Multi-Target Strategies for Enhancing Ceramide Production: A Review of Bioactive Ingredients in Cosmetic Science

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript effectively synthesizes current knowledge on ceramide metabolism, its regulation, and the mechanisms of bioactive ingredients that enhance ceramide production for skin barrier improvement. Following are specific comments:

1. Line 16 (Abstract): The abstract states that "approaches that leverage bioactive ingredients to suppress ceramide degradation and thereby raise stratum corneum ceramide content are being actively investigated". However, the main body of the review does not substantively discuss strategies or ingredients that primarily function by inhibiting ceramide degradation. This claim should either be supported with relevant content or rephrased to accurately reflect the paper's focus on enhancing synthesis.

2. Line 29 / Section 5.1: The abstract claims that "Translational and clinical findings across normal, sensitive, and aging skin demonstrate barrier function improvement and reduced dryness with these actives.". However, Section 5.1 primarily reviews evidence for ceramide-containing moisturizers/formulations in general, not specifically the ceramide-promoting bioactive ingredients detailed in Section 4. This section therefore appears digressive and should be refocused to directly link the clinical outcomes to the actives previously discussed, or its title and scope should be revised accordingly.

3. The section 3 is overly detailed and would benefit from streamlining. Descriptions of immune modulation (e.g., for PPARs/LXRs) are tangential to the core theme of ceramide synthesis regulation and could be condensed or omitted. The focus should be sharpened specifically on the regulation of ceramide synthesis, rather than providing a broad overview of lipid synthesis in general;  The heavy reliance on two review articles from over 15 years ago (Refs 27 & 28) for the PPAR/LXR sections is notable. Incorporating more recent primary research or reviews would enhance the timeliness of this part; The narrative flow of section 3.2 is currently somewhat confusing. Reorganizing this subsection to follow the logical upstream-to-downstream sequence of the AMPK → mTOR → Autophagy axis would significantly improve clarity.

4. Section 4: The mechanistic descriptions for some ingredients in Table 3 are oversimplified or vague, and occasionally contradict the more nuanced discussion in the text. For Eucalyptus leaf extract, Table 3 states "Supports SPT/SMase activity," but the text (4.1) correctly notes this mechanism is not consistently validated and its action is likely indirect. The table entry should be aligned with the cautious tone of the text; For Citrus flavonoids, the mechanism "Antioxidant support → ↑ ceramide-related enzymes" is too general. The text should clarify, if evidence exists, how antioxidant activity mechanistically links to the upregulation of specific enzymes in the ceramide synthesis pathway; Given the manuscript's title ("Multi-Target Strategies...") and the clear delineation of synthesis mechanisms in Section 2, it would be more insightful to categorize the bioactive ingredients according to their primary molecular target or pathway rather than by source (plant/fermentation). This would directly reinforce the "multi-target" strategy framework.
5.  The abstract states the review integrates evidence for "plant-derived and fermentation-based actives." However, Table 3 includes Niacinamide (commonly synthetic/fermentation-derived), Rapamycin (a bacterial product), and Aquatide™ (a synthetic peptide), which do not fit neatly into these two categories. 

6. Reference Accuracy: The clinical findings for Aquatide™ are cited to reference [14], (line 396) which is a general review on ceramide function. Please verify and cite the appropriate primary clinical study to support this specific claim.

Author Response

We sincerely thank you for your thoughtful and constructive comments. We have revised the manuscript accordingly to improve accuracy, clarity, and alignment between mechanistic sections and translational/clinical discussions. Below we provide point-by-point responses.

 

1. Line 16 (Abstract): The abstract states that "approaches that leverage bioactive ingredients to suppress ceramide degradation and thereby raise stratum corneum ceramide content are being actively investigated". However, the main body of the review does not substantively discuss strategies or ingredients that primarily function by inhibiting ceramide degradation. This claim should either be supported with relevant content or rephrased to accurately reflect the paper's focus on enhancing synthesis.

Response:
We thank the reviewer for this careful observation. The initial wording inadvertently implied a focus on degradation inhibition, which was not the manuscript’s primary theme. We have now rephrased the abstract to emphasize enhancement of endogenous ceramide biosynthesis, processing, and remodeling, consistent with the core content of the review.

 

Change in manuscript:

Ceramides are central to stratum corneum barrier organization and hydration. Beyond topical replenishment, ceramide-stimulating strategies increasingly aim to enhance endogenous ceramide biosynthesis, processing, and homeostatic remodeling in coordination with keratinocyte differentiation. In this review, we summarize the three major metabolic routes that shape epidermal ceramide output—de novo synthesis, salvage, and sphingomyelin hydrolysis—and organize representative bioactive ingredients by their primary molecular targets rather than by origin. Specifically, we map ingredients to tractable regulatory nodes, including transcriptional “liposensors” (PPAR/LXR), induction of biosynthetic/elongation and processing enzymes (e.g., SPT, CerS3, ELOVL4), provision of structural substrates and precursors (e.g., linoleate-rich lipids and glycosylceramides), salvage-pathway sphingoid bases that can reshape ceramide subclass output, and metabolic sensing/stress-response pathways centered on AMPK–mTOR–SIRT1/autophagy. Across these mechanisms, agents spanning botanical and fermented extracts, vitamins, sphingoid intermediates, lipid precursors, and pathway modulators (including autophagy-focused probes) have been reported to increase ceramide abundance and, in some contexts, favor barrier-relevant ultra-long-chain species and ω-O-acylceramides that support lamellar organization and the corneocyte lipid envelope. Translational and clinical studies in dry, sensitive, and aged skin generally associate such interventions with improved barrier function and reduced dryness. Aligning ingredient selection with defined biosynthetic and processing checkpoints—and verifying outcomes with lipidomics alongside clinical endpoints—may accelerate the development of evidence-based, ceramide-stimulating cosmetics.

 

2. Line 29 / Section 5.1: The abstract claims that "Translational and clinical findings across normal, sensitive, and aging skin demonstrate barrier function improvement and reduced dryness with these actives.". However, Section 5.1 primarily reviews evidence for ceramide-containing moisturizers/formulations in general, not specifically the ceramide-promoting bioactive ingredients detailed in Section 4. This section therefore appears digressive and should be refocused to directly link the clinical outcomes to the actives previously discussed, or its title and scope should be revised accordingly.

Response:

We appreciate this valuable suggestion. Section 5.1 has been refocused so that the cited clinical and translational evidence is explicitly tied to the bioactive ingredients described in Section 4 (e.g., niacinamide, autophagy-activating peptides, plant-derived PPAR/LXR modulators). The revised section now clarifies how these actives translate into measurable improvements in barrier recovery and dryness.

Change in manuscript:

5.1. Applications in Dry, Sensitive, and Aged Skin

Ceramides are central determinants of epidermal barrier competence, and barrier performance reflects not only total ceramide abundance but also chain-length distribution and subclass composition [5,46,66]. Altered ceramide profiles—such as depletion of very-long-chain species and disturbances in acylceramide-related fractions—have been reported in xerotic, atopic, and aged skin, aligning lipid quality with clinical barrier fragility. In barrier-compromised dermatoses (e.g., atopic dermatitis/eczema), ceramide-containing regimens have shown consistent improvements in symptoms and barrier-related endpoints in systematic reviews and randomized trials, providing translational support for ceramide-dominant barrier care strategies in cosmetically relevant dry or sensitive skin settings [61,67,68]. In parallel, “ceramide-boosting” actives such as niacinamide can augment de novo lipid synthesis (including ceramides together with cholesterol and free fatty acids), supporting a multi-target approach that combines topical lipid replacement with restoration of endogenous biosynthetic capacity [17,53]. In aged or post-menopausal skin, where ceramide quantity and chain-length profiles may decline, combining multi-chain ceramide supplementation with mechanism-based bioactives offers a rational framework to improve persistent dryness and barrier resilience [2,69,70]

 

3. The section 3 is overly detailed and would benefit from streamlining. Descriptions of immune modulation (e.g., for PPARs/LXRs) are tangential to the core theme of ceramide synthesis regulation and could be condensed or omitted. The focus should be sharpened specifically on the regulation of ceramide synthesis, rather than providing a broad overview of lipid synthesis in general; The heavy reliance on two review articles from over 15 years ago (Refs 27 & 28) for the PPAR/LXR sections is notable. Incorporating more recent primary research or reviews would enhance the timeliness of this part; The narrative flow of section 3.2 is currently somewhat confusing. Reorganizing this subsection to follow the logical upstream-to-downstream sequence of the AMPK → mTOR → Autophagy axis would significantly improve clarity.

Response:
We fully agree. Section 3 was condensed to focus strictly on ceramide-relevant regulation, tangential immune-modulation descriptions were shortened, and recent (2023–2024) primary studies were added to replace or supplement older reviews. The subsection 3.2 was reorganized to present the logical upstream-to-downstream flow (AMPK → mTOR → autophagy) for clearer mechanistic continuity.

Change in manuscript:

3.1. Transcriptional Regulation: PPARs and LXRs

PPARs (α/β[δ]/γ) are lipid-sensing nuclear receptors that coordinate keratinocyte differentiation with barrier-lipid synthesis, trafficking, and permeability-barrier repair [27,28]. Activation of all three isoforms increases canonical differentiation markers— including involucrin, filaggrin, loricrin, and transglutaminase-1—in vitro and in vivo, and receptor-deficient models confirm their on-target relevance [27,29]. At the metabolic level, topical PPAR agonists enhance key transcriptional nodes that support ceramide and allied barrier-lipid production, together with lipid trafficking and processing required for lamellar-membrane assembly [27,30]. Isoform- and ligand-dependent differences have been reported; however, the unifying concept is that PPAR signaling couples differentiation to barrier-lipid metabolism [27,31]. PPAR signaling also promotes lamellar-body (LB) biogenesis and secretion: PPAR activation supports lamellar-body–associated lipid transport and secretion, facilitating delivery of sphingolipid precursors for extracellular barrier maturation [27,32]. PPAR activation also enhances post-secretory lipid processing, increasing β-glucocerebrosidase activity and supporting stratum corneum acidification—key steps for efficient conversion of glucosylceramides into ceramides and for lamellar-membrane organization [19,27]. These combined mechanisms accelerate permeability-barrier recovery following acute disruption [31]. Several natural ingredients act through PPAR pathways. For example, oat-lipid extract promotes keratinocyte differentiation and ceramide synthesis via PPAR signaling [33], and caffeic acid enhances differentiation through PPAR-α activation [34]. While PPARs can influence inflammatory tone, this review emphasizes their primary roles as transcriptional “liposensors” controlling differentiation-linked barrier lipid programs [20,27].

LXRs (LXR-α/β) are oxysterol-sensing nuclear receptors expressed across the epidermis [28]. As RXR heterodimers, LXRs regulate epidermal lipid metabolism and differentiation. Upon activation by oxysterols or synthetic agonists (e.g., 22(R)-hydroxycholesterol, TO901317), LXRs induce differentiation markers, restrain proliferation, and activate transcriptional programs that support cholesterol and sphingolipid handling relevant to barrier homeostasis [27]. LXR activation further enhances epidermal maturation and lipid-handling pathways, positioning LXR as an additional transcriptional lever within the multi-target framework for ceramide homeostasis [27,30]. Although most epidermal LXR research emphasizes cholesterol metabolism, LXR signaling intersects sphingolipid pathways at multiple nodes, underscoring its role as a central regulatory hub for ceramide homeostasis [27].

3.2. Cellular Sensing: Autophagy, mTOR, and AMPK

Autophagy is a lysosome-dependent quality-control process that degrades damaged proteins and organelles, recycling substrates essential for homeostasis under stress [35]. mTORC1 suppresses autophagy and favors proliferative programs, whereas its restraint is permissive for differentiation-associated lipid handling [36]. Barrier disruption reduces ATP availability and increases oxidative stress, activating AMPK as a metabolic sensor [37]. Activated AMPK phosphorylates mTORC1 regulators (e.g., TSC2 and Raptor) and suppresses mTORC1 activity, shifting keratinocytes from proliferation toward a differentiation-permissive, lipid-anabolic state [37]. Sustained mTORC1 signaling can therefore be framed as a risk factor for impaired differentiation and disordered lipid assembly, while AMPK activation promotes barrier-oriented programs [37]. mTORC1 suppression permits ULK1-complex activation and initiates autophagic flux, supporting intracellular trafficking required for lipid handling during repair [35,37]. In keratinocytes, autophagy modulation has been linked to lamellar-body formation and lipidomic remodeling, connecting autophagic flux to barrier-lipid delivery and maturation [38]. This framework supports a functional role for autophagy as a “buffer” that helps align lipid delivery and processing with the demands of differentiation and barrier formation [35,38]. In addition, autophagy—particularly lipophagy—can mobilize lipid stores by turning over lipid droplets, thereby supplying fatty-acid substrates that may support ceramide synthesis and barrier lipid remodeling during repair [39]. Consistent with this axis, pharmacologic mTOR inhibition (e.g., rapamycin) activates autophagy and has been reported to increase epidermal ceramide levels in experimental settings [40]. Collectively, AMPK-dependent restraint of mTORC1 and downstream autophagy provide a coherent axis linking energy status to lipid remodeling during barrier repair [35,37-40].

 

3. Section 4: The mechanistic descriptions for some ingredients in Table 3 are oversimplified or vague, and occasionally contradict the more nuanced discussion in the text. For Eucalyptus leaf extract, Table 3 states "Supports SPT/SMase activity," but the text (4.1) correctly notes this mechanism is not consistently validated and its action is likely indirect. The table entry should be aligned with the cautious tone of the text; For Citrus flavonoids, the mechanism "Antioxidant support → ↑ ceramide-related enzymes" is too general. The text should clarify, if evidence exists, how antioxidant activity mechanistically links to the upregulation of specific enzymes in the ceramide synthesis pathway; Given the manuscript's title ("Multi-Target Strategies...") and the clear delineation of synthesis mechanisms in Section 2, it would be more insightful to categorize the bioactive ingredients according to their primary molecular target or pathway rather than by source (plant/fermentation). This would directly reinforce the "multi-target" strategy framework.

Response:
We appreciate this critical point. Table 3 and the associated text have been restructured to align mechanistic descriptions with a multi-target framework. Ingredients are now organized by primary molecular target/pathway (liposensors PPAR/LXR; biosynthetic/elongation enzymes; substrate provision; salvage pathway; metabolic sensing via AMPK–mTOR–SIRT1/autophagy). Each entry now includes evidence strength qualifiers (“reported” / “suggested”) and the tone has been harmonized with the main text.

Change in manuscript:

4. 4. Bioactive Ingredients Modulating Ceramide Metabolism

Recent studies have reported diverse bioactive ingredients that can modulate epidermal ceramide homeostasis and thereby support skin-barrier function [12,17,45-48]. Rather than categorizing these agents by origin, they can be organized by their primary molecular targets, including (i) transcriptional “liposensors” regulating differentiation-linked lipid programs (e.g., PPAR/LXR), (ii) induction of key biosynthetic/elongation enzymes (e.g., SPT, CerS, ELOVL4), (iii) provision of structural substrates and precursors, (iv) salvage-pathway sphingoid intermediates that remodel ceramide subclass output, and (v) metabolic sensing and stress-response pathways centered on AMPK–mTOR–SIRT1 [20,23,28,31,32,35-37,49,50]. Collectively, these mechanisms can enhance not only ceramide abundance but also the differentiation-coupled processing and extracellular organization of barrier lipids required for effective barrier recovery [5,20,24,25]. Table 3 summarizes representative ingredients by primary target and simplified mechanism, while indicating whether evidence is demonstrated or suggested to maintain mechanistic consistency across Section 4.

While the present framework prioritizes a mechanism-first organization, the source of a bioactive (e.g., plant-derived vs. fermentation-derived) can still influence translational interpretation, including compositional reproducibility, standardization, and the availability of in vivo evidence. Plant-derived extracts frequently comprise chemically diverse mixtures, for which batch-to-batch variability and marker-based standardization become key determinants of reproducibility, whereas fermentation-derived materials (including postbiotic- or metabolite-enriched preparations) may offer improved compositional consistency but still require clear identification of active components and dose–response characterization. Importantly, the overall strength of in vivo/clinical support is shaped more by study design, endpoint selection (including subclass-resolved ceramide profiling), and product context than by ingredient origin alone; therefore, we retain the mechanism-first structure while noting origin-related practical considerations where relevant.

 

Table 3. Bioactive ingredients regulating epidermal ceramide homeostasis by primary molecular target.

Mechanistic Category (Section 4)	Representative Ingredient	Primary Mechanism / Molecular Target (Simplified)	Ref.
4.1 Transcriptional liposensors (PPAR/LXR axis)	Oat (Avena sativa) lipids	PPAR-linked differentiation/lipid program activation (reported) → supports coordinated synthesis/handling of barrier lipids and downstream processing; ingredient-specific evidence should be cited	[51]
	Caffeic acid	PPARα-associated differentiation signaling (reported) → may support differentiation-linked lipid handling; ingredient-specific evidence should be cited	[52]
	LXR agonists (conceptual node; not an “ingredient”)	LXR activation (oxysterol-sensing) → transcriptional programs supporting cholesterol/FA/sphingolipid handling relevant to barrier homeostasis	[15,28,31,32,44]
4.2 Enzyme induction / cofactors (biosynthetic & processing enzymes)	Niacinamide (Vit B3)	↑ SPT expression/activity → ↑ ceramides + other SC major lipids (FFA, cholesterol); consistent barrier-supportive effects reported	[17,53]
	Gentiana lutea extract	↑ CERS3/ELOVL4 in keratinocytes (reported) → supports VLC/ULC ceramide production under stress/differentiation conditions	[45]
	Strawberry seed extract (Tiliroside)	↑ SPT/CerS expression (reported) → promotes ceramide biosynthesis in epidermal equivalents	[54]
	Lactic acid / lactic acid isomers	Reported ↑ keratinocyte ceramide synthesis and improved SC lipid/barrier parameters (context-dependent; formulation/pH sensitive)	[48]
	Eucalyptus leaf extract (Macrocarpal A)	Enzyme-node support proposed (e.g., SPT/SMase-related steps reported in some preclinical work) but mechanism remains incompletely validated; avoid over-specific claims unless directly supported	[55]
	Citrus flavonoids (e.g., hesperidin)	Antioxidant/differentiation-supportive actions suggested → may indirectly support ceramide-related enzyme expression; direct enzyme-node evidence is limited	[56,57]
	Fermented citrus peel extract	Reported ↑ FLG and early differentiation-linked programs; sphingolipid-enzyme induction suggested but preliminary	[58]
	Ursolic acid	Reported increase in epidermal/SC ceramides in human skin, including hydroxylated forms (mechanistic target not fully defined; interpret cautiously in formulation context)	[59]
4.3 Precursors & structural substrates (substrate supply / acylceramide-CLE axis support)	Plant-derived GlcCer (oral; rice/wheat/konjac)	Substrate-level support: dietary GlcCer → hydrolysis/remodeling → delivery of sphingoid precursors for epidermal reacylation (conceptual); human TEWL/hydration trial citations should be provided	[60]
	Topical plant GlcCer / topical ceramide formulations	Improves lamellar organization/lipid order (vehicle-dependent); supports barrier function through structural lipid supplementation	[2,12,61]
	Evening primrose oil (linoleate/GLA source)	Provides essential fatty-acid substrates relevant to ω-linoleoyloxyacylceramide/CLE maturation chemistry (biochemical rationale); direct evidence for ω-O-acylceramide increase should be cited if claimed	[62]
4.4 Salvage substrates / sphingoid intermediates (ceramide subclass remodeling)	Sphinganine / sphingosine / phytosphingosine	Salvage-pathway substrate supply → remodeling/shift of ceramide subclasses in differentiating keratinocytes (species- and context-dependent)	[19,22,63]
	Exogenous ceramide (conceptual/optional row)	Exogenous ceramide can serve as a precursor/modulator for endogenous ceramide synthesis and keratinocyte differentiation (reported)	[47]
4.5 Metabolic sensing & autophagy modulators (AMPK–mTOR–SIRT1)	Rapamycin	mTORC1 inhibition → ↑ autophagic flux; reported increase in ceramide synthesis in skin experimental models	[40]
	Resveratrol	S1P signaling/cathelicidin axis reported; may intersect AMPK/SIRT1-autophagy networks (mechanism should match cited data)	[64]
	Aquatide™ / topical autophagy activator (SIRT1-linked)	SIRT1 activation → reported CerS2/3-dependent enhancement of barrier formation/ULC ceramides; clinical/functional outcomes reported for topical autophagy activation	[49,65]

- Abbreviations: PPAR, peroxisome proliferator-activated receptor; LXR, liver X receptor; SPT, serine palmitoyltransferase; CerS, ceramide synthase; ELOVL4, elongation of very long chain fatty acids protein 4; GlcCer, glucosylceramide; VLC/ULC, very-long-/ultra-long-chain; SC, stratum corneum; CLE, corneocyte lipid envelope; TEWL, transepidermal water loss; AMPK, AMP-activated protein kinase; mTORC1, mammalian target of rapamycin complex 1; SIRT1, sirtuin 1; SMase, sphingomyelinase.

 

5. The abstract states the review integrates evidence for "plant-derived and fermentation-based actives." However, Table 3 includes Niacinamide (commonly synthetic/fermentation-derived), Rapamycin (a bacterial product), and Aquatide™ (a synthetic peptide), which do not fit neatly into these two categories.

Response:
Thank you for pointing out this inconsistency. In the revised version, we clarified that the classification scheme is mechanism-driven rather than origin-exclusive. Nonetheless, a short explanatory paragraph was added noting that these actives, while synthetic or microbial in origin, share regulatory targets (e.g., PPAR/LXR activation or autophagy induction) relevant to ceramide homeostasis.

Change in manuscript:

Recent studies have reported diverse bioactive ingredients that can modulate epidermal ceramide homeostasis and thereby support skin-barrier function [12,17,45-48]. Rather than categorizing these agents by origin, they can be organized by their primary molecular targets, including (i) transcriptional “liposensors” regulating differentiation-linked lipid programs (e.g., PPAR/LXR), (ii) induction of key biosynthetic/elongation enzymes (e.g., SPT, CerS, ELOVL4), (iii) provision of structural substrates and precursors, (iv) salvage-pathway sphingoid intermediates that remodel ceramide subclass output, and (v) metabolic sensing and stress-response pathways centered on AMPK–mTOR–SIRT1 [20,23,28,31,32,35-37,49,50]. Collectively, these mechanisms can enhance not only ceramide abundance but also the differentiation-coupled processing and extracellular organization of barrier lipids required for effective barrier recovery [5,20,24,25]. Table 3 summarizes representative ingredients by primary target and simplified mechanism, while indicating whether evidence is demonstrated or suggested to maintain mechanistic consistency across Section 4.

 

6. Reference Accuracy: The clinical findings for Aquatide™ are cited to reference [14], (line 396) which is a general review on ceramide function. Please verify and cite the appropriate primary clinical study to support this specific claim.

Response:
We appreciate this correction. The inaccurate reference [14] was replaced with the appropriate primary, randomized, placebo-controlled clinical trial reporting the moisturizing and autophagy-activating effects of Aquatide™ (Lim et al., J Cosmet Dermatol, 2019).
Change in manuscript:
Citation corrected in Section 5.1 and reference list updated accordingly.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The article provides a clear, well-organized overview of recent advances in ceramide biology and the bioactive ingredients used to support ceramide production in cosmetic applications. The discussion on ceramide biosynthesis and its regulatory pathways (sections 2-3) is fascinating, with up-to-date references and a good level of scientific accuracy. The links between PPAR/LXR activation, autophagy-mTOR signaling, and phytochemical modulators are well articulated. The figures and tables effectively support the text.

 

A few minor changes could further improve the balance and clarity of the review.

• The introduction (lines 37-72) could be slightly simplified to avoid repetition of some concepts.
• In section 4, a more explicit comparison between plant-derived and fermentation-derived active ingredients would help readers understand which classes currently have the most compelling in vivo evidence.
• The future prospects (section 5.3) could also touch on formulation stability and compatibility, which are central considerations for translating these insights into actual products.

Overall, the review is well written, scientifically sound, and clearly relevant to the journal's readers.

Author Response

We sincerely thank you for your positive evaluation and the helpful suggestions to further improve clarity and translational relevance. We have addressed each point below.

 

• Comment 1: The introduction (lines 37-72) could be slightly simplified to avoid repetition of some concepts.

Response: Thank you. We edited the Introduction for concision, reducing repetitive phrasing while preserving the logical setup for the mechanism-first framework and the scope of the review.

Changes in manuscript: Introduction streamlined and tightened for readability while retaining the updated scope statement and review organization.

 

• Comment 2: In section 4, a more explicit comparison between plant-derived and fermentation-derived active ingredients would help readers understand which classes currently have the most compelling in vivo evidence.

Response: We agree this helps translational interpretation. While maintaining a mechanism-first organization, we added an explicit paragraph comparing plant-derived extracts (mixture complexity, batch variability, need for marker-based standardization) vs fermentation-derived materials (potential compositional consistency but still requiring active-component identification and dose–response validation).

Changes in manuscript: Added origin-comparison paragraph in Section 4 alongside Table 3 framing.

 

• Comment 3: The future prospects (section 5.3) could also touch on formulation stability and compatibility, which are central considerations for translating these insights into actual products.

Response: Thank you. We expanded Section 5.3 to include formulation translation considerations, highlighting chemical stability (oxidation susceptibility), solubility and pH-dependent stability, and variability introduced by complex botanical/fermentation mixtures, along with stabilization strategies and packaging alignment.

Changes in manuscript: Added a dedicated formulation stability/compatibility paragraph in Section 5.3.

 

Thanks for your kind consideration.

Sincerely,

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This is a well researched and written review, with very detailed biochemistry pathways.  There are some major gaps that should be corrected.

1. The review omits completely the role of crosslinking of ω-hydroxy ceramides to proteins such as involucrin to form the cornified lipid envelope and provide anchoring for the corneocytes to the lipid matrix.  This leaves the impression that it is just oil/water bilayers that maintain skin hydration, and that just adding more ceramides is good enough.  Without crosslinking, these are temporary solutions (despite short term improvements in TEWL). They should add a section discussing this crosslinking.
2. The authors should add a description of how they chose their papers to include in the review. 
3. Table 3 is misleading and should be removed or significantly qualified.  There are several "extracts" included but little or no description of what component is responsible for the activity.  This does not meet the rigorous standards advocated in Section 5.3 of this review. 
4. The text discussing Table 3  mentions that for several of them, the activity likely has nothing to do with ceramides, but "antioxidant" (L312) or "hydration" (L303) activity.  This should be more clearly stated in the table.
5. There is no discussion of ursolic acid, which has been shown to increase ceramides in human skin, particularly hydroxylated forms (Both et al., Arch Derm Res 293:569, 2002).  This should be added to the discussion of botanicals.

Author Response

We sincerely thank you for your positive evaluation and the helpful suggestions to further improve clarity and translational relevance. We have addressed each point below.

 

Comment 1

Response: We fully agree and have now added a dedicated section describing cornified lipid envelope (CLE) formation, including transglutaminase-mediated crosslinking of ω-hydroxy ceramides to cornified envelope proteins (e.g., involucrin), and the role of acylceramide processing in anchoring corneocytes to the lipid matrix.

We also reinforced this concept in the Conclusion to ensure it is carried through as a core barrier-architecture mechanism, not merely “more ceramides.”

Changes in manuscript: Added/expanded CLE discussion in Section 2.3, with consistent reinforcement in the Conclusion.

 

Comment 2

Response: Thank you. We added a transparent description of our targeted literature search strategy (databases, date coverage through Dec 2025, keyword clusters including CLE/acylceramide terms, and prioritization criteria emphasizing ceramide outcomes and barrier endpoints).
Changes in manuscript: Added “literature search and selection approach” in the Introduction.

 

Comment 3

Response: We agree and therefore substantially revised Table 3 to meet the evidence standards emphasized in Section 5.3. In brief, we:

• Reorganized the table by primary molecular target and explicitly indicate whether evidence is “reported” vs “suggested.”
• Where possible, specified responsible components (e.g., Macrocarpal A for Eucalyptus; tiliroside for strawberry seed extract) and added language to avoid over-specific mechanistic attribution without direct support.
• Explicitly stated when mechanisms are likely indirect (e.g., antioxidant/differentiation-supportive actions), rather than implying direct enzyme activation.

Changes in manuscript: Table 3 revised with qualifiers, component specification where known, and explicit indirect-effect labeling consistent with the main text.

 

Comment 4

Response: Thank you for this valuable addition. We added ursolic acid to Section 4.2 and cited the suggested work (Both et al., 2002), noting the reported increase in ceramides (including hydroxylated forms) while also acknowledging that the precise molecular targets were not fully delineated.

Changes in manuscript: Added ursolic acid paragraph in Section 4.2 and included the citation in the reference list.

 

Thanks for your kind consideration.

Sincerely,

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed my concerns. I have no further comments.