Not All Plants Are Equal: Diet Quality and Inflammation in Vegans and Vegetarians in Urban Poland

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The original article, “Not All Plants Are Equal: Diet Quality and Inflammation in Vegans and Vegetarians in Urban Poland,” submitted to be considered for publication in Nutrients, aims to investigate the association between dietary indices and inflammatory markers.

Minor comments

• Abstract—The authors specify as the aim of the study the evaluation of the association of dietary indices and “reduced inflammatory markers in a large cohort of vegans and vegetarians in urban Poland”. In my opinion, a study with only 198 participants is not considered to involve a large cohort of participants. Secondly, the affirmation implies the idea of a causal effect: plant-based diets lead to a lower level of pro-inflammatory markers (thus inflammation), a formulation that is not appropriate for a cross-sectional study.
• The Introduction provides extensive background and identifies an important gap in knowledge that underlines the need for this research.
• The Materials and Methods section describes the study population. The authors mention a previous study that presented the inclusion and exclusion criteria. I consider them to be important and deserve to be remembered in this research, too.
• A power analysis of the sample size would add a solid background for supporting the results.
• There is a typo error in lines 180-182
• In the Results section, in Table 2, the significant differences between categories are difficult to read. For instance, for IL-6 ≥1.5, do the letters b and c signify a significant difference in the percentage of OV and VG? The same difficulty is in understanding the differences between categories related to the number of lymphocytes.
• The Discussion Section is adequately composed, presenting the main findings of the study and their relationship with other research, the novelty of the article, and the strengths and limitations.

Author Response

Dear Reviewer,
I send detailed responses to your comments in the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Major Comments

The topic is relevant, yet it is not clear why this paper needs to be published. Its novelty with respect to several previous papers including those by Satija et al., Kim et al., and also the recent analysis from the UK Biobank cohort should be explained in more detail. Authors should make an effort to clearly state why a Polish population provides additional evidence for the field or what makes the hPDI unique as compared to other studies published in the European or US populations.

The cross-sectional nature of the study weakens the conclusions regarding cause and effect. The Discussion section only briefly mentions this limitation, and as a result, the interpretation of some results is not convincing. It is crucial to differentiate between association and causation and offer ideas for a future follow-up of this cohort.

No power analysis has been performed to check whether the sample size (n=198) is adequate for finding statistical differences among the three groups.

The analysis did not take into account several important confounders such as BMI, smoking, physical activity, socioeconomic status, and supplementation. Authors should perform a multivariable regression analysis to adjust for these covariates.

The use of non-parametric tests and simple correlations in the paper is an appropriate approach for data that do not follow a normal distribution; however, these statistical tools cannot be used to prove an independent association of an inflammatory biomarker with plant-based dietary patterns and/or the hPDI. The authors should correct for multiple testing (e.g., Bonferroni correction) and use multivariate models (e.g., linear regression or ANCOVA) to control for other covariates.

The finding regarding IL-6 is inconsistent and might indicate a non-specific effect or some lack of sensitivity in the assay. Therefore, more information about this assay would be helpful. For example, what is its limit of detection, what is the variability, was the IL-6 diurnal variation was taken into account, etc. ?

The methods for assessing dietary intake (FFQ and 24-hour recall) are all based on self-recall and are likely to be influenced by recall bias. Therefore, it would be interesting to see if these questionnaires have been validated or show acceptable reproducibility and reproducibility in Polish populations. Moreover, it would be useful to report on how underreporting was minimized and any misclassification has been taken into account.

Construction of the PDI and the hPDI is not explained in great detail. Authors should clarify how each food group/item was scored, weighted, and categorized in the diet quality index. Please add a supplementary table to the paper that would illustrate the scoring matrix with some examples.

Tables 1–4 are too dense to understand. It would be better to move some of them to the supplementary material. Moreover, all these tables lack p-values for the statistical comparisons and effect sizes (e.g., Cohen’s d) where applicable.

The Discussion section is quite long and mostly summarizes the literature. The authors should highlight the key findings of the study and discuss potential reasons for discrepancies between their findings and those of others. For example, IL-6 and CRP data are presented but were found to be unassociated with the hPDI in the US population, which seems very interesting and should be discussed in more detail. It would also be relevant to talk about diet quality indices (US DHQ, AHEI, MED) in the European cohorts and what this Polish population adds.

Diet quality is claimed to modify chronic low-grade inflammation but with no apparent mechanisms to explain this. Antioxidant load, microbiota modulation, short-chain fatty acid (SCFA) production, and so on could be discussed.

There are several grammar mistakes and repeated sentences. For instance, authors mention “In our previous paper [40]” more than 10 times. The paper needs to be polished to improve the language and flow.

The Conclusion section is too long and the statements in it do not reflect the data nature (observational). For instance, “Our data demonstrate that diet quality” should be “Our data suggest that diet quality” or something along those lines.

Author Response

Dear Reviewer,
I send detailed responses to your comments in the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Need to accept.

Author Response

Dear Reviewer,

Thank you very much for taking the time to review this manuscript. I accept all Comments and Suggestions below.