The BGSA consists of three interconnected systems: (1) the CNS, encompassing the hypothalamic–pituitary–adrenal (HPA) axis and autonomic pathways; (2) the gastrointestinal (GI) tract, including the intestinal microbiota and mucosal immune system; and (3) the skin, which serves both as a physical barrier and as an active neuroendocrine and immune organ.

Communication among these systems occurs bidirectionally through neuroendocrine signaling, microbial metabolites, immune mediators, and neural circuits such as the vagus nerve. The HPA axis plays a central role by releasing corticotropin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH), and cortisol in response to stress, thereby influencing gut barrier integrity, altering microbial composition, and regulating skin inflammation.

Acne vulgaris is a chronic inflammatory disorder of the pilosebaceous unit that is increasingly recognized as a multifactorial disease involving the BGSA. Its pathogenesis can be structured into three converging pathways: stress-induced neuroendocrine activation, diet-microbiome-mediated metabolic dysregulation, and neurogenic inflammation.

First, psychoneuroendocrine factors initiate the cascade. Stress activates the HPA axis, resulting in elevated levels of cortisol and CRH. These hormones directly enhance sebaceous gland activity and stimulate the release of pro-inflammatory cytokines such as IL-6 and TNF-α. Crucially, recent findings indicate that local neurogenic inflammation acts synergistically with systemic stress. Elevated concentrations of Substance P have been detected in patients with acne, which amplifies inflammation by stimulating mast cell degranulation and increasing cutaneous innervation density. This creates a feed-forward loop where emotional stress exacerbates local neurogenic signals within the pilosebaceous unit (49, 50).

Second, the gut–skin metabolic interface plays a pivotal regulatory role. Diet serves as a critical regulator, where high-glycemic-load foods and dairy products activate the insulin and insulin-like growth factor-1 (IGF-1) signaling pathway. This pathway is central to the BGSA in acne because IGF-1 not only enhances lipogenesis but also interacts with the mammalian target of rapamycin complex 1 (mTORC1). Gut-derived metabolites reciprocally modulate this mTOR signaling, linking nutrient sensing directly to sebaceous gland function (51). Concurrently, dietary patterns shape the gut microbiome. Gut dysbiosis, characterized by reduced Actinobacteria, increased Proteobacteria, and diminished diversity, has been consistently documented in acne patients (52, 53). Such imbalance facilitates LPS translocation and reduces SCFA production, driving systemic inflammation (19, 54). Specific gastrointestinal associations further validate this link; for instance, a positive correlation exists between Helicobacter pylori infection and acne severity, potentially mediated by bacterial lipase activity and oxidative stress (55).

Finally, these systemic triggers converge to alter the skin microenvironment. Negative emotional states have been shown to correlate with specific microbial shifts (e.g., changes in Cutibacterium and Acinetobacter), suggesting that stress and dysbiosis are not isolated events but act synergistically (56). Elevated serum zonulin levels in acne patients further implicate gut barrier dysfunction (“leaky gut”) as the conduit allowing systemic inflammatory mediators to reach the skin (57). Therapeutic evidence reinforces this integrated model, as probiotic supplementation and low-glycemic diets alleviate severity likely by simultaneously dampening IGF-1 signaling and restoring microbial equilibrium (58, 59). Thus, acne represents a prototypical BGSA disorder where neuroendocrine stress and metabolic dysbiosis converge on the pilosebaceous unit.

Psoriasis is a chronic immune-mediated skin disorder characterized by systemic inflammation and significant psychosocial comorbidities (60, 61). The BGSA drives psoriatic inflammation through a “triangular” interaction involving neuroendocrine dysregulation, gut barrier compromise, and specific metabolite-immune axes (47, 62).

The neuroendocrine arm of the axis is bidirectional. Patients frequently experience psychological distress, which activates the HPA axis and sympathetic nervous system. Beyond cortisol-mediated immunosuppression, gut microbes produce neurotransmitters—including dopamine, serotonin, and γ-aminobutyric acid (GABA)—that regulate neuroimmune pathways. Dopamine, for instance, has been shown to influence T-cell migration and promote keratinocyte proliferation (63). Conversely, proinflammatory cytokines (IL-1β, TNF-α, IL-6) released during psoriatic flares can cross the blood–brain barrier, modulating neuronal and glial activity, potentially driving the high prevalence of depression in these patients (48, 64).

The gut–skin connection in psoriasis is marked by profound dysbiosis and barrier loss. Recent research reveals reduced microbial richness, depletion of SCFA-producing bacteria (e.g., Faecalibacterium prausnitzii), and enrichment of proinflammatory taxa (Firmicutes, Proteobacteria) (9). This dysbiosis functionally impairs the intestinal barrier, evidenced by elevated intestinal fatty acid binding protein (FABP) levels (65). Disruption of the mucus layer facilitates the translocation of bacterial DNA and endotoxins (LPS) into the circulation, triggering a systemic inflammatory response that primes cutaneous immunity (10, 66, 67). Mechanistic proof is provided by animal studies where fecal microbiota transplantation (FMT) from psoriasis patients successfully induces cutaneous inflammation in germ-free mice, whereas interventions like Parabacteroides goldsteinii–derived extracellular vesicles ameliorate it (68, 69).

The most critical mechanistic insight lies in the metabolite-immune convergence, specifically involving the Th17 axis. Gut microbiota metabolites significantly influence host immunity through aryl hydrocarbon receptor (AhR) signaling and histone acetylation. While SCFAs like butyrate promote regulatory T (Treg) cell differentiation (70, 71), tryptophan metabolism shows complex duality: indole-3-lactic acid (ILA) is protective, whereas indoxyl sulfate exacerbates inflammation via Th17 activation (72, 73). Furthermore, Innate Lymphoid Cells Group 3 (ILC3s), which reside in both gut and skin, act as key effectors. Expression of CD200R1 on ILC3s is essential for IL-23–induced STAT3 activation and optimal IL-17 secretion (74), underscoring the complex interplay between microbiota and immune receptors. Microbial metabolites differentially modulate ILC3 function; for example, acetate enhances while butyrate suppresses IL-22 production (75). This gut-tuned ILC3 activity, along with the systemic release of IL-23 and IL-17, completes the pathological loop linking gut dysbiosis to psoriatic plaques. Consequently, therapeutic strategies targeting this axis, from probiotics to stress management, aim to break this self-reinforcing cycle of neuro-metabolic inflammations (76–79).

Atopic dermatitis (AD) is a chronic relapsing inflammatory skin disorder characterized by severe pruritus, epidermal barrier dysfunction, and pronounced microbial dysbiosis. Within the BGSA framework, AD pathogenesis is driven by a unique “vicious cycle” involving dual-site microbial dysbiosis, neuroendocrine stress responses, and metabolic-immune convergence.

Significant alterations in both the skin and gut microbiomes have been documented in AD patients. First, AD is distinguished by simultaneous dysbiosis at two barrier sites: the skin and the gut. Cutaneous dysbiosis is typified by a dramatic loss of diversity and the dominance of Staphylococcus aureus (80). Recent mechanistic insights reveal that S. aureus is not merely a colonizer but a direct driver of neurogenic inflammation. It forms biofilms and releases phenol-soluble modulin α (PSMα), a virulence factor that intensifies barrier damage and, crucially, activates protease-activated receptor-2 (PAR-2) on sensory neurons. This direct microbe-nerve signaling induces non-histaminergic itch, perpetuating the scratch-itch cycle (81). Concurrently, gut dysbiosis presents as a depletion of SCFA-producing bacteria such as Faecalibacterium prausnitzii and Bifidobacterium, accompanied by overgrowth of proinflammatory species including Clostridium difficile and Escherichia coli. This systemic imbalance compromises intestinal epithelial integrity and facilitates the translocation of microbial antigens into circulation (82, 83).

Second, the gut-brain axis regulates cutaneous immunity through specific metabolite signaling. The bidirectional communication is mediated by microbial metabolites, including SCFAs, neurotransmitters, and tryptophan derivatives. SCFAs, particularly butyrate, exert epigenetic control to promote regulatory T-cell (Treg) differentiation and reinforce epithelial barrier integrity. Tryptophan metabolism represents a critical checkpoint in AD: while some indole derivatives exert anti-inflammatory effects through aryl hydrocarbon receptor (AhR) activation, dysregulated metabolism can generate proinflammatory signals via neurogenic pathways (82, 83). Furthermore, gut microbes modulate systemic cytokine production, including IL-10 and IFN-γ, which reciprocally influence neural signaling and stress responses (84).

Third, psychoneuroendocrine factors act as powerful aggravators of this unstable system. Psychological stress activates the HPA axis, resulting in the release of cortisol and CRH. These mediators shift the immune profile towards a Th2-type response (IL-4, IL-5, IL-13), which directly impairs epidermal barrier function and inhibits antimicrobial peptide production. Stress also increases intestinal permeability, facilitating LPS translocation that stimulates innate immune pathways including TLR2 and TLR4. This establishes a self-perpetuating BGSA loop: stress amplifies inflammation and barrier loss, leading to intensified pruritus, which in turn feeds back to aggravate psychological distress (85, 86).

Finally, therapeutic interventions validate this integrated multi-axis model. Microbiome-targeted strategies are evolving from general to specific modulation. Oral probiotics (Lactobacillus and Bifidobacterium) have been shown to restore the Th1/Treg balance and suppress proinflammatory signaling, improving SCORAD scores (87, 88). More precisely, topical “bacteriotherapy” using commensals like Staphylococcus hominis or Staphylococcus epidermidis successfully inhibits S. aureus colonization and decreases PSMα production, directly breaking the neurogenic itch cycle (89). Metabolic interventions, such as dietary galacto-oligosaccharides (GOS), modulate the axis by enriching beneficial microbiota, elevating SCFA levels, and normalizing brain neurotransmitters (90, 91). Emerging approaches like prebiotics, postbiotics, and microbiota transplantation further aim to restore microbial homeostasis and mitigate dysregulation of the gut–brain–skin axis in AD (92).

Rosacea is a chronic inflammatory dermatosis that predominantly affects the central face, presenting with persistent erythema, telangiectasia, papules, pustules, and, in some cases, phymatous or ocular manifestations. Viewed through the lens of the BGSA, its pathogenesis is driven by a distinct neurovascular-microbial circuit, where central neuroendocrine dysregulation and gut metabolic shifts converge on a compromised cutaneous barrier.

First, the neurovascular arm constitutes the primary driver of the “flushing” phenotype. Clinically, neurogenic rosacea is defined by heightened flushing and erythema triggered by skin barrier dysfunction, thermal stimuli, stress, and hormonal fluctuations (93). The biological basis for this lies in central and peripheral neural dysregulation. Functional neuroimaging has revealed altered cerebral and limbic activity in rosacea patients, providing direct evidence of CNS involvement (94). Moreover, shared neuroendocrine pathways between the skin and gut can affect brain function through the autonomic nervous system and the HPA axis (95). This central signaling propagates to the skin via the dysregulated release of neuropeptides such as substance P, CGRP, and vasoactive intestinal peptide (VIP), which promotes vasodilation, mast cell degranulation, and immune activation (96, 97). Crucially, this system forms a bidirectional loop: brain-derived neurotrophic factor (BDNF) and serotonergic signaling link mood dysregulation to skin inflammation (98), while visible facial symptoms heighten psychosocial distress (depression, anxiety), reinforcing the pathological cycle (99).

Second, the gut–skin connection acts as a metabolic amplifier of this inflammation. Epidemiologically, this is supported by the high prevalence of gastrointestinal comorbidities such as H. pylori infection, small intestinal bacterial overgrowth (SIBO), and inflammatory bowel disease (IBD) (100). Microbiologically, next-generation sequencing reveals a specific “rosacea-associated dysbiosis.” While specific signatures vary—Nam et al. (101) reported increased Acidaminococcus and Megasphaera, Chen et al. (102) identified elevated Bacteroides and Fusobacterium, and Moreno-Arrones et al. (4) observed increased Akkermansia muciniphila—the functional consequence is consistent. Metabolomic profiling revealed 56 altered serum metabolites compared with healthy controls; notably, levels of 3, 4-dihydroxyphenylacetic acid were strongly correlated with Bifidobacterium and Lactobacillus abundance. This suggests that gut microbial imbalances do not act in isolation but actively modify the systemic metabolic landscape to drive pathophysiology (103).

Third, these systemic triggers alter the cutaneous microenvironment, leading to secondary microbial dysregulation. Microbial imbalance extends to the skin, centering on Demodex mites. Crucially, it is not just the mite burden but their bacterial cargo that drives heterogeneity. Murillo et al. found that mites from papulopustular rosacea carried higher proportions of Proteobacteria and Firmicutes, while those from erythematotelangiectatic rosacea were dominated by Actinobacteria. Pathogenic genera such as Bartonella and Escherichia were detected exclusively in mites from rosacea patients (104). These findings support the concept that the skin acts as a convergence point where neurovascular signals and gut-derived metabolites alter the terrain, facilitating complex host–microbe interactions that dictate the specific clinical phenotype (105).

Finally, therapeutic strategies targeting the BGSA validate this multi-nodal network. Conventional treatments (metronidazole, doxycycline) are now being complemented by axis-targeted interventions. Probiotics aim to restore regulation by modulating immune responses, suppressing neurogenic inflammation, and reducing vasodilation and TNF-α release (106, 107). Topically, formulations containing probiotic-derived components (e.g., Vitreoscilla filiformis) have been shown to reduce erythema and Demodex density (108). Simultaneously, stress management strategies, including mindfulness, cognitive behavioral therapy, and adequate sleep, have proven effective in interrupting the brain–skin feedback loop and improving overall disease control (109–111).

Emerging evidence indicates that hair follicle biology and disorders such as alopecia areata (AA) and stress-related telogen effluvium can be understood within the integrated BGSA framework. Specifically, the hair follicle functions as a neuroimmune-sensitive “mini-organ” whose cycling is regulated by the tripartite integration of neuroendocrine signaling, microbial ecology, and immune privilege maintenance.

First, the neuroendocrine arm initiates the pathological cascade through stress signaling. Arck and colleagues first proposed that gut microbiota influence neurogenic skin inflammation and hair growth, demonstrating in mouse models that the ingestion of specific Lactobacillus strains attenuated stress-induced damage. Their findings supported a unified gut–brain–skin axis extending to hair follicle cycling and highlighted the role of neurohormonal mediators, including HPA-axis activation, sympathetic signaling, and neuropeptide release, as key links between stress, gut composition, and follicular responses (5). Mechanistically, chronic psychological stress elevates neuropeptides such as CRH and substance P in both the circulation and the perifollicular environment. Since hair follicles express functional CRH receptors (CRHR1 and CRHR2), exposure to these stress mediators directly inhibits hair growth and induces local neuroinflammation, indicating an intrinsic neuroendocrine feedback mechanism within the follicle (112).

Second, gut dysbiosis acts as a systemic modifier of this sensitivity. Gao et al. (113) reported that microbial dysbiosis impairs immune regulation, thereby increasing the host’s vulnerability to psychological stress-induced hair cycle disruption. Further expanding this concept, Feng (114) summarized potential mechanisms involving metabolic and barrier defects: the effects of microbial metabolites such as SCFAs and tryptophan derivatives, combined with the translocation of microbial components due to increased intestinal permeability, create a pro-inflammatory cytokine milieu that negatively affects follicular epithelial cells.

Third, these signals converge to cause the collapse of Follicular Immune Privilege (IP), the central pathogenic event in AA. Stress-induced neuroinflammation and gut-derived systemic signals synergize to disrupt the delicate immune homeostasis of the hair follicle. This collapse is characterized by the infiltration of CD8⁺ T cells and natural killer (NK) cells into the hair bulb, responding to increased expression of MHC I molecules, NKG2D ligands, and danger-associated signals on follicular keratinocytes. Concurrently, this systemic dysregulation promotes Th1/Th17 polarization, resulting in elevated IFN-γ and IL-17 levels that intensify local inflammatory cascades. Thus, the BGSA framework explains how psychological stress triggers autoreactive immune attacks and disturbs hair follicle cycling (112).

Finally, clinical interventions validate the therapeutic potential of this axis. Targeting the gut-metabolic interface has yielded measurable benefits. In a 12-week study, participants with hair loss and a high risk of metabolic syndrome received twice-daily supplementation with a specific probiotic formula. The treatment restored gut microbial balance, promoted hair growth, and reduced stress-related psychological and physiological responses. These results suggest that probiotics can simultaneously modulate microbial and neuroendocrine mechanisms underlying AA (115).

Collectively, these findings outline a coherent framework where targeted probiotics, dietary modulation, and barrier-protective approaches merit systematic investigation as dual-benefit strategies for hair loss.

Vitiligo is a chronic immune-mediated depigmenting disorder characterized by the selective loss of functional melanocytes. The BGSA provides an integrative framework to explain how psychological distress and systemic metabolic shifts synergistically trigger cutaneous autoimmunity and oxidative stress.

First, the neuroendocrine arm initiates the systemic inflammatory cascade through psychological triggers. Chronic stress activates the HPA axis and sympathetic nervous system, modulating the systemic cytokine milieu. Circulating cytokines, including IL-6, TNF-α, and notably IFN-γ, can reach the skin and promote direct melanocyte injury (116, 117). This relationship is inherently bidirectional: visible depigmentation heightens psychosocial distress, which in turn exacerbates HPA axis activation, establishing a self-reinforcing pathological loop. Therefore, the disruption of this axis represents a key mechanism linking psychological stress to immune-mediated melanocyte loss.

Second, gut dysbiosis and altered microbial metabolism serve as systemic amplifiers of the disease. From the perspective of the BGSA, microbial imbalances account for the frequent comorbidity between vitiligo and psychiatric disorders. Gut dysbiosis facilitates the translocation of bacterial endotoxins such as LPS into circulation, leading to systemic inflammation. Crucially, tryptophan metabolism is a central metabolic checkpoint in this axis. Microbial metabolites of neurotransmitters contribute to emotional disturbances, while altered tryptophan metabolism further aggravates psychological dysfunction and modulates systemic immune responses (118). This metabolic dysregulation creates a pro-inflammatory environment that increases the host’s vulnerability to oxidative stress.

Third, these systemic neuroendocrine and microbial signals converge on the skin to drive innate and adaptive immune dysregulation. The convergence of stress-induced IFN-γ and gut-derived inflammatory mediators promotes the recruitment of autoreactive CD8+ T cells to the skin. Oxidative stress, which is central to vitiligo pathogenesis, is further influenced by these intersystemic interactions. The synergy between systemic cytokines and local oxidative stress leads to the collapse of melanocyte homeostasis and subsequent depigmentation (119, 120). Although the molecular details remain under investigation, this systems-based approach highlights the role of the BGSA in connecting emotional states and gut health to localized melanocyte destruction (116, 121).

Finally, therapeutic strategies targeting multiple nodes of the BGSA offer promising clinical outcomes. A multidisciplinary, patient-centered approach is essential for comprehensive care. Modulating the gut microbiota through probiotics, prebiotics, or dietary interventions has shown preliminary benefits in restoring microbial balance and reducing systemic inflammation (52, 122, 123). Simultaneously, psychological support and stress management (e.g., CBT) represent essential components that address the neuroendocrine drivers of the disease (119, 120, 124). Collectively, these interventions underscore the potential of a holistic strategy that not only targets depigmentation but also restores physiological and psychological balance across the interconnected brain–gut–skin network.