Review Reports - Antioxidative and Anti-Inflammatory Mechanisms of Quercetin in the Prevention of Secondary Diabetes Complications

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review comments

Review on the manuscript entitled: "Antioxidative and Anti-inflammatory Mechanisms of Quercetin in the Prevention of Secondary Diabetes Complications" submitted to International Journal of Molecular Sciences.

Major Comments

The manuscript covers a topic that has already been extensively reviewed. The authors should clearly explain what distinguishes the present review from existing reviews on quercetin and diabetes-related complications.
Although the authors state that PubMed was searched, the literature search methodology is insufficiently described. The authors should provide the search date, detailed search terms, literature screening process, inclusion and exclusion criteria, and the rationale for including studies published outside the stated 10-year period. These details are important to improve the reproducibility of the review.
Quercetin has extremely low oral bioavailability and is rapidly metabolized in vivo via glucuronidation and sulfation; it is therefore difficult for the parent compound to reach in vivo concentrations comparable to those used in in vitro cellular experiments. Although the article briefly mentions this issue, it is only touched upon in passing in the section on diabetic nephropathy and is not systematically discussed in the Introduction, General Discussion, or Outlook sections. It is recommended that a short paragraph be added to the introduction specifically addressing the pharmacokinetic limitations of quercetin.
The manuscript does not sufficiently distinguish between quercetin prototypes and derivatives, nor does it differentiate between animal models of type 1 and type 2 diabetes. We recommend adding this information.
The title emphasises "prevention", whereas many of the studies discussed investigate therapeutic or ameliorative effects after diabetic complications have already developed. The authors should clearly distinguish prevention, attenuation, treatment, and reversal throughout the manuscript and reconsider whether the current title accurately reflects the evidence reviewed.

Minor Comments:

Several figure legends are relatively lengthy and could be shortened. Abbreviations should be defined at first use and used consistently throughout the manuscript.
The formatting of Table 1 should be improved. The current presentation includes multiple targets and pathways within individual rows, which makes the table difficult to read. Reorganizing the information into a clearer and more structured format would improve readability.
The terminology used for quercetin, quercetin derivatives, extracts, and combination treatments should be standardized throughout the manuscript to avoid ambiguity.
The reference list should be carefully checked for accuracy and consistency, particularly for studies cited to support specific molecular mechanisms.

Author Response

Major Comments

The manuscript covers a topic that has already been extensively reviewed. The authors should clearly explain what distinguishes the present review from existing reviews on quercetin and diabetes-related complications.

We thank the reviewer for this valuable comment. We acknowledge that few reviews in the past 5 years have discussed quercetin's therapeutic potential in diabetes and its effects on a specific complication. However, this review differs from earlier publications by comprehensively evaluating the molecular mechanisms underlying quercetin's protective effects across five major secondary diabetic complications: diabetic retinopathy, nephropathy, neuropathy, cardiovascular disease, and cataractogenesis. Furthermore, we have highlighted the emerging mechanisms, including ferroptosis, mitochondrial dysfunction, and autophagy that may contribute to quercetin's protective effects. This information has been included in the Introduction section. (lines 102-107).

Although the authors state that PubMed was searched, the literature search methodology is insufficiently described. The authors should provide the search date, detailed search terms, literature screening process, inclusion and exclusion criteria, and the rationale for including studies published outside the stated 10-year period. These details are important to improve the reproducibility of the review.

We thank the reviewer for this valuable suggestion. The primary objective of this review is to provide a comprehensive, critical overview of the molecular mechanisms underlying quercetin's protective effects against various secondary diabetic complications. We agree that adding more information about the literature search strategy would improve the manuscript's transparency and scientific rigor. Accordingly, we have included PubMed search dates, relevant keywords and their combinations, and the general approach used to identify and select studies. Although the review primarily emphasizes studies published within the last 10 years, we included selected earlier publications because they provide foundational mechanistic insights, establish important molecular pathways, or report experimental findings essential for understanding quercetin's therapeutic potential. We have clarified this rationale in the revised manuscript and added inclusion and exclusion criteria (lines 107-117).

Quercetin has extremely low oral bioavailability and is rapidly metabolized in vivo via glucuronidation and sulfation; it is therefore difficult for the parent compound to reach in vivo concentrations comparable to those used in in vitro cellular experiments. Although the article briefly mentions this issue, it is only touched upon in passing in the section on diabetic nephropathy and is not systematically discussed in the Introduction, General Discussion, or Outlook sections. It is recommended that a short paragraph be added to the introduction specifically addressing the pharmacokinetic limitations of quercetin.

We thank the reviewer for this important comment. We agree that quercetin's low oral bioavailability and extensive metabolism represent significant limitations to its clinical translation. As suggested, we have included pharmacokinetic limitations of quercetin and their implications for interpreting preclinical findings in the conclusions section (lines 860-877)

The manuscript does not sufficiently distinguish between quercetin prototypes and derivatives, nor does it differentiate between animal models of type 1 and type 2 diabetes. We recommend adding this information.

We thank the reviewer for this valuable suggestion. As suggested, we have revised the manuscript to clearly distinguish studies investigating the parent quercetin compound from those examining quercetin derivatives (highlighted the sentences in the manuscript). Further, we have now indicated type 1 and type 2 diabetes models in the respective statements. It is well recognized that chemically induced diabetic models, such as streptozotocin- or alloxan-treated animals, are frequently used to investigate insulin-deficient diabetes. In contrast, high-fat diet-associated models, genetic models, and certain combined high-fat diet/streptozotocin protocols are employed to reproduce selected metabolic features of type 2 diabetes.

The title emphasises "prevention", whereas many of the studies discussed investigate therapeutic or ameliorative effects after diabetic complications have already developed. The authors should clearly distinguish prevention, attenuation, treatment, and reversal throughout the manuscript and reconsider whether the current title accurately reflects the evidence reviewed.

We thank the reviewer for this thoughtful and constructive comment. We agree that prevention and treatment represent distinct therapeutic approaches and that these terms should be used carefully when interpreting experimental findings. However, we respectfully propose retaining the term “prevention” in the manuscript title, as the primary focus of the present narrative review is the potential protective role of quercetin supplementation in reducing the development and progression of secondary diabetic complications. Most preclinical studies discussed in this review have investigated quercetin as a dietary or pharmacological supplement administered during diabetes, with the objective of mitigating hyperglycemia-associated oxidative stress, chronic inflammation, advanced glycation end-product formation, mitochondrial dysfunction, and other molecular disturbances that contribute to diabetic complications. These studies predominantly emphasize the protective and disease-modifying potential of quercetin rather than its use as a therapeutic agent intended to reverse established organ damage. Importantly, although quercetin supplementation was initiated after the induction of diabetes in several experimental models, this does not necessarily indicate treatment of an established diabetic complication. Therefore, we believe retaining the term “prevention” appropriately reflects the review's central emphasis on quercetin supplementation as a potential strategy for limiting the development of secondary diabetic complications.

Minor Comments:

Several figure legends are relatively lengthy and could be shortened. Abbreviations should be defined at first use and used consistently throughout the manuscript.

As suggested, we reduced the length of the figure legends and defined abbreviations at first use.

The formatting of Table 1 should be improved. The current presentation includes multiple targets and pathways within individual rows, which makes the table difficult to read. Reorganizing the information into a clearer and more structured format would improve readability.

We thank the reviewer for this valuable suggestion. As suggested, we have revised Table 1 to improve its clarity, organization, and readability. (please see revised table)

The terminology used for quercetin, quercetin derivatives, extracts, and combination treatments should be standardized throughout the manuscript to avoid ambiguity.

As suggested, we have clearly indicated the type of quercetin used in various studies.

The reference list should be carefully checked for accuracy and consistency, particularly for studies cited to support specific molecular mechanisms.

As suggested, we carefully verified the accuracy and consistency of all citations and corrected them as needed.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This is a narrative review focused on quercetin and its possible role in prevention of secondary diabetes complications. The review is structured into five chapters each related to a specific secondary complication of diabetes.

The main problem with quercetin as a potential preventive treatment is the pleitropic effect if individual targets are under investigation. This is obvious from Table 1 that shows quite broad range of possible targets of quercetin. Therefore, the review attempts to point it out in the Conclusion chapter.

Comments:

All the figures show quercetin inhibiting what is downstream from hyperglycemia including, in all instances, oxidative stress, inflammation, and AGE-RAGE. What lies beyond that differs in what type of cell is affected. This is redundant. It would help the Reader if specific signals or pathways affected by quercetin are identified in the Figures, similar to Table 1.

Majority of the chapters lack information on what concentrations of quercetin were used in the experiments. There is a mention on the poor solubility of quercetin and the need for finding more soluble alternatives. Therefore, it would very much help to realize what range of concentrations are attainable in vitro versus in vivo and I suggest the concentrations are mentioned in each paper referenced. This may lead to shortening of the reference list if multiple references show the same.

Another suggestion is to remove and avoid any references to studies involving quercetin derivatives.

Last but not least, there should be a mention in the Conclusion section of the feasible concentration range of quercetin in plasma or tissues. This is important in connection with concentrations used in vitro.

Authors clearly identify the use of AI for language editing. Authors are advised to reveal if AI was also used in compiling, or excluding, data from articles generated through the PubMed search described in the Introduction. I would allow such use but require the acknowledgment.

Author Response

Comments:

We thank the reviewer for this suggestion. As suggested, we have included specific targets affected by quercetin in Figures 3, 4, 5, and 6.

We thank the reviewer for this comment. As suggested, we have included quercetin doses wherever possible. We recognize that the concentrations used in cellular experiments do not necessarily correspond to those achievable in vivo. While in vitro studies micromolar concentrations are often used, oral administration generally yields circulating quercetin-related compounds at nanomolar to low-micromolar concentrations. Furthermore, we agree that administered doses in animal studies cannot be directly equated with circulating concentrations because absorption, metabolism, and tissue distribution vary considerably across experimental models and formulations. We have therefore expanded the discussion of these pharmacokinetic limitations (please see highlighted text, lines 835-877). Regarding the reference list, the studies reporting similar concentrations may provide distinct mechanistic insights, investigate different diabetic complications, and employ different experimental models. Therefore, we have retained references that contribute unique scientific evidence as appropriate.

Another suggestion is to remove and avoid any references to studies involving quercetin derivatives.

We thank the reviewer for this valuable suggestion. We acknowledge that quercetin derivatives may exhibit different physicochemical, pharmacokinetic, and pharmacological properties compared with the parent compound. However, we respectfully believe that including selected studies involving quercetin derivatives and formulations is important for providing a comprehensive understanding of their potential protective effects against secondary diabetic complications. However, as suggested by the other reviewer, we have carefully revised the manuscript to clearly distinguish studies investigating the parent compound quercetin from those involving quercetin derivatives, formulations, and combination treatments. Removing these studies entirely would exclude potentially relevant mechanistic and pharmacological information, particularly concerning approaches designed to address the limitations of conventional quercetin supplementation.

We thank the reviewer for this constructive suggestion. We agree that discussing physiologically achievable quercetin concentrations is essential for assessing the translational relevance of the experimental findings presented in this review. In response to this comment, we have revised the Conclusion section to address the pharmacokinetic limitations of quercetin and the differences between concentrations employed in vitro and those achievable in vivo (lines 866-877).

We thank the reviewer for this important suggestion and for emphasizing the need for transparency regarding the use of artificial intelligence (AI) in manuscript preparation. We clarify that we used AI-assisted tools for language editing, grammatical corrections, improving the manuscript's clarity and readability, and formatting references according to the journal's guidelines. AI was not used to conduct the PubMed literature search, determine publication inclusion or exclusion, extract experimental data, or make decisions about the interpretation of scientific findings.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The author has fully addressed the comments I previously raised and revised the manuscript accordingly. The revised version shows a substantial improvement in quality, and the responses to the previous round of comments are thorough and adequate. I have no further substantive concerns and recommend accepting the manuscript as it stands.