Low Brain Levels of Dietary Polyphenols and Their Conjugates: Reassessing Mechanisms of Alzheimer’s Disease Prevention

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors present a critical assessment questioning the mechanisms of polyphenol action in in vivo settings, particularly considering the relatively high concentrations used in many studies compared with the very low concentrations that are actually detected in the brain. Given the discrepancy between the concentrations used experimentally and those that reach the brain, the manuscript addresses a highly relevant issue. The authors also propose two potential mechanisms of polyphenol action at low concentrations, targeting 67LR and quinone reductase 2, and discuss these potential targets specifically in the context of Alzheimer’s disease. Overall, the manuscript is interesting and relevant; however, several improvements are needed.

Major suggestions

1. Table 1 is not visually appealing or easy to read. The information currently provided below the table should be incorporated into the table itself as separate columns to improve readability. The animal species should be indicated in the table as a separate column rather than in the table description. The mode and frequency of administration should also be specified (e.g., single administration, repeated dosing, and duration of treatment).
   In addition, the main text should discuss what is known about polyphenol accumulation in the brain during prolonged supplementation. The authors should compare concentrations detected in the brain across different species and between acute and chronic administration.
2. Figure 2 and Table 2 relate to the precursor protein 37LRP. The formation of the complete 67LR protein may introduce structural features that are not present in 37LRP. Since the manuscript focuses on 67LR, the relevance of binding to the full-length form should be discussed more explicitly.
3. A substantial portion of the text (Sections 6.1.2–6.1.4) relies heavily on references 170–172. Are there studies from other research groups demonstrating the binding of polyphenols to 67LR? Additional supporting literature should be considered if available.
4. Line 598 – The text should be better organized. The importance of the Aβ–PrPᶜ complex should be explained in the main text before Figure 4, as its role is only described later in Section 6.1.5.
5. Lines 648–652 – This is an interesting statement; however, if the peptide G region is occupied by phenol red, how is Aβ toxicity then achieved? This point requires clarification.

Minor issues

• L120 – “ ...and glucuronide conjugates of intestinal microbila metabolites.“
• L126–134 – References should be added.
• L142 – Quercetin is a flavonol, not a flavanal.
• L144 – Sentence should be corrected: “ Administered quercetin up to 20% absorbed, and its bioavailability is only 2% [65].“
• L181 – “250 ml of green tea” is not an informative unit. Please indicate how many milligrams of green tea leaves were used to prepare the infusion.
• Ref. 103 and 109 appear to be irrelevant.
• L449 –3,4-dihydroxyphenylacetic acid glucuronide
• L770 – EGCG is a galloylated flavan-3-ol (catechin)

Comments on the Quality of English Language

Small improvements are needed

Author Response

Please see the attachment.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This review article provides a comprehensive overview of polyphenols and their potential neurobiological relevance. Overall, the manuscript is informative and well-organized; however, it would benefit from clarification, correction, or additional references to ensure scientific accuracy. The authors are requested to address the following points before the manuscript can be accepted.

1. Line 17: The authors state, “reactive oxidation products have not been detected in tissues.” Several studies, however, report the presence of polyphenol quinones or oxidation intermediates, particularly under oxidative conditions. The authors are encouraged to clarify this statement.
2. Line 21: The statement, “The peptide G region may also act as a redox sensor,” requires further specification. Please indicate whether this is supported by experimental evidence or represents a hypothetical proposal.
3. Usage of “~” with ranges: The manuscript frequently uses “~” with ranges or approximate values (e.g., ~60–70%, roughly ~10–20%, ~3.5 h). It is recommended to use either a range or an approximation, not both. For example, “60–70%” (omit ~) or “10–20% per year.”
4. Line 142: The statement, “Quercetin is a flavanol-type flavonoid,” appears to be a typographical error. Quercetin is a flavonol, not a flavanol. Please correct accordingly.
5. Line 143: The manuscript states, “These glycosides are hydrolyzed by intestinal bacteria,” which may imply that microbial hydrolysis is the sole mechanism. Quercetin glycosides can also be hydrolyzed by intestinal enzymes such as lactase-phlorizin hydrolase and cytosolic β-glucosidase. The authors are encouraged to revise this sentence to reflect both enzymatic and microbial pathways or clarify the statement.
6. Line 202: The statement, “Resveratrol crosses the BBB in vitro only 2% of the plasma concentration,” is unclear. BBB transport in vitro is typically expressed using permeability coefficients or transport ratios rather than plasma percentages. Please clarify the experimental system used (e.g., Transwell BBB model or endothelial cell monolayer assay) and provide the relevant parameters.
7. Line 236: The Authors state, “consumption of two cups of green tea could yield brain EGCG concentrations of roughly 10 nM,” appears speculative. The authors should indicate whether this value is theoretical, model-based, or extrapolated from pharmacokinetic data, and provide appropriate references or methodological details.
8. The manuscript may benefit from a brief introductory overview of general polyphenol pharmacokinetics, including absorption, metabolism, conjugation, and transport processes. This could help readers understand the compound-specific discussions.
9. Line 303: The sentence, “Shah et al convincingly demonstrated…,” is missing punctuation. It should read: “Shah et al. convincingly demonstrated…”
10. Line 348: The term “egalloylate” should be corrected to “degalloylate.”
11. In line 754, the authors state, 'receptor occupancy may be suboptimal…' However, no binding affinity (Kd) values are provided. The authors are encouraged to clarify or support this statement with relevant data.
12. "In line 765, the authors state, “to administer three or four well-characterized dietary polyphenols at low doses in combination.” While this concept is interesting, the section currently lacks supporting clinical evidence and discussion of potential pharmacokinetic interactions. The authors are encouraged to clarify the statement and provide appropriate references.
13. In line 805, the authors state, '67LR binds the AβO-PrPC complex…' Evidence for direct involvement of 67LR in this complex is not yet widely established in the mainstream Alzheimer’s disease literature. The authors are encouraged to clarify this statement and provide strong supporting references.
14. Line 806: The statement, “quercetin, resveratrol, and EGCG bind the same site on 67LR,” would benefit from additional clarification or supporting evidence.

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript can be accepted.