Researchers from Leicester University in the UK who analyzed pooled data from six studies examining links between fruit and vegetable consumption and type 2 diabetes suggest that increasing daily intake of green leafy vegetables such as spinach, cabbage, cauliflower and broccoli, could significantly lower the risk of developing type 2 diabetes and should be investigated further.

However, an editorial comment in the same journal cautioned that while green leafy vegetables show promise, people are advised to concentrate on increasing their overall intake of fruits and vegetables.

You can read how first author Dr Patrice Carter, a research nutritionist in the Department of Cardiovascular Sciences at the University of Leicester, and also a member of the diabetes research team at Leicester Royal Infirmary, arrived at these findings in the 19 August issue of the British Medical Journal, BMJ.

The researchers concluded that their findings add weight to the growing body of evidence linking lifestyle change to prevention of type 2 diabetes, and generally support recommendations to promote the consumption of green leafy vegetables as a way to reduce the risk of developing the disease.

However, they also cautioned that further research is needed before this can be used as a basis for giving specific advice to individuals.

Carter and colleagues conducted a meta-analysis of existing published studies: that is they looked for studies with similar objectives, design, methods of including participants and getting data, and then pooled their results and re-analyzed them as if they had come from one large study.

Their objective was to “investigate the independent effects of intake of fruit and vegetables on incidence of type 2 diabetes”.

They searched several well known databases and lists for articles with key words like “diabetes”, “prediabetes”, “fruit”, and “vegetables”. They also sought expert opinions and reviewed reference lists of relevant articles for further sources.

Although they found nearly 3,500 articles that met the initial search criteria, by the time they had gone through all the quality checks for inclusion in their meta-analysis, only six studies remained, covering over 220,000 participants. However, only two included men, and only four included separate information on intake of green leafy vegetables.

In their summary findings they wrote that:

  • A greater intake of green leafy vegetables was linked with a 14 per cent reduction in risk for type 2 diabetes (hazard ratio 0.86, with 95 per cent confidence interval CI ranging from 0.77 to 0.97, P=0.01).
  • This contrasted with “no significant benefits of increasing the consumption of vegetables, fruit, or fruit and vegetables combined”.

By greater intake, the authors estimated this to be about an extra 120 g a day (1.5 portions in the UK’s system of 80g a portion), of leafy green vegetables, for example cabbage, brussel sprouts, broccoli, cauliflower, and spinach.

Carter and colleagues also found that the reduction of 14 per cent in risk of type 2 diabetes was independent of any weight loss.

They concluded that:

“Increasing daily intake of green leafy vegetables could significantly reduce the risk of type 2 diabetes and should be investigated further.”

In their discussion, the authors wrote they think that fruit and vegetables help prevent chronic diseases because they are rich in antioxidants, and in the case of green leafy vegetables such as spinach, the effect on reducing risk of type 2 diabetes in particular comes from their higher magnesium content.

They stressed that their results support the notion that it is “food” and not the isolated components of food, such as antioxidants, that benefit health and said trials of dietary supplements have not shown strong evidence of preventing disease.

However, in an editorial in the same issue of the journal, Jim Mann, a professor in the Department of Human Nutrition at the University of Otago in New Zealand, and Dagfinn Aune, a research assistant in the Department of Epidemiology and Public Health, at London’s Imperial College in the UK, expressed caution about the findings, pointing to the limited number of studies analyzed.

They agreed that it was probably a good thing to highlight the potential benefit of green leafy vegetables, which could form part of the “five a day” we should all be eating, but they stressed 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,” they added.

“Fruit and vegetable intake and incidence of type 2 diabetes mellitus: systematic review and meta-analysis.”
Patrice Carter, Laura J Gray, Jacqui Troughton, Kamlesh Khunti, and Melanie J Davies.
BMJ, 2010; 341: c4229.
Published online 19 August 2010
DOI: 10.1136/bmj.c4229

Written by: Catharine Paddock, PhD