The rising incidence of chronic inflammatory disorders, particularly autoimmune and type 2 allergic diseases, is linked to the "westernized lifestyle." Atopic dermatitis (AD), a chronic skin disorder, is often an early manifestation of the "atopic march," a progression to other allergic diseases. The epidermis, composed primarily of keratinocytes, forms a crucial barrier against environmental factors. Keratinocyte differentiation involves producing structural proteins and lipids essential for stratum corneum integrity, which is compromised in AD. Short-chain fatty acids (SCFAs), produced by gut bacteria fermenting dietary fiber, modulate immune responses. Lower SCFA levels, particularly butyrate, are observed in children with AD, suggesting a link between dietary fiber, SCFA production, and skin barrier function. This study investigated the hypothesis that a low-fiber diet contributes to skin barrier dysfunction and increased susceptibility to allergen sensitization.

Existing literature highlights the increased global prevalence of allergic diseases and the association with the "westernized lifestyle." Studies demonstrate a strong correlation between impaired epithelial barrier function, increased allergen sensitization, and the development of allergic diseases, including AD. The atopic march, a sequential progression of allergies, is well-documented, starting with AD in infancy and often progressing to food allergies, allergic rhinitis, and asthma. The skin's epidermal barrier plays a crucial role in preventing allergen penetration. Genetic mutations, lipid profile alterations, and environmental factors can disrupt the formation of the cornified envelope, leading to skin barrier dysfunction and AD. SCFAs, primarily butyrate, propionate, and acetate, are metabolites produced from dietary fiber fermentation in the gut. Their immunomodulatory effects have been shown in various inflammatory settings, impacting both local gut immunity and distal organs such as the lungs and skin. The mechanisms include histone deacetylase (HDAC) inhibition, binding to specific receptors (FFAR2 and FFAR3), and influencing cellular metabolism. Studies have shown lower SCFA levels, particularly butyrate, in children with AD or at risk for developing allergies, supporting the hypothesis of a link between dietary fiber intake, SCFA production, and skin barrier dysfunction.

The study employed two experimental models in BALB/c mice. The first model used epicutaneous sensitization with house dust mite (HDM) allergens to induce AD-like skin inflammation (ADLSI). Mice were fed either a low-fiber diet (control) or a high-fiber diet (inulin). The second model used a low-fiber diet with oral administration of different SCFAs (acetate, propionate, butyrate, or water control). Disease severity was assessed by clinical scoring and trans-epidermal water loss (TEWL). Histological analysis (H&E staining) evaluated epidermal thickness and immune cell infiltration. Circulating HDM-specific IgE levels were measured by ELISA. Transcriptomics analysis (RNA sequencing) of lesional skin was performed to identify differentially expressed genes in butyrate-treated mice. Quantitative RT-PCR verified the expression of key genes involved in inflammation, itching, keratinocyte differentiation, and lipid synthesis. Flow cytometry assessed immune cell populations and their activation states in the skin. In vitro experiments using primary human epidermal keratinocytes (HEK) investigated butyrate's direct effects on keratinocyte differentiation. Phase-contrast microscopy, transmission electron microscopy (TEM), and gene expression analysis assessed morphological and functional changes. Lipidomics analysis (UHPSFC/ESI-QTOF-MS and UPLC/ESI-MS/MS) measured cholesterol and ceramide levels. Metabolic profiling assessed the effects of butyrate on keratinocyte metabolism, including fatty acid oxidation (FAO). Experiments with ¹³C-labeled butyrate tracked butyrate uptake and metabolism. Experiments using etomoxir, an inhibitor of FAO, assessed the role of FAO in butyrate's effects on HEK differentiation.

Mice fed a high-fiber diet or supplemented with SCFAs, particularly butyrate, exhibited significantly reduced ADLSI severity, improved skin barrier integrity (lower TEWL), decreased epidermal thickening, and reduced immune cell infiltration compared to controls. Butyrate supplementation also mitigated systemic allergen sensitization (lower HDM-specific IgE). Transcriptomics analysis revealed that butyrate altered the skin transcriptome, affecting pathways related to immunity and barrier function. Specifically, butyrate downregulated pro-inflammatory genes (IL-33, IL-4Rα, S100a8, Cxcl1, IL-20, IL-1β) and altered the expression of nerve-related genes (Areg, Sema3d) associated with itching. Butyrate upregulated genes involved in keratinocyte differentiation (Ivl, Lor, Flg, Lce1m, Clsp) and lipid/ceramide synthesis (Acc, Elovl6, Cers3, Smpd1, B3Galt2). Flow cytometry showed that butyrate dampened early cutaneous immune responses, reducing PD-L2 expression on APCs and decreasing CD4+ T cell numbers and activation. However, this effect was dependent on the route of allergen administration; intradermal administration of HDM bypassed the butyrate-mediated protection. Analysis of skin morphology and barrier function revealed that butyrate increased stratum corneum thickness and loricrin expression, particularly after allergen exposure. Lipidomics showed increased cholesterol and ceramides (particularly EO ceramides) in butyrate-treated mice after allergen challenge. Experiments with FITC-dextran showed butyrate reduced percutaneous allergen penetration. In vitro studies with HEK showed butyrate induced morphological changes characteristic of keratinocyte differentiation (reduced confluence, increased cell size, decreased nucleus-to-cytoplasm ratio). Butyrate decreased RNA and protein synthesis and increased expression of both early and late markers of keratinocyte differentiation. Lipidomics showed butyrate increased ceramide production in HEK. Metabolic analysis revealed that butyrate significantly altered keratinocyte metabolism, increasing mitochondrial FAO and LCFA synthesis. Butyrate increased LCFA uptake by HEK and enhanced FAO activity. Inhibition of FAO by etomoxir abolished butyrate's pro-differentiation effects on HEK.

This study demonstrates a crucial role for dietary fiber and SCFAs, especially butyrate, in maintaining skin barrier integrity and protecting against allergen-induced ADLSI. The protective effects are primarily mediated by butyrate's action on keratinocytes, promoting their differentiation and enhancing the production of structural proteins and lipids crucial for barrier function. The results highlight the gut-skin axis, emphasizing the importance of gut microbiota composition and dietary fiber intake in influencing skin health. The mechanism of action involves butyrate fueling mitochondrial FAO, thereby impacting keratinocyte metabolism and differentiation. The study's findings do not support the hypothesis that butyrate's protective effect is solely due to a direct modulation of immune cells; rather, the improved skin barrier function appears to be the primary mechanism. The observed differences in butyrate's effects compared to previous studies might be due to variations in experimental models and butyrate concentrations.

This research underscores the critical link between dietary fiber, gut microbiota-derived SCFAs, and skin barrier function. Butyrate, through its impact on keratinocyte metabolism and differentiation, plays a vital role in preventing and mitigating ADLSI. The study emphasizes the importance of dietary fiber intake in maintaining skin health and preventing allergic sensitization. Future research could investigate the specific gut microbiota composition responsible for butyrate production, the optimal dietary fiber sources and butyrate doses for therapeutic intervention, and the translational potential of butyrate supplementation for managing AD and other barrier-related diseases.

The study was conducted in a mouse model, and the findings may not be fully generalizable to humans. The study focused on a specific allergen (HDM) and might not encompass the full spectrum of AD triggers. The in vitro studies used primary human keratinocytes, which might not fully capture the complexity of the in vivo skin environment. Furthermore, not all immune cells present in the skin were comprehensively investigated.