The Gut Microbiota and Vascular Aging: A State-of-the-Art and Systematic Review of the Literature

Round 1

Reviewer 1 Report

Overall, it would be good to organize the manuscript more systematically.

This is a review paper on microbiota and vascular aging, however the contents of vascular aging are too poor. The authors need to add more contents of vascular aging.

The contents of the table are too complex. It would be better to change it more simply.

Author Response

Reviewer 1

We responded to the Reviewer’s comments in red/italic, and we cited the text modifications in blue/italic.

Overall, it would be good to organize the manuscript more systematically.

Thank you for the comment. Reviewer 2 also suggested a different organization of the manuscript. However, after a discussion with the Editorial Board we decided to maintain the actual organization, and to revise the manuscript deeply and extensively.

This is a review paper on microbiota and vascular aging, however the contents of vascular aging are too poor. The authors need to add more contents of vascular aging.

Thank you. We are not sure about the Reviewer’s comment. The systematic review is indeed all focused on vascular ageing, as all the selected papers present data on vascular ageing. Therefore, if we understand correctly, we enhanced the description of vascular ageing and its measures in the introduction section, as it can be found at page 2, lines 82-90, as follows:

The main function of the arterial system is to dampen the pulsatility induced by the stroke volume during the systole. This is of pivotal importance as organs with high flow and low resistance (e.g. the heart, brain, kidney) are prone to the side effects of inreased pulsatility.[3] The large arteries, mainly the aorta, due to their elastic properties contribute to preserve continuous and low pulsatile flow into target organs. The progressive stiffening of the arterial tree from the center to the periphery, together with the phenomenon of pressure waves reflection, maintains and even amplifys blood pressure in order to guarantee a correct organ perfusion. Eventually, the peripheral resistances preserve the target organ microcirculation from pulsatility. During the physiological process of ageing, the arterial tree becomes more and more stiff, due to a change in the elastin-collagen ratio and to the deterioration of arterial extracellular matrix. This lead to increased pulsatility at the peripheral level, where target organs may be injured.

The contents of the table are too complex. It would be better to change it more simply.

Thank you for the suggestion. Indeed, the table was hard to read, and we have now a new and simplified version of the table. We also split the main table in two tables (animal and human studies), according to the comments of Reviewer 2.

Reviewer 2 Report

Manuscript ID: jcm-1720807

Arterial stiffness as a common accepted marker for the vascular ageing is found to associate with the composition of gut microbiota. Here in Agnoletti`s review article, a direct correlation and inverse relations of arterial stiffness, microbiota diversity, and abundances of bacteria associated with most fit microbiota composition was found in human and animal studies in the literature. However, due to the lack of large randomized interventional trials in humans, the current literature only shows a stable link between microbiota modification and arterial stiffness in animals. The article is aiming to promote the human interventional study which the microbiota modification could be a treatment of patients with cardiovascular diseases.

The structure of the manuscript is very unusual. The authors have made great efforts to take the background literature as the introduction and the latest progress review, and then take the systematic review as the third part. Depends on the formation requirement of the journal, authors may consider taking the section two, the “State-of-the-Art Review” out from the current manuscript and publish it back-to-back as a separate review article.

Comments for State-of-the-Art Review:

1. Consider to structure of review as:

A. Introduction: Microbiota and hypertension

B.

B.1 The role of SCFAs

B.2The role of Trimethylamine-N-oxide

B.3The role of Lipopolysaccharides

B.4 The role of salt

C.

C.1 Microbiota and exercise

C.2 Maternal heritage and genetic/epigenetic regulation

C.3 Nutrition and stiffness

C.4 Inflammation and immune system in Hypertension

2. In Line 95, add a comma after “Since more than two decades”.

3. Please add a sentence of conclusion or summery of the subsection in after finishing of Line 130.

Comments for Systematic review:

1. In Line 43, please start a new paragraph. And please add one sentence between the two paragraphs to indicating the logics of focusing on microbiota in dealing with vascular ageing.

2. In Table 1, there are multiple places where the first letter of the sentence or items are not capitalized.

3. In Table 1, some context should be further summarized instead of copying the whole paragraph of the referenced, which include but not limited to 4/6/12 of 32. Tables should be a way to present the summarized information in any review articles, even for the tables that are included in the supplementary documents.

4. In Line 282, please also indicated the numbers of studies for human and for animals in those 24 screened studies. And maybe divide the current Table 1 into tow tables, one for animal study, the other for humans.

Author Response

See the uploaded file

Author Response File: Author Response.pdf

Reviewer 3 Report

This manuscript from Agnoletti and colleagues provides a comprehensive review of the literature linking microbiota with vascular stiffness.

 Overall, this is a tour de force, but it is difficult for a general reader to follow.

 There are some suggestions for improvement..

 The statement that “CVD is mainly driven by high blood pressure (BP),” while commonly stated, is not necessarly true.  There is an association but causality may be reversed. As with all associations, the direction of causality may vary.  The authors should acknowledge the possibility of reverse causality – that systemic inflammation from other (e.g. metabolic) causes stiffens the artery wall (the authors describe the mechanisms for this phenomenon in lines 142-158), and it is well known that arterial stiffness causes hypertension, thus inflammation may be causing hypertension not the other way around.. In any case, this analysis is still highly relevant because gut microbiota, by causing arterial stiffening, are increasing cardiovascular disease, including hypertension.

 In a number of places, it’s difficult to interpret the findings without more help from the authors.  For instance, “fiber and acetate supplementation…increased Bacteroides a. prevalence.”  Without knowing whether that species is beneficial, it’s hard to interpret.As another example  “exercise reduced Bacteroidetes, while it increased Firmicutes, Proteobacteria, and 223 Actinobacteria in mice.”  Is this beneficial or harmful?  And another example: “hepatic GPx level was higher in SPF than GF and BF mice.”

 There are some paradoxes that seem unresolved.  Clostridia is associated with benefit in the garlic study, but with worse stiffness in the exercise study. Similarly, in Line 173,If TMAO increases CV risk, and it requires an intact microbiota to be formed, then it sounds like an intact microbiota would be an adverse situation. But in other places, it sounds like an intact microbiota is protective.

 MINOR ISSUES

There are some language issues that could require careful correction.

 F:B Ratio seems to be undefined

 Line 99

It would be helpful to define the cardiorenal weight index

Line 292 The term consensual seems to be mis-used.

 Line 316. Od should be Ob

 This sentence is ambiguous 341-342

 Line 369  In children with chronic kidney 369 disease (G1, 9.5 years-old; G2-G3, 13.7ys),

What is G1, 2, and 3?

Line 377 is confusing.

Line 390. What is “augmentation index”?

 The Table is quite comprehensive, but difficult to follow because the Microbiota and results columns appear to fuse from one study to the next.

Author Response

See the uploaded file

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The manuscript is well revised to the reviewer's opinion. There are no further comments.

Author Response

We thank the Reviewer

Reviewer 2 Report

The manuscript has been improve significantly, I appreciate the author`s efforts. I hope readers or audience will find this publication be helpful and informative to their research or studies.

Author Response

We thank the Reviewer

Reviewer 3 Report

The authors have addressed this reviewer's concerns, though the writing still requires grammatical editing, and a few paragraphs could still use a closing sentence with a concluding interpretation.  

Author Response

We want to thank the Reviewer for the interest on our manuscript.

In the present revised version of the manuscript, we include small sentences at the end of several paragraphs as requested, and we revised the English grammar.

We think that our manuscript has further improved.