Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Reply to ‘Healthy dietary indices and risk of depressive outcomes: a systematic review and meta-analysis of observational studies’

We recently read with considerable interest the paper entitled “Healthy dietary indices and risk of depressive outcomes: a systematic review and meta-analysis of observational studies”, by Lassale et al. [1]. We commend the authors for this important research and agree that the data are of considerable public health importance. However, upon reading the paper, we noted a number of methodological limitations with the conduct of this meta-analysis.

First, the authors state in the statistical analysis section on page 2: “For each dietary score (exposure variable), we conducted separate meta-analyses dependent on study design (cross-sectional vs. longitudinal)”. However, it appears from the actual reporting of the results in the figures that the authors pooled data from cross-sectional, case−control and cohort studies together (in addition to separate pooled effect sizes) and produced an overall effect size for all study design types. For example, in Fig. 1, in which the pooled effect size of cohort studies was 0.67 (95% CI: 0.55−0.82) and that of cross-sectional studies was 0.66 (95% CI: 0.35−1.24), the paper also reported a combined pooled effect size (OR = 0.69; 95% CI: 0.59−0.82) [1]. We suggest readers focus predominantly on the pooled effect sizes for the distinct study designs rather than on the overall pooled effect size, given that it is not methodologically correct to pool the study designs in this way.

Second, the authors appear to pool the evidence using ORs and not RRs, which is not in line with the common recommendations in meta-research [2,3,4]. Since HRs are time dependent, they should not be mixed with RRs/ORs (that are not time dependent) [5]. Other authors have reported that the use of RRs instead of ORs should be preferred for better accounting for the different estimates used [6]. Of importance, when an outcome is common (i.e. >10% in the unexposed group), the ORs usually exaggerate the RRs, making findings statistically significant when they are actually not [2]. Contrary to these assumptions, the authors reported the data as ORs, probably introducing a bias in their findings; RRs should have been reported instead.

Third, almost all the results reported in the discussed work are heterogenous (as I2 ≥50%). However, no meta-regression or sensitivity analysis for explaining these results has been reported. For example, the authors pooled together the studies using depressive symptoms and depression, but the agreement between depressive symptoms and clinical depression is often low−moderate [7]. The PRISMA guidelines [8], followed by the discussed review (page 2) [1], suggests to explore the sources of heterogeneity with appropriate statistical methods, such as meta-regression or sensitivity analyses [8].

Finally, the authors reported that “Estimates (beta and standard error) from studies that used depressive symptoms (outcome) as a continuous variable were converted into log odds ratios by multiplying by a factor 1.81 and then exponentiated”, citing the seminal work of Chinn [9]. However, Chinn reports that this conversion is useful when the data are reported as continuous (in his work as SMDs) and not as a result of a linear regression analysis (i.e. as beta) [9]. This concept is also accepted by the Cochrane group in Chapter 12 [10, 11].

In conclusion, we commend the authors on undertaking this work and welcome the findings of this important meta-analysis, but we believe that this paper has a number of potential methodological limitations that preclude any definitive answer on this topic.


  1. 1.

    Lassale C, Batty GD, Baghdadli A, Jacka F, Sánchez-Villegas A, Kivimäki M, et al. Healthy dietary indices and risk of depressive outcomes: a systematic review and meta-analysis of observational studies. Mol Psychiatry. 2018;24:1094.

    Article  Google Scholar 

  2. 2.

    Viera AJ. Odds ratios and risk ratios: what’s the difference and why does it matter? South Med J. 2008;101:730–4.

    Article  Google Scholar 

  3. 3.

    Sedgwick P. Relative risks versus odds ratios. Br Med J. 2014;348:g1407.

  4. 4.

    Schmidt CO, Kohlmann T. When to use the odds ratio or the relative risk? Int J Public Health. 2008;53:165–7.

    Article  Google Scholar 

  5. 5.

    Moayyedi P. Meta-analysis: Can we mix apples and oranges? Am J Gastroenterol. 2004;99:2297.

    Article  Google Scholar 

  6. 6.

    Cummings P. Methods for estimating adjusted risk ratios. Stata J. 2009;9:175.

    Article  Google Scholar 

  7. 7.

    Matias AGC, Fonsêca MdA, Gomes MdLdF, Matos MAA. Indicators of depression in elderly and different screening methods. Einst (São Paulo). 2016;14:6–11.

    Article  Google Scholar 

  8. 8.

    Liberati A, Altman DG, Tetzlaff J, Mulrow C, Gøtzsche PC, Ioannidis JPA, et al. The PRISMA statement for reporting systematic reviews and meta-analyses of studies that evaluate health care interventions: explanation and elaboration. PLoS Med. 2009;6:e1000100.

    Article  Google Scholar 

  9. 9.

    Chinn S. A simple method for converting an odds ratio to effect size for use in meta-analysis. Stat Med. 2000;19:3127–31.

    CAS  Article  Google Scholar 

  10. 10.

    Schünemann H, Oxman A, Vist G, Higgins J, Deeks J, Glasziou P on behalf of the Cochrane Applicability and Recommendations Methods Group et al. Chapter 12: Interpreting results and drawing conclusions. Cochrane Handbook for Systematic Reviews of Interventions Version 5; 2011.

  11. 11.

    Ioannidis JPA. Why most published research findings are false. PLoS Med. 2005;2:e124.

    Article  Google Scholar 

Download references

Author information



Corresponding author

Correspondence to Nicola Veronese.

Ethics declarations

Conflict of interest

The authors declare that they have no conflict of interest.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Veronese, N., Smith, L. Reply to ‘Healthy dietary indices and risk of depressive outcomes: a systematic review and meta-analysis of observational studies’. Mol Psychiatry 25, 3119–3120 (2020).

Download citation

Further reading


Quick links