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Growth arrest-specific protein 6
Protein
Growth factor involved in hair follicle signalling and cellular processes relevant to hair cycle regulation.
Gut microbiota
BiologicalProcess
The community of gut microorganisms that modulates immune signalling, inflammation, and microbial metabolites with downstream effects on skin barrier function and disease risk.
Gut-skin axis
BiologicalProcess
The bidirectional communication system linking gut microbiota, immune signalling, and microbial metabolites to skin inflammation, barrier function, and disease susceptibility.
Hair follicle
AnatomicalStructure
Complex miniature organs embedded in the dermis and subcutis that cycle through growth (anagen), transition (catagen), and rest (telogen) phases throughout life.
Hair loss
MedicalCondition
Umbrella term for conditions causing increased hair shedding or thinning, ranging from temporary telogen effluvium to progressive androgenetic alopecia.
Hallmarks of ageing
BiologicalProcess
A framework of twelve interconnected biological processes - organised into Primary, Antagonistic, and Integrative tiers - that collectively produce the structural decline of ageing tissue.
Hard water
ChemicalSubstance
Water with elevated dissolved calcium and magnesium content. Disrupts skin barrier function through four mechanistically independent pathways.
Headache
MedicalSignOrSymptom
Non-GI side effect of tirzepatide showing significant variation by injection site, with thigh injection reducing incidence by nearly half.
Heparan sulfate proteoglycans
BioChemEntity
Glycosaminoglycan-bearing proteins distributed across the dermal ECM and basement membrane that store and regulate growth factors including FGF2, FGF7, FGF10, and VEGF in a protected on-site depot.
High sugar and processed food intake
BiologicalProcess
Dietary pattern of elevated sugar and processed food intake that leads to increased blood glucose and potential skin effects.
Histidine
ChemicalSubstance
An essential amino acid with a uniquely skin-specific metabolic pathway. Filaggrin is particularly rich in histidine, and its degradation converts histidine to trans-urocanic acid.
Hyaluronic acid
ChemicalSubstance
A glycosaminoglycan that forms the water-retaining ground substance of the dermis and maintains the mechanical environment fibroblasts need for collagen synthesis.
Hyaluronic acid lip fillers
MedicalTherapy
Hyaluronic acid lip fillers are injectable cross-linked HA gels used to augment lip volume, define the vermilion border, and restore lip proportion lost through ageing or asymmetry.
Hyperpigmentation
MedicalCondition
Hyperpigmentation describes the localised or diffuse overproduction or abnormal distribution of melanin in the skin. It is not a single condition but three mechanistically distinct presentations.
Hypothalamic–pituitary–adrenal axis
BiologicalProcess
The body’s central stress‑response system, driving CRH→ACTH→cortisol signalling systemically and via a parallel cutaneous HPA axis that regulates skin immunity, barrier function, and repair.
Ileal brake
BiologicalProcess
The ileal brake is a physiological feedback mechanism by which unabsorbed nutrients in the small intestine triggers the release of satiety hormones that suppress appetite.
Inflammageing
BiologicalProcess
Chronic low-grade sterile inflammation accumulating with age through senescence, cGAS-STING, and NF-κB amplification — impairing tissue repair and driving structural skin ageing.
Injectable Platelet-Rich Fibrin
MedicalTherapy
Low-spin blood preparation preserving platelets, leucocytes, and stem cells within a fibrin matrix that sustains growth factor release over 7-14 days.
Insulin
Protein
A β‑cell–derived anabolic hormone that regulates glucose uptake, lipid storage, and tissue growth, with chronic hyperinsulinaemia producing metabolic effects before overt hyperglycaemia.
Insulin resistance
MedicalCondition
A state of reduced insulin responsiveness driven by DAG–PKC signalling and impaired fuel switching, linking metabolic overload to systemic glucose dysregulation.
Insulin signalling
BiologicalProcess
The PI3K–AKT signalling pathway triggered by insulin receptor activation that drives glucose uptake, suppresses hepatic glucose output, and restrains adipose lipolysis.
Insulin-like growth factor 1
Protein
A growth factor produced primarily by dermal fibroblasts and dermal papilla cells, with roles spanning fibroblast proliferation, keratinocyte migration, collagen stabilisation, and angiogenesis.
Integumentary system
AnatomicalSystem
The body's outermost organ system, comprising the skin and its appendages: hair, nails, sebaceous glands, and sweat glands.
Interleukin-13
Protein
Inflammatory cytokine that suppresses production of barrier proteins including filaggrin and claudin-1.
Interleukin-33
Protein
IL‑33 is an epithelial‑derived “alarmin” cytokine released from stressed or damaged barrier tissues, including keratinocytes in the skin.
Interleukin-4
Protein
Inflammatory cytokine that suppresses barrier protein production, perpetuating barrier dysfunction during inflammatory skin conditions.
Interleukin-6
Protein
A pleiotropic cytokine with context-dependent pro- and anti-inflammatory effects determined by which receptor complex mediates signalling.
Inulin
ChemicalSubstance
A fructooligosaccharide prebiotic fermented by S. epidermidis to support SCFA‑driven acidification, AMP tone, and barrier integrity while forming a hygroscopic film that reduces TEWL.
Irritant lip plumping agents
ChemicalSubstance
Topical actives that trigger TRP‑channel‑mediated neurogenic inflammation to produce transient lip swelling; effects are short‑lived and increase barrier disruption with repeated use.
Kallikrein related peptidase 5
Protein
A trypsin-like serine protease expressed by keratinocytes that acts as the master initiating enzyme of epidermal desquamation.