A questionnaire survey found that women suffering from abnormal bowel movements have many skin problems such as a high frequency of dry skin. Although there are similarities between the structure and barrier function mechanism of the gut and skin, experimental data are insufficient to show an association between the intestinal environment and skin conditions. Phenols, for example phenol and p-cresol, as metabolites of aromatic amino acids produced by gut bacteria, are regarded as bioactive toxins and serum biomarkers of a disturbed gut environment. Recent studies have demonstrated that phenols disturb the differentiation of monolayer-cultured keratinocytes in vitro, and that phenols produced by gut bacteria accumulate in the skin via the circulation and disrupt keratinocyte differentiation in hairless mice. Human studies have demonstrated that restriction of probiotics elevated serum free p-cresol levels and harmed skin conditions (reduced skin hydration, disrupted keratinisation). In contrast, daily intake of the prebiotic galacto-oligosaccharides (GOS) restored serum free p-cresol levels and skin conditions in adult women. Moreover, a double-blind placebo-controlled trial demonstrated that the daily intake of fermented milk containing the probiotic Bifidobacterium breve strain Yakult and prebiotic GOS reduced serum total phenol levels and prevented skin dryness and disruption of keratinisation in healthy adult women. It is concluded that phenols produced by gut bacteria are one of the causes of skin problems. Probiotics and/or prebiotics, such as B. breve strain Yakult and/or GOS, are expected to help maintain a healthy skin by decreasing phenols production by gut microbiota. These findings support the hypothesis that probiotics and prebiotics provide health benefits to the skin as well as the gut.
Keywords: gut environment; keratinisation; prebiotics; probiotics; skin conditions.