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Median-centered dietary indices do not accurately classify exposure to the Mediterranean diet

European Journal of Clinical Nutrition volume 66, page 974 (2012)Cite this article

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Sánchez et al.1 found that adherence to the Mediterranean diet (MD) at baseline and increase in the adherence over time were associated with better health-related quality of life (QoL). However, exposure misclassification stemming from a median-centered dietary index may not accurately reflect the relationship between MD and QoL.

The authors created a score to assess adherence to MD by comparing a participant's consumption for an MD component with the group median at each time point. Consequently, adherence is not a function of an individual's dietary change since baseline, but rather a reflection of a person's diet relative to the group. If group consumption changed over time, a person's MD adherence score could change even if diet remained constant. This bias would differentially affect individuals near the middle of the distribution, who would be more likely to fall on either side of a shifting threshold. Moreover, combining the adherence values from year 2 and 4 to create a composite score fails to accurately capture exposure time to MD. An individual with increased adherence to MD at year 2 that is then maintained has a different length of exposure than someone who maintains adherence at year 2 but increases it at year 4. Moreover, individuals who are at either ends of the distribution may be less likely to have decreased adherence at the low end or increased adherence at the high end, simply because they are already consuming an extreme diet. The dietary index method does not capture the changing probability of likelihood with changing levels of adherence. As written, this article does not adequately classify adherence to MD over time and subsequently may not accurately reflect the true relationship between exposure to MD and health-related QoL.

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References

  1. Sánchez PH, Ruano C, de Irala J, Ruiz-Canela M, Martínez-González MA, Sánchez-Villegas A . Adherence to the Mediterranean diet and quality of life in the SUN Project. Eur J Clin Nutr 2012; 66: 360–368.

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  2. Hu FB . Dietary pattern analysis: a new direction in nutritional epidemiology. Curr Opin Lipidol 2002; 13: 3.

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  1. Department of Nutrition Gillings School of Global Public Health and School of Medicine, Chapel Hill, NC, USA

    L P Smith

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  1. L P Smith
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Correspondence to L P Smith.

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Smith, L. Median-centered dietary indices do not accurately classify exposure to the Mediterranean diet. Eur J Clin Nutr 66, 974 (2012). https://doi.org/10.1038/ejcn.2012.73

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  • DOI: https://doi.org/10.1038/ejcn.2012.73

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