The Gut, Hormone, Skin Axis in Skin Health and Ageing: A Scoping Review and Integrative Dermatological Perspective

This project is a scoping review investigating the relationship between gut health, incretin and endocrine signalling, systemic inflammation, and cutaneous ageing or skin health outcomes. The review focuses on the proposed gut–incretin–hormone–skin axis and its potential relevance to acne, comedogenesis, post-inflammatory hyperpigmentation, melasma, skin barrier dysfunction, dryness, sensitivity, wrinkles, laxity, and dermal collagen–elastin ageing. The purpose of this review is to map and synthesise the current evidence linking gut microbiota, diet, microbial metabolites, GLP-1, GIP, glucagon, insulin/IGF-1, estrogen, and inflammatory pathways with epidermal and dermal skin outcomes. Rather than attempting to prove that “gut health improves skin” as a broad claim, this review aims to clarify which biological mechanisms are supported by evidence, which skin outcomes have been most studied, and where the evidence remains indirect or emerging. The review will be conducted using a scoping review design, informed by JBI guidance for scoping reviews and reported in accordance with PRISMA-ScR. Search reporting will be structured using PRISMA-S principles where applicable. The Population–Concept–Context framework will be used to define eligibility criteria. The population will include humans with acne, inflammatory skin disease, pigmentary disorders, barrier dysfunction, dry or sensitive skin, or age-related skin changes, as well as healthy individuals with measured skin biophysical outcomes. Mechanistic animal or cell studies may be included as supplementary evidence where they clarify biological pathways relevant to gut-derived metabolites, incretin hormones, inflammation, glycation, or collagen–elastin ageing. The core concept of the review is that gut health and hormonal/metabolic signalling may influence skin not through a direct cosmetic “beauty” pathway, but through systemic effects on inflammation, oxidative stress, glycaemic control, insulin/IGF-1 activity, gut barrier function, short-chain fatty acid production, bile acid signalling, estrogen metabolism, and incretin hormone regulation. These pathways may affect sebaceous activity, follicular hyperkeratinisation, inflammatory acne, post-inflammatory melanogenesis, epidermal barrier function, dermal extracellular matrix remodelling, glycation, and fibroblast ageing. The review will include evidence from dermatology, microbiome science, endocrinology, nutrition, metabolic medicine, and aesthetic dermatology. Relevant study types may include randomised controlled trials, observational studies, systematic reviews, scoping reviews, clinical intervention studies, translational studies, and selected mechanistic studies. Areas of interest include dietary glycaemic load and acne, dairy and whey protein exposure, probiotics and prebiotics, gut dysbiosis in inflammatory dermatoses, short-chain fatty acids and barrier function, GLP-1 receptor agonists and gut microbiota, dual GLP-1/GIP agonism, glucagon-related metabolic signalling, estrogen and the estrobolome, advanced glycation end-products, oxidative stress, and dermal collagen or elastin changes. Expected outcomes of the review include a mapped evidence table showing which gut, dietary, incretin, endocrine, inflammatory, and metabolic pathways have been studied in relation to specific skin outcomes. The review will also identify areas where evidence is strongest, such as acne, atopic dermatitis, inflammatory skin disease, and diet–insulin/IGF-1 signalling, as well as areas where evidence is currently weaker or indirect, such as pore size, dermal laxity, wrinkle reversal, and direct collagen–elastin improvement from gut-directed interventions. A further expected outcome is the development of an integrative conceptual model describing the gut–incretin–hormone–skin axis as a systemic skin-reactivity and metabolic-ageing pathway. This model proposes that gut dysbiosis, high glycaemic load, low fibre intake, insulin resistance, adipose inflammation, oxidative stress, glycation, and hormonal imbalance may lower the threshold for visible skin dysfunction. Conversely, improved gut and metabolic health may raise skin resilience by reducing inflammatory and glycaemic pressures that contribute to acne, pigmentation persistence, barrier fragility, dryness, sensitivity, and dermal matrix ageing. This review is expected to clarify that gut-directed, diet-directed, and incretin-related interventions should be understood as adjunctive systemic skin-support strategies rather than stand-alone dermatological or cosmetic anti-ageing treatments. The anticipated conclusion is that gut health may support skin ageing and skin quality indirectly by reducing the biological conditions that make skin more inflamed, reactive, pigmented, dry, or structurally aged. Preliminary exploratory searches have been conducted to refine the topic, identify relevant concepts, and develop candidate search terms. Formal database searches, deduplication, screening, data extraction, and synthesis will be conducted after protocol registration. The final review will aim to provide a transparent map of current evidence, identify mechanistic and clinical research gaps, and propose future study designs using objective skin measurements such as acne lesion counts, transepidermal water loss, corneometry, melanin index, MASI, sebumetry, cutometry, ultrasound, optical coherence tomography, reflectance confocal microscopy, inflammatory biomarkers, hormonal markers, and gut microbiome profiling.