Green Leafy Vegetables Cuts The Risk of Diabetes by 14%

A British meta-analysis found that increasing the daily intake of green leafy vegetables can reduce the risk of Type 2 diabetes…

According to Patrice Carter, a PhD student at the University of Leicester, consuming 1.35 servings of these vegetables per day was associated with a 14% reduction in risk compared with consuming only 0.2 servings (HR 0.86, 95% CI 0.77 to 0.96, P=0.01).

A trend also was seen suggesting a benefit for consuming greater quantities of fruits and vegetables overall, although this was not statistically significant.

The incidence of Type 2 diabetes has been rising dramatically during the past two decades, with diet being a major contributor, but evidence thus far has been conflicting as to whether increasing consumption of fruit and vegetables would have an impact.

To sort through this evidence, Carter and colleagues conducted a systematic review and meta-analysis, identifying six studies that met their inclusion criteria.

The studies included a combined population of 223,512 subjects who were 30 to 74 years of age. Length of the studies ranged from 4.6 to 23 years.

Comparisons of the lowest versus highest intake of fruit and vegetables found these hazard ratios for Type 2 diabetes:

Vegetables only, HR 0.91 (95% CI 0.76 to 1.09, P=0.32)
Fruit only, HR 0.93 (95% CI 0.83 to 1.01, P=0.27)
Fruit and vegetables overall, HR 1 (95% CI 0.92 to 1.09, P=0.97)
The researchers found considerable heterogeneity between the studies, so they carried out a sensitivity analysis, examining multiple factors that could be sources of bias.

They found no influence of study location, despite one study having been conducted in China, where the traditional diet is high in fruit and vegetables. Heterogeneity also could result from differences in food group classifications, with some studies defining green leafy vegetables as lettuce, spinach, and kale, while others included Chinese greens.

The researchers pointed out that the food group of green leafy vegetables actually includes brassicas, such as cabbage and cauliflower, Compositae such as lettuce, and herbs such as parsley and dill.

They listed possible reasons why fruit and vegetables might help prevent chronic disease, among them the antioxidant effects of beta-carotene, vitamin C, and polyphenols, as well as the magnesium and polyunsaturated fatty acid content.

However, they noted that the benefits of these compounds are likely to result from consumption in foods rather than as dietary supplements, with disappointing results having been seen in earlier trials of supplements for disease prevention.

The importance of overall nutrition was echoed in an editorial by Jim Mann, PhD, of the University of Otago, in Dunedin, New Zealand, and Dagfinn Aune, BSc, of Imperial College in London, which accompanied the meta-analysis.

"Although some studies have shown associations between individual vegetables and fruits and coronary heart disease, stroke, and some cancers (for example, allium vegetables and stomach and colorectal cancer, tomatoes and prostate cancer), most current recommendations focus on food groups as a whole rather than magic bullets," they wrote.

The editorialists noted that although it may be "reasonable" to draw attention to the potential benefits of green leafy vegetables, the more important message is for increasing fruit and vegetable intake overall.

"The findings are also a useful reminder to clinicians that giving dietary advice may be just as beneficial, if not more so, than prescribing drugs to patients at risk of chronic disease," wrote Mann and Aune.