The prevalence of type 2 diabetes is currently estimated at 6.4% worldwide and is predicted to rise¹. There is evidence to support lifestyle modification programs to successfully prevent or delay the onset of type 2 diabetes². Several lifestyle interventions have promoted increased intake of fruit and vegetables. In addition, high intakes of fruit and vegetables have been linked to a reduction in risk of cancer and cardiovascular disease. Diabetes is a strong, independent risk factor for cardiovascular disease and often the conditions exist together, sharing common modifiable risk factors. Therefore, we aimed to investigate the independent effects of fruit and vegetable consumption on the incidence of type 2 diabetes.

The electronic databases MEDLINE, EMBASE, CINHAL, BNI and the Cochrane library were searched for both Medical Subject Headings and keywords on diabetes, prediabetes, fruit and vegetables. Prospective cohort studies with an independent measure of fruit and vegetable intake by self-report and data on type 2 diabetes incidence were selected for inclusion in the review. All data on fruit and vegetable intake was standardised into servings per day. Random effects meta-analysis was used to estimate overall hazard ratios and 95% confidence intervals for the association between fruit and vegetable intake and risk of type 2 diabetes.

Increasing green leafy vegetable consumption decreases risk of type 2 diabetes

The search identified 3,446 articles. Six studies met all the inclusion criteria^3-8; four included data on the consumption of green leafy vegetables^3-6. The combined population resulted in 223,512 participants, aged between 30 to 74 years. We specifically looked at the lowest versus the highest intake values. The meta-analysis did not show any significant reductions in risk of type 2 diabetes for increased consumption of fruit, vegetables or fruit and vegetables combined, however there was a trend towards benefit from their increased consumption. All studies which examined the intake of green leafy vegetables showed a reduction in risk of type 2 diabetes with greater consumption. Summary estimates showed that those with highest intake as compared to the lowest intake had a 14% reduction in risk (p=0.01) of type 2 diabetes (hazard ratio 0.86, 95% confidence interval 0.77 to 0.96). Several differences existed between the studies in our review, therefore we carried out a sensitivity analysis to explore potential sources of bias. We examined sex of participants, location of study, quality of article, length of follow up, and how intake of fruit and vegetables were grouped. No significant interactions existed between these variables and therefore results should be drawn with caution. Measuring true dietary intake is very difficult. Food frequency questionnaires are open to criticism and to both random and systematic errors; such errors could have lead to an underestimation of the true effect of fruit and vegetables.

The review demonstrated that increasing the amount of green leafy vegetables in an individual’s diet may help to reduce the risk of type 2 diabètes.

Potential mechanisms by which green leafy vegetables may confer health benefits

Fruit and vegetables could potentially prevent chronic disease due to their antioxidant content; our results support this as green leafy vegetables have high levels of vitamin C and beta carotene, both potent antioxidants. There are a number of other potential mechanisms by which green leafy vegetables may confer health benefits. They have high magnesium content; a recent meta-analysis found magnesium intake to be inversely associated with incidence type 2 diabetes¹⁰. Green leafy vegetables are good sources of α linolenic acids¹¹. Intake may influence the composition of the fatty acid composition of the phospholipid bilayer which is related to insulin sensitivity. Further investigation into the potential beneficial mechanisms of green leafy vegetables is required.