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Epidemiology and Population Health

The protective effect of obesity on mortality among those with (or without) CVD cannot be fully explained by collider-stratification bias

International Journal of Obesity volume 45, pages 918–919 (2021)Cite this article

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We thank Stovitz et al. for their interest in our paper [1] and to bring up an important issue. Collider bias may play a role in obesity paradox, but the protective effect of obesity on mortality cannot be completely justified by collider-stratification bias [2]. An unmeasured confounder (such as a gene) for the effects of CVD on death can cause selection bias for the effects of obesity on death among those with (or without) CVD, but the bias should be huge to be able to change the direction of the effects of interest, which is unlikely in our study for the following reasons:

(2) We adjusted for family history of premature CVD, as a surrogate of genes affecting CVD in the analysis. This confounder was associated with 58 and 33% increase in hazard for incident CVD and all-cause mortality after adjustment for confounders among Iranian population [8]. So any concern about the likely effect of unmeasured genetic confounder was dispelled by including family history in the model.

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Fig. 1: Collider-stratification bias for the effect of BMI on death.

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Authors and Affiliations

  1. Department of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran

    Nazanin Fekri & Mohammad Ali Mansournia

  2. Prevention of Metabolic Disorders Research Center, Research Institute for Endocrine Sciences, Shahid Beheshti University of Medical Sciences, Tehran, Iran

    Farzad Hadaegh & Azra Ramezankhani

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  1. Nazanin Fekri
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  2. Farzad Hadaegh
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  3. Azra Ramezankhani
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  4. Mohammad Ali Mansournia
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Correspondence to Mohammad Ali Mansournia.

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Fekri, N., Hadaegh, F., Ramezankhani, A. et al. The protective effect of obesity on mortality among those with (or without) CVD cannot be fully explained by collider-stratification bias. Int J Obes 45, 918–919 (2021). https://doi.org/10.1038/s41366-021-00756-y

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