Diet (Effect size = 0.12)

This post is part of a series looking at the influences on attainment described in Hattie J. (2009) Visible Learning: a synthesis of more than 800 meta-analyses relating to achievement. Abingdon: Routledge. The interpretation of Hattie’s work is problematical because the meaning of the different influences on achievement isn’t always clear. Further context here.

Working my way through the influences I’ve skipped a few that didn’t look terribly interesting, had low effect size, or had nothing to do with what happens in schools but I have had a little look at Diet (Effect size = 0.12) because I am surprised this is so low. At the college where I used to work, which was in a typical deprived coastal-urban setting, we had plenty of students who hadn’t been terribly successful at GCSE and were doing Level 1 and 2 courses to try to improve their qualifications. Amongst this group it wasn’t unusual to find that a student’s breakfast had been 1.5 litres of Coke and a Monster, which I always found pretty stunning. I think I would rather have White Russian on my Cornflakes than have to face drinking that lot in the morning!

I’ve tended to go along with the general opinion that Coke and Red Bull is likely to have a significant effect on learning performance and have this vague memory of various studies having shown that a balanced diet and a proper low GI breakfast leads to significantly better concentration during the school day. That certainly seems to be the opinion of leading nutritionists successful chefs appointed as government advisers. However I’m not sure that proper scientists would agree that the caffeine was a major problem. On the other hand, blood glucose levels and/or particular additives or nutrients might be a different matter.

I work quite closely with Professor Marcus Grace who, as well as tutoring on the Secondary Science PGCE at Southampton, is one of the significant figures involved in the LifeLab project. I really ought to get round to asking him about this – there is so much research expertise in the School of Education Studies that I need to work on tapping into! When I get round to doing that I’ll update this post; meanwhile, what evidence is Hattie basing his d=0.12 on?

There is one meta-analysis, Kavale and Forness (1983). I can only access the abstract but it’s clear that despite the missing clause in Hattie’s summary, the meaning that I had assumed he intended does match this meta-analysis. Equally it is clear that this is very specifically looking at ADHD and not children without this diagnosis. Essentially this paper states that the studies analysed do not provide evidence to support the earlier hypothesis that dietary changes could have a positive effect on ADHD symptoms. I’m guessing that the outcome measure was not academic achievement, but more likely some behavioural measure, which reminds me again that Hattie seems rather blasé about what his meta-analyses are measuring.

A quick trawl for more recent work suggests to me that things may have moved on, with this Schab and Trinh (2004) meta-analysis dealing only with double-blind, placebo-controlled trials getting d=0.21-0.28. Again there is this issue of whether or not Hattie’s 0.40 average effect size is the correct bar for comparison. With double-blind, placebo-controlled trials, it shouldn’t be. The methodology ought to make the inherent effect of the intervention zero and these authors are clear that their meta-analysis does show that artificial food colours affect ADHD. Having said that, when the trials were separated into groups according to who was reporting on the effects, teachers couldn’t detect any difference in behaviour but parents could. That’s not parents’ wishful thinking because of the double-blind; it might have rather more to do with the difficulty kids have in shifting their teachers’ expectations. Stevens et al. (2011) is a review of the literature, including both the meta-analyses mentioned above. They reach a similar conclusion but picking up the suggestion in Shab and Trinh that the effect might be restricted to only a proportion of children with an ADHD diagnosis (10%-33%). However the Bateman et al. (2004) study on the Isle of Wight involving Southampton academics and a further study (and a smaller one from the USA cited on p.286 in Stevens et al.) suggest quite strongly that artificial food colourings affect all children (well – young ones at least).

Since writing this post I’ve come across this Harvard Mental Health Letter reviewing the relationship between diet and ADHD. It includes the findings from the Schab and Trinh (2004) meta-analysis but also some other research. The conclusions are similar – that some artificial food colourings do seem to have an effect on at least a proportion of children, which probably means that reducing exposure is a good thing. It also suggests that increasing Omega-3 essential fatty acids and micronutrients might just help too. A final point is that the research on the effect of sugar on behaviour suggests there is no link (but of course the link with obesity and Type II diabetes is only too obvious). But the strongest message is that the usual recommendations for a healthy diet apply to all children.

Anyway, this isn’t something for day-today teaching. There are all sorts of issues around ADHD (like whether it is a useful diagnosis, whether drug treatments are a good idea, and so on) and even if all children are susceptible to artificial food colourings it’s possibly something teachers might helpfully be aware of but it isn’t going to affect what we do in our classrooms. I again find myself wishing that Visible Learning was narrower in its breadth and deeper in it’s depth but it’s been an interesting evening educating myself. Next, I’m going to jump to Time on Task (Effect size = 0.38) because I want to look at this in relation to a paper by Professor Daniel Muijs (another big hitter from the Southampton School of Education Studies) that suggests Time on Task is one of the most important influences on achievement.

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4 thoughts on “Diet (Effect size = 0.12)

  1. Pingback: Index to series on Hattie’s Visible Learning | docendo discimus

  2. Pingback: A closer look at two more Effect Sizes by Dodiscimus | ollieorange2

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