more nutrient rich per serving than many foods⁽²⁶⁾.

Fruits and Vegetables, Fiber and Glycemic Control

Results from studies that have evaluated fiber and blood glucose control are inconsistent. Large amounts of fiber appear to be necessary for beneficial effects on glycemia, hyperinsulinemia and plasma lipids in people with diabetes. A recent study⁽²⁷⁾ addressed the potential issue of acceptability of a high fiber diet. In a randomized, crossover study, 13 patients with type 2 diabetes consumed a standard ADA diet (24 g total fiber) or a high fiber diet (50 g total fiber), achieved by including fruits and vegetables and grains, especially those high in soluble fiber. Results indicated that the high fiber diet improved glycemic control compared to the ADA diet, and patients accepted the high fiber diet with few side effects.
The fiber content of fruits and vegetables may also contribute to a lower glycemic index and hence glycemic load of the diet. A recent metaanalysis indicates that the use of this technique can provide an additional benefit over that observed when total carbohydrate is considered alone⁽²⁸⁾. Although scientists worldwide do not agree that using the glycemic index is the best way to plan carbohydrate intake for individuals including those with diabetes^(29,30), the potential health benefits of fruits and vegetables for those with diabetes are becoming obvious.

Fruits and vegetables, antioxidants and diabetic complications

Newer research is revealing additional benefits of fruit and vegetable consumption for health in diabetes. Elevated blood glucose levels can lead to oxidative stress. This damage may play an important role in the development of complications in diabetes, such as lens cataracts, kidney disease and neurological disease⁽³¹⁾. Animal studies suggest the mechanisms by which antioxidants in the diet may help counteract this damage⁽³²⁾. Data from the Third National Health and Nutrition Examination Survey (1988-1994) (NHANES III) showed that adults with metabolic syndrome have suboptimal concentrations of several antioxidants in the blood, as well as a lower consumption of fruits and vegetables compared to adults without metabolic syndrome⁽³³⁾. Further research is needed to determine the role of antioxidants for improving glycemic control and/or exerting antioxidant activity.

Conclusions

Overall the body of evidence strongly supports that healthy diets which include ample fruits and vegetables, whole-grains, and high-fiber foods without excess fat or calories, in conjunction with regular physical activity—can improve control of blood sugar among individuals with type 2 diabetes and may offer some protection against its development.