A 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.