Why I won’t be eating two more portions a day

The health service, nutritionists, the government and your parents are always telling you to eat more fruit and veg.  Its sound advice, it really is, but how can we actually quantify the amount of fruit and veg a person should eat?  The answer is we can’t.  At least not accurately enough to warrant telling people exactly how many ‘portions’ we should be eating in a day.  The ‘5-a-day’ message is a reasonable one though.  Even if it does seem to suggest there is a set amount of fruit and veg the average person should eat in a day, the real message is plainly eat more fruit and veg because it’s good for you.

So why then do I turn on the TV this morning to find the news barking about how a new study has shown we need to eat 7 portions of fruit and veg a day to cut the risk of dying of common diseases?

The news comes from a study carried out by researchers at University College London who analysed questionnaire data collected by the NHS on people’s diet and lifestyle.  Essentially, what has been reported is that the study indicates that the more fruit and vegetables people ate, the less likely they were to die, at any given age.  In other words – fruit and veg is good for you – not a particularly new message.  So why, oh why, then did they have to go and quantify how much you should be eating to avoid death?  It’s just not possible, and not just because it’s difficult to interpret real world quantities from a self-filled questionnaire, but because the study itself has flaws which make it impossible.  This is not a criticism of the research but more a facet of this type of study which should be respected.

Snap-shot

The first thing to note about the study is that it collected data from participants from 2001-2008.  Although this is better than a single measurement it still only represents a relatively short amount of time in a persons life. This isn’t particularly useful when a healthy lifestyle is something you have to maintain rather than just be healthy at the exact moment you were asked.  This isn’t to say that this kind of study isn’t useful but its definitely a limitation worth considering.

Confounding factors

The other big factor which needs to be taken into account is other lifestyle and environmental factors which affect general mortality.  The participants in the study are likely to be exposed to a wide range of different factors such as smoking, drinking, exercise, where they live, what their job is etc.  You can try to take these into account when analysing the data but you’ll never be able to remove them from the equation – particular when you are dealing with a large scale cross-sectional study.  There’s also the issue that healthy eaters tend to live healthier lifestyles.  So how do we know that the benefits of 7-a-day aren’t because those people go to the gym more often or don’t smoke?

Risk reduction paradigm

It’s also worth mentioning that lifestyle interventions that reduce risk against anything need to be assessed alongside the impact of other lifestyle factors.  This is important so we can see the ‘weighting’ of each factor on risk reduction, rather than each in isolation, and as such be able to work out how much impact a certain intervention has to an individual.  For example, if you eat 7-a-day but you are also a heavy smoker; does eating 7-a-day even have an impact on your risk of cancer or heart disease?  The effects of eating 7-a-day, over say 5-a-day, could be so small that the increased risk from smoking makes it irrelevant.  In fact, if you are a heavy smoker it might not be any benefit to eat fruit and veg at all.

Remarks

The BBC have been cautious over touting a 7-a-day message which is great to see but I haven’t yet seen the full media coverage and I expect there will be some which completely overstate the results in this study.  I don’t think the study authors should have allowed a 7-a-day message to accompany their research because I think they are a long way off showing that eating 7-a-day has any benefit over 5-a-day.  Whichever is the case, the important message is that a healthy diet and a healthy lifestyle are going to be your best bet at having some influence over how and when you die, but I wouldn’t get too hung up on it.

Full study article (Open Access, yay!): http://jech.bmj.com/content/early/2014/03/03/jech-2013-203500.full

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4 thoughts on “Why I won’t be eating two more portions a day

  1. Just read the article and your comments. Both are interesting. I agree that promoting 7+ portions of fruits and vegetables seems not meaningful given the fact that a vast majority of the population cannot accurately recognize portion size. Besides, their data showed that participants in other categories (3-5,5-7 portions/d) also had lower risk of all mortality and cause-specific mortality compared with people in the reference group (<1), so it's kind of misleading to say you should eat 7+ portions per day to have a lower risk of death. The more the better, but no need to panic if you couldn't achieve 7+.

    What I don't agree include 1) this is not a cross-sectional study, because participants were followed from the date of completion of the survey to the date of death or the end of follow-up (the first quarter of 2013). It's not a typical cohort study either, since new participants joined in the cohort every year (I didn't read the paper about the study design, so please correct me if I'm wrong). 2) residual confounding is a possible limitation, not only in this study, but in all observational epidemiological studies. Unless a strict intervention study is designed, you can always argue that other unmeasured factors may account for the association observed.

    • Hi and thanks for leaving a comment.
      I think the study is cross-sectional in the sense that data on fruit and veg intake was only collected once from each participant at one point between 2001-2008. I don’t think data was collected at regular intervals to assess average consumption prior to death or end of study. However, I may be wrong so please let me know if I am and I can make some changes!

      And yes I agree that residual confounding is present in all epidemiological studies, but because in this particular study there are so many other factors that could be playing a role and would have a big impact on mortality, I thought it was worth mentioning. I wasn’t suggesting that this study is the only one with limitations!

      • Good to know. I consider the study a prospective cohort study design based on their follow-up, although all information was collected only once at baseline. Nice comments anyway.

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