Gut-skin axis: Emerging insights for gastroenterologists-a narrative review

Table 1 Key microbial metabolites and their effects on the gut-skin axis

Metabolite	Produced by	Effects on gut	Effects on skin
SCFAs (e.g., butyrate)	Firmicutes (e.g., Faecalibacterium)	Enhances gut barrier, anti-inflammatory	Promotes tregs, reduces inflammation
p-Cresol	Clostridium difficile	Increases gut permeability	Decreases keratinocyte differentiation, causes xerosis
Indole derivatives	Tryptophan metabolism	Modulates intestinal immunity via AhR	Influences skin inflammation and barrier repair
Zonulin	Enteric epithelial cells	Increases intestinal permeability ("leaky gut")	May indirectly increase risk of inflammation-related dermatoses
Serotonin	Enterochromaffin cells/gut flora	Gut motility, mood regulation	Implicated in pruritus and neurogenic inflammation
Trimethylamine-N-oxide	Gut bacteria metabolizing choline/carnitine (e.g., Lachnoclostridium)	Increases systemic inflammation and oxidative stress; implicated in metabolic dysfunction	May contribute to skin aging and inflammatory dermatoses (indirectly via systemic effects)

SCFA: Short-chain fatty acid.