Skin-directed Solius UVB light therapy increases vitamin D levels and alters intestinal microbiome and metabolome homeostasis.

The recent global rise in immune and inflammatory diseases, such as psoriasis and inflammatory bowel diseases, has been linked to Western society-based changes in lifestyle and environment. These include decreased exposure to sunlight in the ultraviolet B (UVB) spectrum, leading to impaired production of 25-hydroxyvitamin D. In addition, many of these diseases are linked to dysbiotic changes in gut microbiota composition and function. Clinical overlap between skin and gut diseases, along with growing evidence for a bidirectional "skin-gut axis," supports the concept that exposing the skin to UVB light may impact the gastrointestinal tract including the microbiome. At present, exactly how UVB exposure influences gut microbial and metabolomic profiles and the role that baseline vitamin D status may play in these relationships remains unclear. To test this, we fed mice either a vitamin D sufficient or deficient diet and exposed them to Solius UVB light therapy (targeted peak of 293 nm) either acutely (single exposure) or repeatedly (six exposures) over 14 days. A single exposure modestly increased circulating vitamin D levels, whereas repeated exposures significantly increased levels to that seen in mice fed a vitamin D sufficient diet, and caused skin photoadaptation without histological injury. Notably, repeated Solius UVB light therapy induced significant changes in the composition of the gut microbiota, coinciding with marked fecal metabolome alterations, particularly in mice fed the vitamin D deficient diet. These fecal metabolic changes included increases in short chain fatty acids, as well as bile acid and tryptophan derivatives. In contrast, serum metabolomic responses were less marked, suggesting that Solius UVB light therapy induction of overt metabolic shifts within the intestinal tract is not merely reflective of systemic responses. Together, these findings demonstrate that skin-directed Solius UVB light therapy can be delivered safely, to induce significant changes in the composition of the gut microbiota and its associated metabolites, in a manner that depends on repeated exposures and baseline vitamin D status. Further studies exploring the ability of Solius UVB light therapy skin exposure to promote intestinal homeostasis and health are thus warranted.