The latest study to make the headlines suggests that eating five portions of fruits and vegetables a day is simply not enough and that it needs to be at least seven or even more. What are we to make of this? The short answer is “not much” and here’s why.
Firstly the study asked participants to report their food consumption for a single day and this is obviously problematic. You may have had an unusual intake of vegetables on the day you were asked to report or you may have had a day eating junk because you were travelling. The researchers will tell you that because the study population was large, this would not have introduced any significant bias, but it’s not always easy to exclude. The next problem is self-reporting food intake. How many people know what a portion of fruit and vegetables is and how accurate do you think they are at telling you what they had the day before? We all exhibit a certain degree of “recall bias” in which we tend to under-estimate bad behaviours and over-estimate good ones. So we invariably under-report the number of chocolate biscuits we ate and over estimate how long we spent on the exercise bike. It’s quite likely that participants in this study over-estimated their consumption of fruits and vegetables, though again the researchers will tell you they “adjusted” the data to take this into account.
What they did not take into account (because they didn’t ask about it), was how much physical activity the study subjects engaged in. This is a glaring omission. Numerous studies show that those who participate in regular, moderate physical activity, have a much lower risk of heart disease and cancer than their more sedentary counterparts. So it could well be – indeed it seems likely – that those who ate more fruits and vegetables every day, also had higher levels of physical activity and it was these benefits which were observed in the study, not the effects of fruit and vegetable intake.
The problem with this study is that the methodology – the way the study was designed and carried out – is inadequate to support the claims being made. Nowadays, of course, researchers are under pressure to publish their results and to achieve a maximum “impact factor”. So if you can get your results reported on TV or Radio 4 it’s likely to improve your standing in the next round of research funding – and the media will choose things that look interesting or controversial and publicize them. This is not the way to carry out scientific research or to promote public health. It leads to health claims which are routinely over-stated because they are simply not supported by the evidence. Worse still, it leads to public apathy in the face of an unremitting torrent of contradictory advice which most people will understandably want to ignore. In this case, they would be right.
Dr David Ashton MD PhD
8th April 2014