How fruity should you be?

| August 5, 2014

flickr_ubelong-volunteer-abroad 12767496505_306afe0a6e_nA couple of months back the BBC reported a new study which questioned existing advice to eat five portions of fresh fruit and vegetables a day.  Five was not enough according to the study authors, it should be seven.  I really do try each day to eat five portions. Where was I going to find the time and space for these extra two portions?  But this looked like a sound study published in a respected academic journal, with data from over 65,000 people.

But hang on a minute.  This is a study based on observational data with no attention whatsoever to selection bias. That is, the observations are of people who cram in seven portions of fruit and vegetables a day, are health nuts who exercise three times a day,  and follow it up with a bracing cold bath and a quick yoga session.  The BBC also quotes the sensible sounding Professor Tom Sanders, at the School of Medicine, King’s College London, who says it was ‘already known’ that people who said they ate lots of fruit and vegetables were health conscious, educated and better-off, which could account for the drop in risk. Exactly, Tom. So, it is not clear why the BBC is pegging the article  on this apparently erroneous finding. A better headline would have been ‘UK academic blasts study for erroneously mistaking correlation for causation’.

But the BBC redeemed itself last week by reporting a systematic review published in the British Medical Journal which concludes that five portions a day really is enough. More than five has no additional health benefits.

Why should I believe this ‘new research’, as the BBC calls it, when it was misleading me back in April to eat seven portions?  I should believe it because it is not new research at all.It is something better: it is a systematic review.

Why is a systematic review so great? It is great because the study team did an extremely comprehensive search for all of the studies of this topic they could possibly find, which ended up being over 7,000 research papers. They then screened them all for quality, and only kept those which contained credible evidence of a causal link.  They finally kept only 16 of the 7,000 studies.  Turning to the original systematic review , I see the authors are also using observational data. But they use only the estimates that adjust for confounders, which can potentially deal with selection bias on observables, though not entirely. The systematic review pools together all these findings to get a single estimate based on over 800,000 people.  And they present a very nice graph which shows how the risk of dying is reduced by eating more fruit and vegetables, but the effect clearly plateaus between four and five portions. So, actually four would be okay and five is better. But more than five will just result in flatulence not fitness.

The rise of evidence-based medicine was closely associated with the rise of systematic reviews.  Doctors’ decisions on treatment options need to be based on evidence from thousands of patients, not just the dozens they have seen. We can and should apply the same principles to development policy.  And there are hundreds of reviews already available to development policy makers.  Check out the 3ie systematic review database and learn about what works in development programmes based on evidence from reviews, and not just single studies. Systematic reviews can inform better policies that can lead to  better lives.



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2 Comments on “How fruity should you be?

  1. Hannah Kikaya

    Systematic reviews are great in theory – but what about publication bias? Giving the results of systematic reviews greater weight in the evidence ladder can serve to amplify the distortions that favour publication of positive findings. This issue is a particular concern in health and development where few studies or papers make their way into the scientific record. If all program evaluations were systematically published and accesible to researchers – rather than remaining inaccesiably archived in the libraries of program funders – the results of systematic reviews would be a lot more useful for improving decision-making.
    Hannah Kikaya
    Editor, Strengthening Health Systems

    Reply
    1. Howard WhiteHoward White Post author

      Dear Hannah

      A good SR does include unpublished studies, which is another reason to prefer them over reading single studies. Search strategies certainly do include gray literature, including agency reports which don’t make their way into journals. Study teams should also contact sector experts regarding on-going and unpublished studies. However, I do agree there are two challenges. First is studies which never saw the light of day, which may be because of adverse findings. Ex ante registration of study protocols will allow study teams to identify such studies and so follow up with the primary study researchers. For researchers in international development, the 3ie registry is available: http://www.3ieimpact.org/evaluation/ridie/

      The second issue are agency studies for specific projects which don’t enter the public domain just because it has not been agency practice to make these studies public, no matter if the findings are positive or negative. This practice is changing, though further lobbying in this respect is of course useful.

      Thanks for the comment

      Howard White

      Reply

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