August 19, 2013

Sympathetic magic again

Once again, the Herald is relying on sympathetic magic in a nutrition story (previous examples)

1. Walnuts: These nuts look just like a brain, so it makes sense that they’re packed with good stuff for your grey matter.The British Journal of Nutrition reported that eating half a cup of walnuts a day for eight weeks increased reasoning skills by nearly 12 per cent in students. 

There’s no way that the appearance of a natural food could possibly be a guide to its nutritional value — how would the walnut know that it’s good for human brains, and why would it care? Pecans, which look a bit like brains, don’t contain the levels of n-3 fatty acids that are supposed to be the beneficial component of walnuts, and fish and flax seeds, which do contain n-3 fatty acids, don’t look like brains.

The story gets two cheers for almost providing a reference: searching on “British Journal of Nutrition walnuts reasoning skills” leads to the paper. It’s a reasonable placebo-controlled randomised experiment, with participants eating banana bread with or without walnuts.  The main problem is that the researchers tested 34 measurements of cognitive function or mood, and found a difference in just one of them.  As they admit

The authors are unable to explain why inference alone was affected by consumption of walnuts and not the other ‘critical thinking’ subtests – recognition of assumption, deduction, interpretation, and evaluation of arguments.

The prior research summarised in the paper shows the same problem, eg,  one dose of walnuts improved one coordination test in rats, but a higher dose improved a different test, and the highest dose didn’t improve anything.

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Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »

Comments

  • avatar
    Chris Hughes

    Definitely one for our old friend the Journal of Irreproducible Results and Weak Positive Correlations

    I wonder if the old UK satire show “Spitting Image” must take some responsibility for this one the sketch”The President’s Brain is missing”
    a puppet of Ronald Reagan finally finding a walnut

    12 months ago Reply

  • avatar
    Det Mackey

    What looks most like brains is brains and has a strong correlation with nausea. Publish that.

    11 months ago Reply

  • avatar
    Michelle Gosse

    Maybe the authors of the paper should have eaten some walnuts before they wrote up their conclusions section. :)

    Rather than doing a statistical fishing trip, shouldn’t there be some logic in the underlying biological mechanism of action that would lead one to undertake particular cognition tests and not others? So there should be some plausible biochemical explanation for the results rather than reliance on a p-value?

    I am wondering – why walnuts? Also, looking at figure 1 in the article, it would appear that banana bread containing walnuts may offer less protection against H1N1 flu than placebo banana bread. Sadly, the authors did not test this hypothesis.

    11 months ago Reply

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