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The not so clean plate club: food self-served won’t always result in food eaten

International Journal of Obesity volume 39, page 376 (2015)Cite this article

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In a recent article, Wansink and Johnson1 combined data from 14 studies and on the basis of this conclude that in adults ‘about 92% of self-served food is eaten’. On the basis of this they suggest that we can extrapolate how much food will have been eaten in previous studies that measured self-served portion size alone.

When combining continuous outcome data (in this case percentage eaten) from multiple studies, it is best practice to individually weight study-level estimates, usually according to their sample sizes, and base conclusions on this. Without study weighting, studies with smaller sample sizes are over-represented. In one small study (n=35) in the present analysis, around 92% of self-served food was consumed.2 Conversely, in another study with over twice the amount of participants (n=88) close to 50% of food was consumed3 and in another study (n=85) similarly low consumption percentages were observed.4 Because Wansink and Johnson’s 92% headline estimate is aggregated and also not weighted according to the sample sizes and percentages of food eaten for each individual study (n=14), it is unlikely to accurately reflect the actual average consumption percentage of the 853 adults and may be an overestimation. In line with this although the 92% estimate in adults is the headline estimate provided by the authors, there is mention in the results section and Table 1 that when 13 of the 14 studies ‘are weighted by their sample sizes’, the mean percentage consumed is lower—83.7%. The lack of confidence intervals for the quoted percentages also makes it unclear how precise the estimates provided are likely to be. See Supplementary Material for further examples of potential problems associated with the unweighted aggregate estimates used by Wansink and Johnson.1

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References

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Authors and Affiliations

  1. Department of Psychological Sciences, University of Liverpool, UK

    E Robinson, E J Boyland, J A Harrold & J C G Halford

  2. Department of Biostatistics, University of Liverpool, UK

    S Nolan & C Tudur-Smith

Authors
  1. E Robinson
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  2. S Nolan
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  3. C Tudur-Smith
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  4. E J Boyland
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  5. J A Harrold
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  6. J C G Halford
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Correspondence to E Robinson.

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Supplementary Information accompanies this paper on International Journal of Obesity website

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Robinson, E., Nolan, S., Tudur-Smith, C. et al. The not so clean plate club: food self-served won’t always result in food eaten. Int J Obes 39, 376 (2015). https://doi.org/10.1038/ijo.2014.197

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