Eating Green Leafy Vegetables May Lower Risk for Type 2 Diabetes

From Medscape Medical News

Laurie Barclay, MD

August 20, 2010 — Eating green leafy vegetables may lower the risk for type 2 diabetes, according to the results of a systematic review and meta-analysis reported in the August 20 issue of the BMJ.
"High intake of fruit and vegetables has been associated with a reduced incidence of cancer and cardiovascular disease," write Patrice Carter, from University of Leicester, in Leicester, United Kingdom, and colleagues. "Diabetes is a strong independent risk factor for cardiovascular disease, and often the conditions exist together, sharing common modifiable risk factors. As yet no firm conclusions have been made as to whether increasing intake of fruit and vegetables can decrease the risk of type 2 diabetes itself, given the abundance of conflicting evidence within the literature."
To determine the independent effects of fruit and vegetable consumption on the incidence of type 2 diabetes, the reviewers searched MEDLINE, EMBASE, CINAHL, British Nursing Index, and the Cochrane Library for terms involving diabetes, prediabetes, fruit, and vegetables. They also asked experts about their opinions and searched bibliographies of retrieved articles. Inclusion criteria were prospective cohort studies with an independent measure of intake of fruit, vegetables, or fruit and vegetables, as well as data on the incidence of type 2 diabetes.
Of 6 studies meeting selection criteria, enrolling a total of more than 220,000 participants, 4 studies also provided separate data regarding intake of green leafy vegetables.
Greater consumption of green leafy vegetables was associated with a 14% reduction in the risk for type 2 diabetes (hazard ratio [HR], 0.86; 95% confidence interval [CI], 0.77 - 0.97; P = .01), based on summary estimates.
Increasing intake of vegetables, fruit, or fruit and vegetables combined did not significantly reduce the risk for type 2 diabetes.
"Increasing daily intake of green leafy vegetables could significantly reduce the risk of type 2 diabetes and should be investigated further," the study authors write.
Limitations of this meta-analysis include difficulties in determining dietary intake, significant heterogeneity among the included studies, and unmeasured confounding. In addition, only a small number of studies were included in the meta-analysis, which may have prevented detecting a benefit of combined fruit and vegetable consumption for reducing type 2 diabetes risk.
"Our results support the evidence that 'foods' rather than isolated components such as antioxidants are beneficial for health," the study authors conclude. "Results from several supplement trials have produced disappointing results for prevention of disease, in contrast with epidemiological evidence.
Further investigation is warranted to understand the mechanisms involved in the proposed relation between green leafy vegetables and risk of type 2 diabetes."
In an accompanying editorial, Jim Mann, from the University of Otago in Otago, New Zealand, and Dagfinn Aune, from Imperial College London, in London, United Kingdom, warn that increasing overall fruit and vegetable intake should still be emphasized. However, dietary recommendations for 5 portions of fruit and vegetables should certainly include green leafy vegetables.
"The meta-analysis suggests that an additional one and a half UK portions (roughly 120 g) daily of green leafy vegetables (for example, cabbage, brussel sprouts, broccoli, cauliflower, and spinach) has the potential to reduce the risk of diabetes by 14% independently of any effect of weight loss," Drs. Mann and Aune write. "Although it may be reasonable to draw attention to the potential benefits of green leafy vegetables, which could be incorporated as one of the five recommended portions of fruit and vegetables a day, we must be careful that the message of increasing overall fruit and vegetable intake is not lost in a plethora of magic bullets. 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."


BMJ. 2010;341:c4229. Published online August 20, 2010.