PONE-D-20-23675

C-reactive protein and risk of cognitive decline: The REGARDS study

PLOS ONE

Dear Dr. Cushman,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration by 2 Reviewers and an Academic Editor, all of the critiques of both Reviewers must be addressed in detail in a revision to determine publication status. If you are prepared to undertake the work required, I would be pleased to reconsider my decision, but revision of the original submission without directly addressing the critiques of the 2 Reviewers does not guarantee acceptance for publication in PLOS ONE. If the authors do not feel that the queries can be addressed, please consider submitting to another publication medium. A revised submission will be sent out for re-review. The authors are urged to have the manuscript given a hard copyedit for syntax and grammar.

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Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented. 

Reviewer #1: Partly

Reviewer #2: No

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2. Has the statistical analysis been performed appropriately and rigorously? 

Reviewer #1: Yes

Reviewer #2: Yes

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3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: No

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4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: No

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The association between CRP and cognitive function has been extensively studied. Yet, findings are conflicting and the current study has the advantage of a large national, biracial sample that was followed up for a relatively long duration.

I have several comments:

Abstract:

1. please add values to the reported results. It seems like subheadings were omitted in line with the journal formatting but the sentences were not revised accordingly (line 3- "To examine...", results should start with words such as "We found that..." (line 39). In addition, the sentence in line 43 ("Findings suggest...") is redundant.

Methods:

2. Why did the authors exclude participants who had one or more errors on the Six-Item Screener? This may limit external validity. Please explain.

3. Why was the 90th percentile chosen as CRP cutoff? Did the authors identify a threshold there? Please provide a rationale.

Results:

4. Line 203 "adjusting for covariates did not change the results". Although p-value remained significant, the effect sizes did become smaller. Please revise the sentence accordingly.

Discussion

Some important literature seems to be missing. For example, Boydoun et at (PMID 30356710) explored the association between CRP and cognitive decline in Whites and African Americans. In addition, there are multiple studies demonstrating an association between CRP and brain function and structure (e.g. PMID 29304217). These may help explaining the underlying mechanisms.

minor comments:

Line 109 "After fully adjustment..." please rephrase as other confounders may exist that the authors weren't aware of or were not available for this cohort.

Line 243: rate decline (change to "rate of decline")

Reviewer #2: The manuscript “C-reactive protein and risk of cognitive decline: The REGARDS study” concerns an interesting theme related to aging. However, some alterations are needed in order to improve the quality of the manuscript. English use and writing are acceptable, but could still be refined. For example, past and present tense are mixed throughout the text (past tense is preferable). The use of a single measure of CRP is an important limitation of the study, given that authors say several times that this would be a marker of “systemic inflammation”, what is not true. This should be carefully revised, as also the affirmation that “effects” were found (longitudinal analyses provided no significant findings, and cross-sectional associations cannot infer causality). Specific comments to each section are given below.

Introduction

1. The topic of the study is not only relevant to the population of the United States, but worldwide. By starting the introductory paragraph of the manuscript specifically mentioning the United States, the authors seem to restrict their own interest to this country. I suggest editing the sentence to make it more general.

2. One important point about the relationship between inflammation and cognitive decline is that, mainly, it has been shown within the status of low-grade inflammation, which may have very different metabolic implications compared to acute inflammation. CRP may be used as a marker of low-grade inflammation, if repeated measures are used. This was not the case of the present study. So, in fact, authors have used a very general marker of non-specific inflammation to investigate such potential pathway affecting cognitive function. This has to be clearly stated in the manuscript since the beginning, and also recognized as a limitation of the study.

Methods

3. The topic “Design and procedures” should mention the maximum duration of follow-up.

4. Lines 83-84: Details about how the Stroke Belt is defined could be briefly presented. Again, authors need to consider that their manuscript will reach readers from many countries, not only from the USA.

5. Why classifying CRP as “low” or “elevated”? Naming those categories like this give the idea that both would be “not ideal”. If authors test the hypothesis that increased CRP would be associated with cognitive decline, then “elevated” (or “high”) could be one category, compared to “normal” values.

6. In the Methods, authors recognize that literature used a distinct cutoff for CRP, for what can be defined as “chronic systemic inflammation” (or low-grade inflammation). By using a very distinct cutoff based on other criteria (genetic ancestry affecting CRP in different races), authors in fact distance themselves from exploring low-grade inflammation (what would also demand repeated CRP measures, by the way). So, again, the fact that chronic systemic inflammation is not being investigated needs to be made very clear throughout the manuscript.

7. Lines 146-151: If fasting glucose, nonfasting glucose, total cholesterol, low density lipoproteins and high density lipoproteins were measured by the investigators, their analytical procedures should be reported, as done for CRP. Detailed information should be provided for blood pressure, weight and height assessments.

8. Were body weight and height also measured in duplicate as done for blood pressure? This information should be made clear. If they were not, this should also be reported as a limitation of the study.

9. Please check with the Editor if the Journal has any specific orientation about how to mention race / skin color categories. I suggest avoiding the use of the terms “blacks” and “whites”.

Results

10. Line 192. If categories of nutritional status based on BMI are given in the results, they should have been previously defined in the Methods.

11. Line 194. The word “greater” may suggest the idea of something that is positive, good. I suggest using “higher” instead of “greater” (here and all over the text) to avoid this misinterpretation.

Discussion

12. In the first paragraph, please indicate the population of the study (adults aged ≥45 years).

13. Line 221-223: Significant findings with baseline data do not infer causality, so the word “effect” should not be used. “Strong” could also be avoided. I suggest replacing “While the effect of continuous increments of CRP on baseline memory” by “While the associations between continuous increments of CRP and baseline memory”, and replacing “the effect remained strong” by “the association remained significant”.

14. Lines 227-228: Authors say that “the effect of CRP on baseline cognition was robust”, but cross-sectional associations did not test effect.

15. Lines 244-245: Avoid repeating “midlife” in the same sentence: “take place before the age of 65, in midlife given various studies suggesting that age-related cognitive accelerates during midlife”.

16. Authors did not discuss the fact that their single measure of CRP may have influenced the findings. As said before, a unique measure does not allow inferring the state of low-grade inflammation.

17. Another point that may have contributed to distinct findings compared to literature was the use of different cutoffs.

18. Line 260: Authors mention “systemic inflammation”, but again, they did not measured systemic inflammation. Please revise.

19. Lines 309-310: I do not agree with the conclusion, since authors again insist in saying that “systemic inflammation” was measured, but it was not. Also, the findings of the present study are not “strong evidence that systemic inflammation as measured with CRP has a deleterious effect on cognitive level”. Effect (and so, causality) is measured in longitudinal analysis, and your longitudinal analyses did not provide significant results to support this affirmation. The significant cross-sectional associations cannot infer any causal relationship, so authors should not call it “deleterious effect”.

20. Only 8 out of the 56 references of the manuscript are recent (published after 2016). I suggest including more recent publications about the topic.

Tables and figures

21. Table 1. I suggest including the “n” to all cells in which only the % is presented (categorical variables).

22. Table 1. Were all variables available to all 21,782 participants? If not, variables with missing values should present their exact “n”.

23. Table 1. The differences in baseline characteristics according to race should be given (either if there were, or in the footnote in case there were not).

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6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

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Reviewer #1: No

Reviewer #2: No

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

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Please submit your revised manuscript by February, 2021. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

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• A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.
• An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

We look forward to receiving your revised manuscript.

Kind regards,

Stephen D. Ginsberg, Ph.D.

Section Editor

PLOS ONE

Journal requirements:

When submitting your revision, we need you to address these additional requirements.

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at

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2. Thank you for including your ethics statement: 

"REGARDS was approved by Institutional Review Boards of all participating institutions and all participants provided written informed consent. Interviewers were trained to identify participants answering questions in a manner suggesting lack of comprehension, and such participants were not included further. Potential participants who were able to respond to telephone questions provided verbal consent, which was followed by written consent at an in home visit.".   

a. Please amend your current ethics statement to include the full name of the ethics committee/institutional review board(s) that approved your specific study.

b. Once you have amended this/these statement(s) in the Methods section of the manuscript, please add the same text to the “Ethics Statement” field of the submission form (via “Edit Submission”).

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3. We note that you have indicated that data from this study are available upon request. PLOS only allows data to be available upon request if there are legal or ethical restrictions on sharing data publicly. For information on unacceptable data access restrictions, please see http://journals.plos.org/plosone/s/data-availability#loc-unacceptable-data-access-restrictions.

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a) If there are ethical or legal restrictions on sharing a de-identified data set, please explain them in detail (e.g., data contain potentially identifying or sensitive patient information) and who has imposed them (e.g., an ethics committee). Please also provide contact information for a data access committee, ethics committee, or other institutional body to which data requests may be sent.

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We will update your Data Availability statement on your behalf to reflect the information you provide.

PONE-D-20-23675R1

C-reactive protein and risk of cognitive decline: The REGARDS study

PLOS ONE

Dear Dr. Cushman,

Thank you for resubmitting your work to PLOS ONE. Please make the corrections posed by Reviewer #2 so I can render a decision on this manuscript.

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Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: (No Response)

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2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented. 

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously? 

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: No

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5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: The quality of the manuscript was improved. Please find some minor comments below.

Line 74. The word “in” appears twice. Please delete one.

Please include in the limitations of the study the fact that weight and height were not measured in duplicate.

There are still several sentences using “greater”, for which “higher” would be more appropriate.

There are still some sentences in which the word "effect" must be avoided, because of describing cross-sectional findings (example: line 241).

Line 267-269: Authors say that “recent studies have reported (…) greater risk of dementia in later life [6]”. However, reference 6 is not “recent”, since it was published in 2002.

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7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #2: No

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

==============================

Please submit your revised manuscript by February, 2021. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

• A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.
• A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.
• An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

We look forward to receiving your revised manuscript.

Kind regards,

Stephen D. Ginsberg, Ph.D.

Section Editor

PLOS ONE

C-reactive protein and risk of cognitive decline: The REGARDS study

PONE-D-20-23675R2

Dear Dr. Cushman,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

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Kind regards,

Stephen D. Ginsberg, Ph.D.

Section Editor

PLOS ONE